The steps of the dance: 3. Swing Phase

SWING PHASE

The goal of early swing is to get airborne and accelerate the leg forwards on a trajectory that will allow it to overtake the torso by mid-swing. While it is essential that the foot should accelerate in early swing, it should be borne in mind that it must decelerate in late swing if it is to have zero horizontal velocity relative to the ground at foot-strike. It might seem at first sight that the need to match an energy consumptive acceleration with a deceleration that will also consume energy should encourage us to be conservative in the generation of acceleration. However, this would be a very misleading conclusion. Our ability to generate adequate forward acceleration of the foot in early swing determines our ability to maintain a particular target speed.

The crucial role of acceleration of the leg in early swing
To understand why forward acceleration of the leg in early swing is crucial, we need to return to basic biomechanical principles. In the earlier posts in this series in which we considered the implications of Newtonian physics we reached the conclusion that cadence should be high and time on stance should be short. Except at very slow speeds, cadence should be near the limit determined by the optimum speed of contraction of muscles. Observation of elite runners suggests the optimum is a cadence in the range 180-200 strides per minute. Elite athletes employ a cadence in this range for all except very slow paces.

Furthermore, time on stance should be as short as can be tolerated, after allowing for the fact that ground reaction forces and risk of tissue damage increase dramatically as time on stance becomes very short. Elite athletes tend to spend only about 50-100 milliseconds on stance, with the longer times being applicable in long events where protection of muscles from damage due to repetitive impacts in important. Apart from these relatively small variations, cadence and time on stance are fairly consistent over a range of paces extending from 1500K pace to marathon pace. Therefore, over this range of paces, the major variable that increases as pace increases is stride length.

As shown on the calculations page accessed via the side bar, the work that must be done against gravity (per unit of time) is determined by cadence, time on stance and body weight. The energy required to lift the body is not directly influenced by stride length. However, increase in stride length must be matched by an increase in the amount of acceleration required to bring the foot forward fast enough to support the body at foot fall. Thus, it is ability to accelerate the leg in early swing phase (and then decelerate it again in late swing phase), that is the main determinant of our ability to maintain a high pace. So how should we do this?

Breaking contact with the ground
In late stance the elastic recoil of quadriceps, augmented by concentric contraction, has imparted an upward impulse to raise the centre of gravity and hip extension has preloaded the hip flexors (e.g. psoas). As the body rises, an active contraction of hamstrings lifts the foot from the ground. Contraction of the hamstring alone, when the hip is already extended, will produce flexion at the knee, pulling the foot up wards behind the line from foot to hip. While this is the path of the foot observed in many athletes, if the main goal is to accelerate the leg forwards, the hamstring contraction should be accompanied by hip flexion.

Accelerating the leg
Fortunately, the preloading of the hip extensors (i.e stretching associated by eccentric contraction) during hip extension in late stance can be utilized to facilitate a powerful recoil associated with concentric contraction of the hip flexors that accelerates the leg forwards.

Deceleration of the leg
However, the price paid for this powerful forward acceleration is the need for a powerful deceleration in late swing, provided by an eccentric contraction of the hip extensors. This is stressful for the hamstrings, and suggest that exercises such as hip swings might play a useful role in conditioning the body during training.

As the hip extensors decelerate the leg, the lower leg and foot should be allowed to swing down to that the knee is only mildly flexed, in preparation for footfall. The combination of contraction of hip extensors and relaxed un-flexing of the knee present a challenge. Because the hamstrings cross both hip and knee joint, pure hamstring contraction to decelerate the leg would prevent the relaxed swinging of the knee. Therefore it is essential to use gluteus maximus to assist in the deceleration of the leg. In addition, some contraction of the quadriceps might also be used to un-flex the knee, but this should be done very sparingly, as vigorous contraction of quadriceps at this stage is likely to result in over-striding.

In summary

Contraction of the hamstrings will help break contact with the ground as the body rises under the influence of the upwards impulse generated by recoil and quadriceps contraction in late stance. However, the ability to accelerate the leg forwards in early swing phase (and then decelerate it again in late swing phase), is the main determinant of our ability to maintain a high pace. Rapid forward acceleration of the leg in early swing might be achieved by employing the preloading of the hip flexors (e.g. psoas) that occurred during late stance to facilitate a powerful contraction of the hip flexors. However, this must be matched by a deceleration produced by contraction of hamstrings and gluteus maximus in late swing, allowing the foot to drop to the ground with the knee slightly flexed and travelling with approximately zero horizontal velocity relative to the ground.

The Steps of the Dance: 2. Stance

Preamble

The early articles in this series examined the constraints that Newton’s laws of motion place on the way in which we run. The most recent article, posted on 31^st March examined the question of how we should orient the joints and tension the muscles at foot fall in order to capture the energy of impact as elastic potential energy, while minimizing the risk of damage to muscles and other tissues. This article examines the actions that occur during stance.

STANCE

Following foot fall the foot remains stationary on the ground, anchored by friction, while the COG passes over the point of support. Then as the COG continues forwards the hip extends until lift-off. We will use the term early stance to describe the period from foot fall to point where COG passes over point of support, and late stance for the period from the passing of the COG over point of support to lift off.

The preceding article in this series discussed the mechanisms by which a substantial portion of the energy of impact at footfall is converted to elastic potential energy, stored in quadriceps, calf muscles and the connective tissues of the foot. During late stance the elastic potential energy is recovered and contributes to the generation of the impulse required to get airborne at lift off.

 

While the main task that must be performed during stance is the generation of the upwards impulse required to get airborne, there are also a number of important subsidiary actions, including:

- generation of horizontal GRF.

- subjecting the hip flexors to eccentric contraction preparing them for the task of accelerating the leg forwards in early swing phase

- subjecting the hip rotators to eccentric loading in preparation for the rotation around the long axis of the body during swing phase

- preventing the unsupported hip from dropping and tilting the pelvis.

These actions are mostly performed automatically when running, but understanding them is important to allow us to develop strength and resilience in the muscles and other connective tissues, and to identify faults that might cause injuries. In particular, it is important to identify the muscles that undergo a large change in length during the gait cycle, as flexibility exercises might profitably be employed to maintain as these muscles and their tendons in a pliant state. In contrast, those muscles which exert large forces over relatively short distances are generally better maintained in a stiffer state.

 

Generation of the upwards impulse.
The only upwards directed external force acting on the body is the vertical component of Ground Reaction Force (vGRF) and hence this force is responsible for lifting the body. vGRF is a reaction by the ground as it resists compression by a downwards directed force exerted though the foot. It should be noted that pulling the foot towards the hip (like pulling on ones own boot-straps) cannot lift the body.

About 50% of the energy required to become airborne might be derived from elastic recoil of quads and calf muscles. The knee had been initially slightly flexed at footfall and then flexed even more as the impact energy was absorbed in early stance; as this flexion occurs the quadriceps undergo a moderate eccentric contraction , then in late stance, recoil augmented by the concentric contraction of the quadriceps straightens the leg, and imparts an upwards impulse to the body.

Similarly, the recoil in the calf abolishes the mild dorsiflexion of the ankle that had developed by midstance and re-establishes the mild degree of plantar flexion present at footfall. The pronation of the foot in mid-stance is replaced by slight supination.

 

Generation of horizontal impulse
As the hip extends in late stance, the predominantly downwards directed forces exerted by the leg on the ground inevitably have a small backward directed component, which is resisted by friction, thereby generating a forward directed horizontal GRF. This will propel the body forwards against wind resistance – but unless there is a strong head wind, this forward propulsion is usually excessive and the excess must be matched by braking in early stance (as discussed when we addressed the question of where the foot must land at footfall). Apart from the contribution that overcomes wind resistance, the impulse due to horizontal GRF does not achieving any useful purpose. Therefore this impulse should be minimized as far as is feasible by lift-off from stance which is as rapid as possible (bearing in mind that stance must be long enough to allow the generation of adequate vertical impulse without necessitating very high and potentially damaging vGRF

 

Preloading the hip flexors
Immediately after foot fall the hip extensors (mainly gluteus maximus and the hamstrings) will undergo a degree of eccentric contraction as impact forces are absorbed. In mid and late stance, the elastic energy stored during eccentric contraction will be released in conjunction with moderate concentric contraction of the hip extensors. In addition there will be a strong impetus to passive extension of the hip as momentum carries the COG forwards beyond the anchored foot. Thus there is an almost effortless extension of the hip which stretches the hip flexors, priming them for a powerful concentric contraction in early swing phase that will help propel the foot and leg forwards to overtake the torso. Because the hip flexors, predominantly psoas undergo a major change in length, these muscles should be maintained in flexible, pliant state. The hip flexors are perhaps the most important muscles for a runner to maintain in a flexible, pliant state.

Preloading the hip rotators
In late stance, as the torso moves forwards leaving the support foot behind, not only is there passive extension of the hip but there is also a passive (external) rotation of the hip about the long axis of the body because the hip on the supported side is tethered via the leg to the ground while the other hip is free to move with the torso. Thus the internal rotators of the hip are subjected to an eccentric contraction which primes them for an internal rotation after lift-off that will help increase stride length.

 

Supporting the pelvis
The unsupported hip tends to drop, causing the pelvis to tend swing inwards towards the supporting leg. This adduction of the hip must be opposed by the hip abductor muscles of the supporting leg. The main abductors are gluteus medius and gluteus minimus, assisted by tensor fascia lata (TFL), a long muscle running down the outside aspect of the thigh from the rim of the pelvis to attach to the tibia via the iliotibial band (ITB). If gluteus medius is weak, TFL is called upon to bear too much of the load in prevent the pelvis from tilting. Excessive tension of the iliotibial band creates a risk of friction at the point where the ITB passes adjacent to the bony protrusion (femoral condyle) on the lateral aspect of the knee. Excessive eversion of the ankle, which tilts the tibia (shin bone) outwards at the knee also increases the friction on ITB. Increased friction may lead to painful inflammation (ITB syndrome).

 

 

In summary

The quads and calf muscles undergo eccentric contraction in early stance thereby storing much of the energy of impact as elastic energy. In late stance, recoil of these muscles, aided by moderate concentric contraction provides the impulse required to accelerate the body upwards against gravity. The other major muscle action during stance is hip extension. Some of the impact energy is absorbed in an eccentric contraction of the hip extensors (gluteus maximus and hamstrings) which subsequently recoil while undergoing concentric contraction in late stance. Hip extension is further promoted by passive extension generated by the momentum of the torso. The resulting hip extension pre-loads the hip flexors preparing them for a powerful hip flexion to accelerate the leg forwards after lift-off. Other important actions are external rotation of the hip that preloads the internal rotators, and hip abduction that prevents tilt of the pelvis.

In the next article in this series we will examine the actions occurring at lift-off and during swing phase.

The steps of the dance: foot fall

Preamble

In the first part of this series of articles (posted in mid March 2008) we examined the way in which of Newton’s laws of motion constrain the way in which we run. This section of the series will examine the ways in which we should orient our joints and contract our muscles to run efficiently and safely in accord with the constraints of Newtonian mechanics.

It should be noted that the conclusions we draw should guide the way we judge running style objectively, such as when examining a video recording, or using specialized equipment such as a force plate or electromyography. These conclusions should not dictate the way in which we attempt to control our muscles consciously when running, because it is impossible to focus simultaneously on everything that matters, and furthermore, muscle actions that require precise timing are more effective when controlled automatically via habit than by imposition of conscious control. If we have not yet acquired the required habits, drills performed with a greater degree of conscious control might help establish the required automatic control. The issue of what perceptions we should attend to when actually running will be dealt with in a subsequent section on the psychodynamics of running.

 

Summary of Newtonian principles

In the first section of the series devoted to Newtonian mechanics, we saw that the laws of conservation of conservation of momentum and angular momentum led to the following principles:

P1) On a level surface in the absence of wind resistance, no direct net propulsive force is required to keep the body moving at a constant velocity. However, energy is required to lift the body against gravity to compensate for free fall during airborne time, and also to accelerate each leg forwards in early swing phase so that it can overtake the torso and provide support at the next footfall.

P2) When the leg is angled downwards and backwards in late stance, there is a forward-directed horizontal ground reaction force (GRF) that will propel the body forwards. In the absence of wind resistance on a level surface, this must be balanced by a backward directed horizontal GRF at some other part of the gait cycle. This will generally be provided by the braking effect in the first half of stance, provide the point of support in front of the body’s centre of gravity (COG) at that stage. Unless the foot lands in front of the COG there will be no compensation for the acceleration of he body in the late stance and hence , it will be impossible to remain in control at constant velocity. A face down crash will occur.

P3) Any force that pulls the foot forwards towards the torso in early swing phase must be balanced by a compensating force that pulls the foot backwards towards the torso after the foot has passed beneath the torso. As a result, the foot will be travelling at the same speed relative to the torso (and therefore the same speed relative to the ground if torso moves at constant velocity) at foot-strike and was the case at lift-off. If velocity relative to the ground is zero at lift off, then velocity relative to the ground will be zero at foot fall.

P4) Any external torque applied to the body at some point in the gait cycles (eg gravitation torque that arises when the COG is not aligned over the point of support) must be compensated for by an oppositely directed external torque applied at some other stage of the gait cycle. (In general, gravitational torque provides angular acceleration in a face forwards and down direction in late stance, and this must be compensated by a torque producing head back and downwards directed torque in early stance.

P5) Mean vertical GRF during stance is equal to body weight x stride duration/time on stance. This equation must be satisfied to ensure that the average upwards force over the full gait cycle exactly matches body weight.

Because Newton’s laws are immutable for bodies of human scale moving at running speed, we cannot maintain a constant speed if any of these principles are violated

 

Essentials for efficient safe running

These principles led us to the following conclusions about how we should run for optimum efficiency and safety (posted 22^nd March, but repeated here for convenience).

C1) High cadence is beneficial

C2) Time on stance should be small compared with airborne time (though at very slow speeds total energy cost actually increases as time on stance decreases while high cadence is maintained)

C3) If time on stance is substantially shorter than airborne time, vGRF will be at least several times body weight

C4) The impulse require to lift the body against gravity must be provided by a downwards push of the leg against the ground.

C5) Skilful orientation of joints and tensioning of muscles at foot fall is required to minimize abrupt rise in vGRF

C6) Landing in front of the COG is inevitable to balance the effects of the forward directed hGRF acting in the second half of stance.

C7) The majority of the impulse required to accelerate the legs forward past the torso and then decelerate them to provide support at foot fall, is best provided by internal muscle action that pulls the foot forward towards the torso in early swing and backwards towards the torso in late swing.

These recommendations apply whatever running style we choose to adopt. The goal of the current article is to begin to address the question of how we should orient our joints and tension our muscles to achieve these recommendations. The connective tissues of the body (muscles, tendons, ligaments) are viscoelastic, which means that their stiffness depends on the rate at which force is applied. Therefore, it is virtually impossible to make accurate predictions about the exact consequences of any specific muscular action. The conclusions reached in this section are tentative and must be tested against experience. Furthermore it is likely that what works best for one individual in one circumstance might not be best for other individuals, or indeed even for the same individual under different circumstance.

 

Assumptions regarding behaviour of musculo-skeletal tissues

In order to proceed we need to understand how musculo-skeletal tissues react to forces. There is an extensive body of knowledge from material science and physiology that provides the principles and information needed to understand the behaviour of body tissues, but in this article, we will merely present some of the principles as assumptions. In a subsequent post, we will examine the evidence justifying these assumptions. The assumptions are:

A1) Musculo-skeletal tissues are more likely to be damaged by forces that are applied abruptly. For example, Simpson and colleagues in their chapter in Exercise and Sport Science (edited by WE Garrett & DT Kirkendall) provide references to several studies that indicate that the rate of impulsive loading might determine the risk of degenerative changes to cartilage.

A2) Tissue rupture can occur after many repeated applications of a relatively small force which is below the threshold required to cause tissue rupture on a single application. This is why repetitive strain injuries arise in tendons after thousands of repetitions of the same small impact such as is generated when playing a piano, and why metal fatigue caused ships in World War 2 Atlantic convoys suddenly broke up and sank in mid-ocean after many repeated relatively minor impacts with ocean waves.

A3) Muscles develop greater force when a concentric contraction (in which the muscle shortens as it exerts force) follows an eccentric contraction (in which the muscle is stretched by an external force while the contractile process within the muscle exerts an opposing force. This is known as the stretch shortening cycle (SSC). The effectiveness of the SSC depends on the period of time between eccentric contraction and the subsequent concentric contraction (known as the amortisation time). This probably depends on circumstance such as the size of the muscle and the stiffness of the muscle during the eccentric contraction, which in turn depends on the rate at which the external force builds up, because muscle and tendon is viscoelastic.

A typical manoeuvre that illustrates the SSC is the drop jump in which the body drops from a height (typically 50-100cm) and then rebounds into the air. On initial impact, some of the energy of impact is stored as elastic energy and then recovered as the muscle recoils. As the height of the drop increases, the height of rebound increases up to a certain optimum value and then decreases. At the optimum, the maximum amount of elastic energy is recovered in the recoil. Data collected by Ishikawa and colleagues demonstrates that the duration of the eccentric contraction of the quadriceps is around 100 milliseconds when the maximum rebound is achieved. If the leg were stiffer at impact, the optimum would be achieved at a shorter time. If it were less stiff, the optimum would be achieved at longer time. Thus, we will assume that the optimum amortisation time for human leg muscles when running is in the range 50-150 milliseconds; shorter values will apply when the leg is stiffer on impact.

The muscle actions of the gait cycle are many and the relationships between them are complex. In particular, the actions we perform at lift-off from stance place strong constraints on where and how we land at the next foot-fall. We will start by considering each section of the gait cycle in isolation and than address the question of how it should all fit together

 

FOOT-FALL

The goals

There are three goals to be achieved at footfall:

1) maximizing the capture of impact energy as stored elastic energy that can be recovered at a later stage of the stance phase,

2) minimizing the risk of tissue damage; and

3) avoiding over-striding.

 

In general a stiff spring stores elastic energy more efficiently and delivers it for re-use more rapidly than a soft spring. The connective tissues of the legs are viscoelastic which means that stiffness will be greater when the tissue is subjected to a rapidly rising force. Therefore, for maximum mechanical efficiency of elastic recoil, the joints and muscles should be deployed so that impact forces are absorbed rapidly, and the time on stance should be should be correspondingly short to allow efficient recovery of the elastic energy.

However risk of tissue damage, both short and long term, is likely to be greater if vGRF rises rapidly (assumption A1 above). Impact will stretch the quadriceps and calf muscles. This stretching is likely to cause microscopic tearing of muscle fibres, which is the most likely explanation for delayed onset muscle soreness (DOMS) experienced the day after heavy exercise. Furthermore, immediately after substantial use muscles suffers a loss of power that lasts for one or two days, consistent with the possibility that microscopic tearing has weakened the muscle. In long events, such as a marathon or ultra-marathon, in which the legs will be subjected to thousands of repeated impacts, it is especially important to minimise microscopic tearing.

Furthermore, in the case of runners who have recently adjusted their running style or are currently unfit, the risk of microscopic tearing is likely to be increased because the adjustment of muscle tension and joint position is likely to be less finely tuned.

Thus there are a variety of circumstances in which minimization of risk of tissue damage might take precedence over achievement of maximal mechanical efficiency. Under such circumstances landing might be softened so as to avoid abrupt rise in vGRF. Fortunately, under such circumstances the time optimum time scale for recovery of elastic energy will be increased, so provided time on stance is adjusted appropriately, it is nonetheless possible to retain at least a moderate level of mechanical efficiency. Thus, one consequence of the viscoelastic nature of human connective tissue is the possibility of adjusting rate of rise of GRF so as to reduce risk of tissue damage without serious loss of efficiency.

The third goal to be achieved at foot fall is avoidance of over-striding. Although many experts since (and perhaps even before) Gordon Pirie have emphasized the importance of avoiding over-striding, it is not easy to define what is meant by over-striding. For the purpose of this discussion, we will define over-striding as landing further forward than is necessary to counteract the inevitable forward directed GRF that arises from the backwards angling of the leg during the second half of stance when duration on stance is optimal for the circumstances.

One consequence of the forgoing considerations is that the optimum foot fall depends on circumstances. There is no single universally applicable description of how the joints and muscles should be deployed. While this flexibility allows for adjustment according to priorities, it might seem daunting that there are many possibilities. Fortunately, as we will see in the finals section of this series of articles, most of the required adjustments are automatic provided we focus on a few essential features.

 

Orientation of the joints

The ankle and foot
The foot human is exquisitely designed for the absorption and distribution of elastic energy to structures such as the medial longitudinal arch, and it seems sensible that when running the foot should be allowed to fall in a manner that utilizes this design, except under circumstances such a sprinting, where a high degree of stiffness of the leg is required to ensure very rapid return of the stored elastic energy. Because our main concern in this article is with middle and long distance running we will focus on the mechanism for transferring the load to the medial arch.

As the foot falls the leg must be angled towards the mid-line of the body to ensure that COG advances in a straight line. Hence the foot strikes on the lateral side. The ankle should be very slightly plantar flexed and the initial point of support a little forward of mid-foot. The foot rolls inwards (pronation) so that the load is transferred towards the medial side, and the plantar flexion of the ankle is relaxed so that the heel touches the ground, to minimise the stress on the Achilles tendon. Otherwise, the Achilles tendon would be required to support a downwards force typical several time body weight via a cantilever. There is debate about the degree to which force should be transmitted to the ground via the heel. In the manual for the Pose Method, Dr Romanov states that the heel should only lightly brush the ground. My own belief is that under circumstances where avoidance of damage to tendons and muscles is a priority (eg during a marathon or ultra-marathon) it is best to allow the heel to bear appreciable weight.

As the COG passes forward of the point of support, the ankle should remain approximately neutral but because the leg is now angled backwards, the point of support moves forward in the under surface of the foot and the foot should roll outwards slightly (supination) as recoil occurs. The process of pronation followed by supination transfers weight to the medial arch and then allows recoil that employs the stored elastic energy to provide an upwards impulse. The degree of pronation should be modest to ensure that the elastic energy is not dissipated prior to recoil.

The knee
The knee should be at least slightly flexed at footfall to ensure that the quadriceps absorbs much of the energy of he impact. If the quadriceps is quite highly tensioned prior to impact, the energy of will be absorbed rapidly. This is appropriate when sprinting but for middle and long distance runners, there should be less tension in the quadriceps allowing a somewhat greater knee flexion and more gradual absorption of elastic energy.

Where should the foot fall?

Unless wind resistance is sufficient to counteract the forward directed GRF generated in the second half of stance, the point of support at foot fall must be in front of the COG in order provide adequate braking to prevent uncontrolled acceleration. As implied in the definition of over-striding given above, the point of support at foot fall should be the minimum distance necessary to compensate for that part of the forward GRF that is not counteracted by wind resistance. The longer the duration of stance the greater the horizontal GRF and the hence the further forward the point of support should be a footfall. Thus, time on stance determines where the foot should fall.

What is the optimal time on stance? It can be shown that the energy consumed in generating the forces that elicit the horizontal ground reaction forces is proportional to velocity squared multiplied by time on stance (see calculations page – to be presented early April). At moderate and high speeds the energy cost of generating these horizontal forces become appreciable compared with the energy costs of compensating for free fall when airborne, so it is desirable to keep time on stance as short as possible. However if time on stance is too small vertical GRF becomes damagingly high and furthermore there is a risk that stored elastic energy will not be recovered efficiently.

Thus, the optimum time on stance will depend on the relative priority of speed versus minimization of damage to tissue. As discussed above in the section on assumptions, the available evidence suggests that elastic energy can be recovered with times on stance ranging from around 50 to 150 milliseconds, depending on how rapidly GRF rises.

When speed is the priority, the knee should be held fairly stiffly at foot fall leading to rapid rise in GRF and rapid accumulation and then release of elastic energy. Time on stance will be as short as 50-60 milliseconds. This time on stance will generate vGRF that is 4-5 times body weight at cadence 180 /min. As approximately half of the time on stance is spent with the point of support in front of the COG, footfall should occur approximately 25 -30 milliseconds before COG passes over the point of support. This footfall should be about 10 cm in front of the COG at a speed of 4.1 m/sec (equivalent to 4 min per Km).

When minimization of tissue damage is the priority, (eg in a marathon or ultra-marathon or for a runner with less skill in controlling a rapid rise in vGRF) it would probably be prudent to land with foot fall up to 50-60 milliseconds before the COG passes over support. Provided the knee is allowed to flex somewhat more to give a relatively soft landing, the rate of uptake of elastic energy and its subsequent recovery can be slowed sufficiently to match the time on stance. Mean vGRF will only be about twice body weight.

 

Horizontal speed of the falling foot

To minimize shearing forces in the foot, horizontal speed of the foot relative to the ground at foot fall should be near to zero. In fact provided the forwards impulse delivered while the leg was angled backwards in the latter part of the previous stance was exactly matched by the braking impulse in the first part of that stance (plus wind resistance), and also providing the accelerating impulse during early swing was matched by a braking impulse during later swing, the speed of the foot will automatically be adjusted to zero at footfall. Note that swing phase and stance phases impulses must be balanced separately, since only the stance phase horizontal forces exert a net effect on the body as a whole, and these must therefore be balanced (after allowing for wind resistance) to achieve a constant forward motion of the body.

 

Summary

At foot fall, the ankle should be slightly plantar flexed, and after initial impact on the lateral side of the foot a little in front of mid-foot, the load should be transferred to the medial arch by a small degree of pronation. The location of point of support on the sole of the foot should move slightly backwards and the heel should touch the ground before the point of support moves forward again after the COG passes over it. Finally mild supination promotes recoil releasing the stored elastic energy. The degree of stiffness of the knee joint should be adjusted according to the relative priority of speed versus protection against tissue damage. At foot fall the point of support should be in front of the COG by the minimum amount needed to provide braking to compensate for the forwards impulse delivered when the leg is angled down and backwards in late stance. This amount will depend on duration of stance, but typically the time from footfall to when the COG passes over the point of support should be in the range 25-50 milliseconds, with the shorter durations being appropriate when speed in the priority and longer duration appropriate when minimizing risk of tissue damage is the priority. At foot fall, the horizontal speed of the foot relative to the ground should be zero. This will be achieved automatically if the pairs of horizontal forces acting during the various phases of the gait cycle are well matched.

The muscle actions required in late stance and during swing phase will be discussed in future blogs.

Running: a dance with the devil

Running is becoming airborne.

The essence of running is becoming airborne. When a human wants to increase speed while walking, he or she can increase stride rate or stride length. Beyond a certain stride rate, muscle contraction becomes inefficient because force is generated by a ratchet-like interaction between actin and myosin molecules within the muscle fibre, and the speed of this ratchet action is limited by the time it takes to make and break chemical bonds. Beyond a certain stride length, efficiency falls due to poor leverage of muscles on awkwardly angled legs. So the only practical option for further increase in speed is to increase stride length by becoming airborne for part of each stride. Thus we make the transition from walking to running.

Becoming airborne requires energy to propel us upwards against gravity. Once we are airborne, our body inevitably experiences a downwards acceleration of 9.8 metres/sec/sec (32 feet/sec/sec) due to gravity. The energy used to raise the body is now converted to kinetic energy that must be dissipated on impact with the ground. While a single impact following a fall of a few inches is unlikely to do much damage, minor impact repeated thousands of times creates a risk of repetitive strain injuries to connective tissue or even to stress fracture of bones such as the metatarsals in the feet or the tibia (shin bone). Thus, while running can be both graceful and efficient, it is also an energetic and risky form of locomotion. Not surprisingly, many runners suffer injury.

The deal with the devil
Thus running is a dance with the devil – gravity. We spend energy raising ourselves against this demon and then are at risk of injury as we are flung back to earth. However, in the force of impact, there is the sniff of a deal with the devil. If instead of dissipating the impact energy destructively at foot-fall we can capture it as elastic potential energy by the stretching of muscles and other connective tissues, this elastic energy might subsequently be recovered to propel us upwards at lift-off. The muscle contraction energy required to lift our bodies is reduced and the jarring effect of impact is diminished.

The process of capturing impact energy as elastic energy and sustaining it as we prepare for lift-off requires exquisitely controlled tensioning of muscles and angling of joints. Releasing it at the right moment and in the correct direction requires exquisite timing. Fortunately our brains learn to do this automatically in infancy and childhood, so for the most part, we can run tolerably well without thinking about it. However, whether due to bad habits of posture acquired sitting in an office chair, to de-conditioning of the muscles of the feet due to wearing shoes, or simply the fact that nothing in either the evolution of the species or the experiences of childhood prepared us for the monotonous repetitive impacts produced by running for miles on a paved surface, few people run naturally with optimum efficiency or adequate safety. Therefore, we need to learn how to run. This is the introduction to a series of three articles that will address the question of how to run efficiently and safely.

The laws of the dance
In our dance with the devil both he and we are constrained by the laws of motion. We cannot violate these laws. If we try to we are likely to waste energy and/or injure ourselves. In this article we will examine the physical mechanics of running. We will identify the constraints imposed by the laws of Newtonian physics. These laws are immutable (at least for bodies of human scale moving at running speed) and therefore, they provide a clearly defined framework that must be taken into account irrespective of personal choice or opinion.

The steps of the dance
In the second article, we will examine the biodynamics of running; that is, the optimum way to use of muscles, connective tissues and joints to execute the movements required to become airborne, to maintain forward momentum and move our legs forward to provide support at footfall; and to avoid injury on impact. Because of the complexity of the human body, it is virtually impossible to take into account all of the factors that might determine the outcome of a particular action, so the proposals are more speculative. They should be tested against experience, but it is not easy to generalise from a single test because individual differences in body constitution and in circumstances can lead to different outcomes. Therefore, the proposals in this section should be taken with a pinch of salt

The mind of the dancer
The third section will deal with the psychodynamics of running: the intentions, beliefs and perceptions that allow us to perform the steps of the dance. It is impossible, and in any case counterproductive to try to consciously manage each muscle contraction when running. We can only attend consciously to a single perception at one time, so we need to identify the aspects of our running on which it is most helpful to focus consciously. Fortunately, as we shall see when we consider the constraints imposed by the laws of mechanics, the magnitude and direction of the impulses delivered at lift-off place tight constraints on the location and impact of footfall. Furthermore, we have well developed automatic mechanisms that regulate footfall. Therefore, most of our conscious focus should be on the lift-off.

Perception is a product of sensory information entering the brain and of predictions generated within the brain. The predictions are shaped by prior beliefs. What we perceive does not necessarily correspond exactly with what an external observer or a video camera might record. We ourselves can shape our perceptions. Some schools of running technique, such as Pose (Pose Method of Running, Nicholas Romanov, Pose Tech Corp 2002) appear to encourage perceptions that are contrary to the laws of physics, and in particular encourage the perception that freely available propulsive energy is provided by gravity. The Pose Method provides many valuable insights into good running style. The perception that gravity provides freely available energy for propulsion might be beneficial insofar as it might discourage unnecessary and wasteful muscular effort, but in my opinion, it leads to internal contradiction and confusion in the mind of the runner. Therefore, the goal of this article is to develop perceptions that are consistent with the biomechanics of running based on physical laws and biodynamics.

The conversion of intention into action is guided not only by perception but also by a more tenuous but crucial mental attribute: confidence. It is confidence that allows conscious perception to be integrated with automatic processes to produce the exquisite control of force and timing necessary to run well. One way to acquire confidence is to place faith in a guru. The other is to place faith in principles derived from understanding of the laws of physics and from sound biodynamic theory. The ambitious goal of this set of articles is to provide a foundation for such confidence. However, it should be emphasised that the material presented is a preliminary effort at assembling such principles. The main direct evidence supporting them is my own experience as a runner. I am not a coach. My experience should not be assumed to apply to others and before changing one’s running style it is advisable to consult a qualified coach.

(Subsequent articles in this series will be posted over the next few days)

Perceptions, preconceptions and ‘free-falling up hill’

One of the most dramatic moments of the recent Pose Clinic in Loughborough occurred when Dr Romanov selected a volunteer from among the group of Pose novices for a special experience. The novice was instructed to close his eyes and run, Pose style, while Dr Romanov guided him by holding his wrist. At the time we were gathered on the edge of a grass playing field at the base of a steep grassy slope, perhaps inclined at 1 in 5. Dr Romanov set-off leading the novice along the edge of the field and then turned up the slope. At the top, they turned and ran down again, finally turning back along the edge of the playing field to where we were assembled. The novice was allowed to open his eyes and asked where he had run. He indicated a level course along the field. He was incredulous when informed that he had been to the top of the hill.

Then we all had an opportunity for a similar experience. My guide wove a sinuous path to throw me off the scent. I concentrated on pulling my foot from stance as rapidly as possible on each stride, and avoided focus on foot-fall, as we had been instructed the previous day. At times I had the sense that we were traversing across a slope because I perceived that one foot was landing lower than the other, and occasionally I was aware that we were going downhill because I received a slight jolt at foot-fall. When I finally opened my eyes, I was just as amazed as the first novice had been to discover just how much hill climbing I had done without perceiving any extra effort.

So what was there to learn from this amazing experience? The first lesson is that perception can be quite different from reality. The second lesson was that removing preconceptions can make things easier. Perhaps much of the effort we perceive when running up hill comes from our preconception that running up hill is effortful. The third implied lesson is that Pose method makes running up hill easy. Chapter 32 of the ‘Pose Method of Running’ deals with running up and down hills. When describing uphill running on page 210, the authors, Dr Romanov and John Robson, state: ‘The momentum of your running plus the forward lean allows gravity to continue to work for you – you literally free fall uphill.’ To the amazed novice experiencing the blind-fold hill running session, the conclusion seemed inescapable. Pose made hill running easy.

I believe that at least two of these three conclusions are true, so it is worth stepping back and looking a little more closely at each.

Perception and reality

With regard to the first conclusion that perception can be quite different from reality, Dr Romanov had pointed our several times during the weekend that we can only be conscious of one thing at a time. (This of course is the source of a magician’s magic). We had been instructed that when running Pose style we should focus solely on a rapid pull of the foot from stance. This is almost certainly excellent advice because it minimises the waste of energy and increased risk of injury associated with actively pushing downwards at footfall. Not surprisingly, we failed to perceive the premature footfall and shorter stride when running up-hill, and similarly failed to perceive the slightly delayed footfall when running downhill. So the practical conclusion is that when running we need to identify what we should choose from among the possible things we could focus on, to achieve our goal. Focus on the pull from stance appears to be a good choice.

Preconceptions

Most people find running up hill effortful even when they decrease their speed. Dr Romanov emphasises that when running up hill the stride should be shortened so that perceived effort remains constant. That is what he did when leading the novice up the hill. When deprived of an unhelpful preconception, running uphill with an appropriately reduced stride-length requires no additional effort.

‘Free-falling uphill’

The third implied conclusion that Pose method makes uphill running easier because ‘the momentum of your running plus the forward lean allows gravity to continue to work for you – you literally free fall uphill’ requires closer inspection. Once you are moving at constant velocity, the main influence that keeps you going when on a flat surface is indeed momentum. As discussed several times on my blog, most recently in yesterday’s post, I believe that gravitational torque cannot provide propulsion, either on the flat or up hill, simply because any torque applied at some point in the gait cycle must be cancelled by an opposite torque at some other point in the cycle, if we are to avoid a face-down crash. When running up-hill work must be done against gravity. There is no plausible source other than muscular effort. If the guide sets the pace such that perceived effort is constant due to deceased stride length, less muscular work is required per stride to accelerate the leg from stance to overtake the advancing torso. The energy saving (per stride) provides the energy necessary to raise the centre of mass up the hill. Unfortunately we require more strides to cover the same distance compared with running on the flat. The rate of energy expenditure in not increased but the duration is.

So, the amazing experience of blind-fold hill running tells us something useful about perception and also about preconceptions, but unfortunately we didn’t ‘literally freefall up hill’ as implied in ‘The Pose Method of Running’

Pose Clinic with Dr Romanov

I have just returned from a very enjoyable and informative weekend Pose Running Clinic led by Dr Romanov. I learned a lot of useful and thought provoking things about efficient running during the weekend, and will post my thoughts on some of these topics over the next few weeks. Meanwhile, in this posting I will summarize some of my main impressions.

It was a delight to meet Dr Romanov. The meeting confirmed that he is a charismatic person with many very valuable insights in to running. Previously in this blog I have expressed doubts about some of the biomechanical principles underlying Pose, and indeed I continue to have these misgivings. However, these misgiving need to be interpreted in light of the distinction which Dr Romanov himself makes between the psychological reality and biomechanical principles of efficient running. Psychological reality refers to the state of mind that facilitates the performance of efficient running. The biomechanical principles are the principles of Newtonian mechanics that govern the way in which we run.

On this weekend clinic, the main focus was on the psychological reality. In general, focusing the mind on biomechanical principles when running is unhelpful because conscious attempts to manage all aspects of running style while running interfere with the automatic mechanisms by which our body reacts to the forces acting on it. Indeed our perceptions of what we are doing can in some circumstances by quite different from the biomechanical events.

Two illustrations of this are provided by the Pose concept that gravity is a source of free energy, and the concept of gravitational torque.

Free energy?
In Dr Romanov’s book ‘The Pose Method of Running’ (2002 edition), chapter 12 is entitled ‘ The Free Falling Concept’. In that chapter, on page 62, he states ‘The great runner is not impervious to gravity; instead he taps into it as a readily available source of free energy. In the same way that the tremendous force of gravity inevitably draws a free falling sky-diver towards Earth, we can appropriate the force of gravity to run further, faster and with less effort.’ From the biomechanical point of view, this statement is at best misleading, and at worst simply wrong. When running on a level surface, the gravitational potential energy at the end of each gait cycle is the same as at the beginning of that cycle. Gravitational potential energy is proportional to height about the Earths surface. Therefore, gravity cannot be a source of free energy for a runner on a level surface. However, the important psychological issue is that a belief that we need to push off from strongly from the ground at the end of the stance phase in each gait cycle in order to run fast is very likely to lead to serious inefficiency and risk of injury by promoting over-striding. My weekend at the Pose Clinic has helped me refine my ideas about the best way to define ‘over-striding’, but that will have to wait for a later posting. The crucial point for the present discussion is that focus on correct biomechanical principles can lead to an unhelpful psychological approach. In contrast, the belief that gravity is a free source of energy might promote a less forceful push-off, thereby minimising the risk of over-striding.

Gravitational torque
Dr Romanov emphasises strongly that when the long axis of the body is leaning forwards, the force of gravity generates a torque that provides forward propulsion. One of my calculations presented in the side bar of this page suggests that the amount of angular momentum generated by gravitational torque is probably small compared with the angular momentum that arises from the fact that a body moving horizontally forwards will inevitably begin to rotate in a ‘head forwards and down’ direction around the point of support when the foot is held stationary on the ground. My calculation was based on model that is only valid for a rigid body that does not extend at the hips. In fact, appreciable extension at the hip is inevitable in the latter part of stance. At some point in the future I will present a more precise calculation taking account the effect of hip extension. However, I do not believe the more accurate calculation will lead to a substantially different conclusion, but in any case, the magnitude of the increase in angular momentum due to gravitational torque is not the most important issue. If we are to avoid an ever-increasing rotation in a face forwards and downwards direction, the effect of gravitational torque must be reversed by some equal and opposite torque at some point in the gait cycle. Hence, gravitational torque cannot provide forwards propulsion. This is the biomechanical reality. However, the psychological reality is that forward lean leads to increased speed. In fact this is most apparent when a sprinter accelerates from the starting blocks. The biomechanical fact is that it is not gravitational torque that provides the propulsion; it is the muscular actions that moves the legs forwards rapidly enough to avoid a face-down crash. However, focus on forward driving action of leg muscles is generally counterproductive, so the concept that gravitational torque provides propulsion might be a useful psychological reality.

Proprioception and the point of support at foot-fall
The Pose Method places a very helpful emphasis on the role of body perception in efficient running. Dr Romanov emphasises that the proprioceptive sensations from the muscle and joints are crucial for accurate timing and direction of the muscle actions during running; especially for the action of pulling the foot from the ground at the end of stance. However, even here there can be a disparity between subjective experience and objective reality. At the Pose Clinic, it was stated that one should aim for the sensation that the foot lands beneath or even behind the body’s centre of gravity (COG). This statement is reinforced by the statement on page 311 of ‘The Post Method of Running’, that landing ahead of the body is a common error. However, the biomechanical realty that a torque imparted at some time during the gait cycle must be reversed at some other point in the gait cycle makes it almost mandatory that the point of support at foot-fall must be in front of the COG. When support is in front of the COG, the gravitational torque acts that tends to pull the body backwards and down, thereby compensating for the ‘face forward and downwards’ torque applied when the support is behind the COG. Though it seems a shame to be obliged to suffer this braking effect, it is imperative to avoid a face down crash. Rather enigmatically, this is implicitly accepted in the Pose Method criterion that the period on stance should consist of one video frame (33 milliseconds) from footfall to mid-stance (the Pose position when the point of support, COG and shoulder are aligned in an almost vertical orientation) and one video frame with between mid-stance and lift-off from stance. So there is a discrepancy between the recommended subjective perception and one of the observable criteria for good Pose Method running.

Time on Stance
For me the most thought provoking issue raised at the Clinic was the recommended duration on stance. I have previously suggested, on the basis of my own experience, that unless a runner is quite fit, he/she should be cautious about having period on stance of less than 120 milliseconds. My own experience has been that for runs of 15K or longer, if I spend less than 120 milliseconds on stance, I experience some pain over the metatarsal heads the following morning. However, using the technique recommended at the Pose Clinic I found that I was able to reduce my time on stance to 80 milliseconds while feeling that my footfall was fairly light. On the first day of the clinic I spent 20 minutes or less running with time on stance around 80 milliseconds, and the next morning I was aware of mild pain under my metatarsal head. However, on discussing this problem with Pose Coach, Mark Hainsworth, and also with Dr Romanov himself, I came to appreciate that it is likely that I can avoid this metatarsal pain if I allow my heel to rest lightly on the ground during stance. Contrary to the impression created by all of the diagrams in ‘The Pose Method of Running’, Dr Romanov does actually recommend lowering the heels to the ground. I have not yet had the opportunity to combine the technique I learned at the Clinic with lowering the heels to the ground, but once I have had the opportunity to test this, I will review my previous blog posting and perhaps revise my conclusions regarding time on stance.

The Clinic raised many other interesting issues; I hope to deal with several of these on my blog in the next week or two

Does leaning help us run faster?

In my blog of Jan 10^th I discussed the fact that we cannot obtain free energy from gravity while running on a level surface. Both the Pose Method of Running (http://posetech.com) and Chi running (http://www.chirunning.com) advocate a forward lean from the ankles, on the grounds that such a lean promotes unbalancing which supposedly helps the runner capture the hypothetical supply of free gravitational energy. The reality of this source of energy appears to be confirmed by the experience that you can speed up if you lean more. So, if gravitational free energy is an illusion, do we go faster if we lean more, and if so, why?

Initial acceleration
The first point is than lean certainly helps us get started from rest. A sprinter driving from the blocks leans forward in a seriously unbalanced position and is forced to swing the legs forward powerfully to prevent a face down crash. Even when a long distance runner starts from a standing position, it is probable that a transient forwards lean creates the initial unbalancing that evokes the commencement of forward movement.

Maintaining a steady velocity
Once we are moving forwards at a steady velocity, momentum ensures that the torso continues forwards relative to the foot while the foot is on stance, so an unbalancing rotation of the body forwards and downwards will occur without the need for input from gravity. The second calculation on the calculations page (see the side bar) demonstrates that at least during the early part of the time on stance, when the amount of lean is small, and the hip is not far from a neutral position, any contribution from gravity to the rotational motion associated with unbalancing is much less than the contribution from forwards momentum in a runner moving at a moderate speed. I suspect that this will remain true even up to the largest degree of lean occurring in a long distance runner running at steady pace. Irrespective of whether the unbalancing arises primarily from the effect of linear momentum or from gravity, the forward and downwards rotation will result in lean and raises the question of whether or not deliberately accentuating the lean will cause us to go faster.

Lean might lead to increased speed by two mechanisms.

 

The impulse from horizontal ground reaction force.

In the final part of the stance phase, lean will result in the leg pressing down obliquely on the ground. This oblique push will have a backward directed horizontal component that will in turn lead to a forward directed ground reaction force (GRF). Force plate measurements confirm this, revealing a forward directed GRF typically lasting around 100 milliseconds before the completion of lift-off and reaching a peak value that is typically 0.45 times body weight (Cavanagh and LaFortune, (Journal of Biomechanics, 13, 397-406, 1980). This forward directed GRF will impart an impulse to the foot that will tend to propel the foot and lower leg forwards.

On the calculations page (see side bar) I have estimated the increase in forward momentum of the foot and lower leg after lift-off from stance, and compared this with the impulse imparted by the horizontal component of GRF measured by Cavanagh and LaFortune. This computation is only intended to provide an imprecise estimate of the acquired momentum. It reveals that the horizontal GRF observed in runners who land on the mid-foot is sufficient to generate about 90% of the forward momentum attained by the foot and lower leg following lift-off from stance. It should be noted that runners using different running styles might generate differing amounts of horizontal GRF, though in fact Cavanagh and LaFortune found very similar forward directed GRF in heel-strikers during this late part of the stance. (As expected, during early stance, the heel-strikers showed an additional sharp peak of vertical GRF suggesting a sharp and potentially damaging loading of the structures of the leg).

Thus it appears plausible that the forward directed horizontal GRF generated when the leg presses obliquely downwards in the late part of the stance delivers a sufficient impulse to the foot to provide for the majority of the momentum gained by foot and lower leg during lift-off.

 

Reflexive pull
The unbalancing associated with leaning will elicit a reflex that pulls the leg forwards to prevent a face-down crash. Thus, any additional force required might be generated by a reflex action, or indeed by a voluntary pull . In particular contraction of the hamstrings, supported perhaps by some action of hip flexors, will pull the foot towards the hip, thereby proving both horizontal propulsion of the foot and lower leg and also vertical elevation.

 

It should be noted that in addition to gaining forward momentum, the foot and lower leg will gain gravitational potential energy as they are lifted vertically. This lift will be generated by an upwards force supplied in part by the vertical component of GRF (which will include a contribution from the reaction to elastic recoil by achilles and calf muscles) and also by the vertical component of the active pulling of foot towards the hip.

Conclusion
Increasing the lean will increase both of these two effects (the impulse derived form the from horizontal GRF and the tendency for unbalancing to elicit an active reflexive pull). Thus deliberately increasing the lean should result in the legs moving forwards powerfully enough to sustain a higher speed of running.

 

Gravity and running

 

‘The great runner is not impervious to gravity; instead he taps it as a readily available source of free energy.’ Nicholas Romanov, founder of the Pose Method (in ‘Pose Method of Running, 2004 edition p62,)

‘All you’re doing is this (focussing on the central needle – the core of the body); gravity is doing the rest. You let gravity do its job and you get out of the way. The only thing the legs are for is for momentary support’ Danny Dreyer, founder of Chi Running,
http://www.youtube.com/watch?v=e-zrH6IOTQI

 

How should we to interpret these key statements of the theoretical foundations for Pose method and for Chi running. These statements cannot be literally true for running on a level surface. They violate the law of conservation of energy. As an object moves, the change in potential energy due to gravity is proportional to the change in height (http://en.wikipedia.org/wiki/Gravitational_potential_energy). If the height of the body’s centre of gravity at the end of each stride is the same as that at the end of the previous stride, there is no change in gravitational potential energy. The law of conservation of energy requires that any energy supplied by gravity at some part in the gait cycle must be re-paid at some other point in the gait cycle, and gravity cannot do the work required to keep us running.

My own belief, reinforced by the opinions of experts such as Dr Tim Noakes of Cape Town University, is that Dr Romanov has a remarkably good intuitive grasp of good running style, and hence his teachings should not be abandoned wholesale because one of his key theoretical statements appears mechanically unsound. So, how are we to interpret these statements about gravity?

If they cannot be accepted as literal truth, do they have any truth? Consider the saying: ‘the way to a man’s heart is via his stomach’. If this statement appeared in a training manual for aspiring cardiac surgeons, it would lead to surgical catastrophe, but in a ‘good home-makers guide’ it might lead to domestic bliss.

Dr Romanov, in an article posted in 2003 on PoseTech (http://www.posetech.com) makes it clear that he does expect the muscles to play a part: ‘By the pose method philosophy, muscles should just assist gravity in pulling us forward. Which doesn’t mean, of course, that we don’t need muscular strength, on the opposite, with the increase of the portion of gravity work we need much more skilful and powerful muscles to handle gravity.’ Maybe this implies that he intends us to adopt a mind set more analogous to that of the romantic homemaker rather that of the cardiac surgeon reading a training manual. The meaning might be in the image created rather than the literal truth. The image created might help harness muscular strength efficiently.

So what image might be created by the statement that we must get out of the way and let gravity do the work? By implying that the body will continue to progress forwards with only relatively minimal guidance from muscles, these statements encourage us to avoid pointless muscle action which is both wasteful and potentially injurious. ‘When you master the Pose method you will experience the incredible Lightness of Running’ (Pose Method of Running, 2004 edition, p 42). In fact because of Newton’s first law of motion, (a moving body will continue to move in a straight line at constant velocity unless acted upon by a force) a direct propulsive force is not required to maintain a constant pace when running on a level surface in the absence of wind resistance. However, because we run upright on two legs rather than rolling like a billiard ball, we do have to move our legs rapidly forwards in each stride if the body is to remain supported in an approximately upright orientation. The goal of efficient running is to move the legs forward rapidly enough to prevent a face-down fall in a way that uses minimal energy with minimal risk of injury. A focus on minimizing muscle action makes sense.

Dr Romanov emphasizes the incredible lightness of running – in other words, minimizing the impact of gravity which tends to pull us to earth with a thud. So if we are to achieve this incredible lightness we need to understand the points within the gait cycle at which gravity is likely to play a strong role.

Free-fall while airborne
Gravity is at its most remorseless when we are airborne. It is inevitable that we will fall freely during this time. Gravitational potential energy is converted to kinetic energy that will bring us forcefully into contact with the ground at foot-strike. At this point, some of the energy can be captured in the stretching of the quadriceps muscles, provided the knee is slightly flexed, and in the muscles and tendons of the foot and calf, provided we land on the ball of the foot. These are features of the Pose style. The stored energy can be recovered by elastic recoil as the foot is lifted from the ground at the end of stance. However, some energy will inevitably be lost, so one of the cardinal goals of efficient running is to minimise the amount of free fall. Because the speed of falling increases with longer duration of fall, less energy will be lost by a series of frequent short airborne periods than by series of fewer longer airborne periods, of the same total duration. Hence high cadence is essential. For a mathematical proof of this, see the calculations page in the side bar.

Footfall
What about the foot fall itself? Because the leg begins to fall while the body is already falling, gravity is unable to pull the foot down relative to the trunk. A very light muscular action is in fact required. However, consciously forcing the foot down creates a grave danger of excessive force. The task of applying the slight force to bring the foot down while simultaneously adjusting muscle tension in a way that establishes sufficient tension in the muscles of the thigh to stabilise the knee is beyond conscious control. Fortunately, by virtue of learning to walk in the omnipresence of gravity, we have already acquired a great deal of automatic skill in summoning the right amount of force. If we are applying too much force, foot-strike will generate a thudding sound. However, the incredible lightness of running described by Dr Romanov cannot be attained by conscious adjustment of muscle tension; rather it is done by listening to the sound of foot-strike and simply aiming to reduce the thud to a light patter. Our brain has a remarkable capacity to adjust muscular activity to produce the goals we set for it.

While on stance
Once the foot is on the ground, gravity serves one useful goal: it tends to anchor the foot in place and minimizes the risk of slipping. However, now our body will begin to rotate in a head forward and downwards direction because our trunk is carried forwards by momentum, while the foot is stationary. This rotation unbalances the body and after a short period, will stimulate a reflex action that lifts the foot from the ground and initiates the next swing forwards. This lift-off can be promoted by conscious effort, and indeed a strong intentional pull is essential if we wish to run fast.

As the long axis of the body rotates away from vertical during the early part of the stance phase, gravity will have a component at right angles to the axis of the body and therefore, will tend to increase the speed of rotation. Dr Romanov suggests that this effect of gravitational torque offers a free source of energy. However by virtue of the law of conservation of energy, any energy gained from gravity at this point of the gait cycle must be paid back at some other point in the gait cycle. Furthermore, according to the calculation presented on the calculations page (in the side panel), the amount of acceleration due to gravity is relatively small, at least until the lean becomes quite marked.

The gravitational torque induced by the transient marked lean as a sprinter drives from the starting blocks almost certainly plays an important part in generating the rotational motion that stimulates rapid and forceful lift-off from stance. However, in my opinion, during steady running at the more moderate pace of the long distance runner, the contribution of gravitational torque to destabilization is likely to be small relative to the contribution from forward momentum. Even if the degree of lean is sufficient to produce an appreciable contribution of gravitational torque to the de-stabilization, this energy would have to be paid back, and it cannot be regarded as a free source of energy. So, I suspect that gravitational torque generated while on stance plays at most a very small part in running.

Conclusion
Where does this leave the teaching of Dr Romanov and Danny Dreyer regarding gravity. The crucial importance of their contributions is their emphasis on running lightly. In practice, they offer several very useful suggestions for achieving this. In Pose Method, the recommendations are:Run with a high cadence to minimise free fall. Land with the knee flexed, and on the ball of the foot, to absorb the energy of impact in quadriceps, and the tendons and muscles of the foot and calf. (But allow the heel to touch the ground to prevent excessive load on the Achilles tendon). Spend a short time on stance to promote efficient recovery of this stored energy through elastic recoil. And if your ears tell you that you are not running lightly, simply focus your mind on achieving lightness. The brain will do the rest.

How should the foot be lifted from the ground?

When running on a level surface in the absence of wind resistance, momentum carries us forwards. According to Newton’s first law of motion, maintaining a constant velocity does not in itself require any input of energy. However, we do have to put energy into moving our legs forward quickly enough from one stance to the next to avoid a face down crash.

The process of moving the leg forward from stance to take up stance again with the foot beneath the body about 300 milliseconds later can be subdivided into three segments: lifting the foot off the ground, swinging it forward and allowing it to fall to the ground. These three phases merge into each other, but nonetheless, each has its own characteristic role in the gait cycle, and it is worthwhile to try to examine the requirement of each phase.

Because it is wasteful of energy and potentially injurious to land any more than a slight distance in front of the centre of gravity (COG) the swing forwards should be as passive as possible (though maybe when we want to run very fast, we do need to put a bit more energy into the swing). Because the foot needs to be going backwards relative to the COG at foot-strike, it is also best if the foot-fall is a passive as possible, though of course, substantial muscular contraction is required to stop the knee collapsing on impact.

The involuntary muscular effort required to prevent the knee collapsing is not directed at moving the leg forward, so the major muscular effort to move the leg forwards must be provided in the lift off phase. In this post we will consider only this phase. In later posts I will return to the question of what should happen in the other phases; though from the point of view of conscious effort, the answer is probably: ‘very little’.

There are various different ways we might use our muscles to lift our foot from the ground. Let us start with the issue of how we might lift the left foot from the ground while standing stationary. It is instructive to consider three ways in which we might do this. I will describe the intentions, sensations and results of each of the three ways of doing it:

 

(1) Start with feet side by side; then think ‘Lift the ankle to the hip’. The left foot travels almost perfectly straight up, with only a slight initial swing backwards, so that the mid-point of the left instep passes just behind the medial malleolus of the ankle of the right foot. I feel a contraction in my hamstrings, and also a weaker amount of activity is hip flexors, especially sartorius (a thin strip of muscle that runs down and across the front of the thigh.)

 

(2) Start with feet side by side, think ‘lift the knee’; The hip flexors, especially rectus femoris, the large muscle on the front of the thigh, contracts; the foot comes almost straight up, but with slight forward swing, and the instep passes just in front of medial malleolus on the right ankle.

 

(3) Start with the active foot about 12-15 inches behind the support foot, with the ankle in a neutral position; think: ‘Lift foot to hip’. The foot travels upwards just a little behind the direct line to the hip, as the hamstring contracts. This feels as if it is an almost pure hamstring action. I have no awareness of contraction of either sartorius or rectus femoris when I do this.

 

Which of these is nearest to the required action when running? We can rule out action (2) immediately. First of all, several studies in which electrodes attached to the muscles have been used to record muscular activity during running of runners reveal relatively little activity in the quadriceps during the lift-off (see for example, Pilutsky and Gregor, Journal of Experimental Biology, vol 204, pages 2277-2287, 2001); although some studies such as the ingenious study by Modica and Kram (Journal of Applied Physiology, vol 98, pages 2126-2131, 2005) do indicate that the hip flexors might play some role. While it may be that the runners who participated in these studies were not using the most efficient style, we would be unwise to try anything that is too far different from what runners typically do. We can reasonably assume that at even in modern society we are not so far removed from the natural human condition that modern man (or woman) has made a really radical shift from the optimum running style. Our goal in seeking an efficient style is largely to remove any relatively minor inefficiency that might have crept in as a result of modern life style (and shoes). Therefore, it seems sensible to assume that hip flexors play at most a relatively minor role.

So we are left to consider methods (1) and (3), both of which rely largely on the hamstrings. Brief consideration of the consequences of spending at least the minimum reasonable time on stance suggests that (3) is most appropriate.

When on stance, the body will necessarily rotate in a ‘head forwards and downwards’ direction, as a result of forward momentum that carries the body forwards while the foot is anchored. If we are moving at 4 metres per sec, immediately after foot-strike the COG will continue to move forwards at 4 metres per sec, while the foot is stationary. Assuming the COG is approximately 1 metre off the ground, the line joining COG to the foot must start rotating at 4 radians per second (a radian is the ‘natural unit’ of angle and is approximately 57 degrees). Assuming a cadence of 180 strides per minute, the total duration of each stride is 333 milliseconds. Unless we are prepared to suffer ground reaction forces exceeding 3 times our body weight, we must remain on stance for at least one third of the gait cycle in order for our weight to be supported, so we must spend at least 110 milliseconds on stance. During this time, our COG will have moved forward about 0.4 metres (somewhat more than a foot.). That is why when I described method (3), I had specified starting with the left foot about 12-15 inches behind the right foot. In fact, most runners will remain on stance longer than 110 milliseconds; the foot will be even further behind at lift off and the hip will be further extended. However, the crucial point to make at this point is that if we land with the foot under the COG (which is desirable to minimize wasteful braking), by time of lift-off from stance, the hip must be moderately extended. A pure hamstring pull without hip flexor action will now bring the foot up towards the hip without any action of the hip flexors.

In general, action of the hip flexors would increase the risk that we will over-stride at the next foot fall, unless we actively employ the hamstring in the late swing phase to arrest the swinging leg. It seems inefficient to use hip flexors at lift off, and then have to apply hip extensors in late swing to correct for this. Perhaps if we are aiming for maximum speed, we might have to be prepared to spend a bit more energy to get the swing moving forwards more quickly, at the price of needing to do extra work later in the swing to arrest it, but this appears to be an expensive way to run.

So these considerations suggest that a pure hamstring pull starting from a position of moderate hip extension is the most economical way to perform the lift off. Maybe active hip flexion is necessary if we want to run really fast, but this active flexion will come at extra cost.

I think that these considerations have led us to an action largely consistent with the proposal of Dr Romanov (founder of the Pose technique) who states that the pull should be performed by the hamstring. However, I am a little puzzled by one of the drills that Dr Romanov recommends in order to help us learn the correct direction of the pull. In the tapping drill, he recommends starting with the feet side by side, and as far as I can establish, the recommended action is very similar to what I have described in action (1). It should be noted that my knowledge of Pose comes only from Dr Romanov’s book ‘Pose Method of Running’ and from information on the PoseTech website (www.posetech.com). I am looking forward to my first workshop with Dr Romanov in March of this year, and maybe I will understand the tapping drill better after that.

So for the time being, I think that if one wants to learn the correct muscle action for economical lift-off using a drill performed when stationary, it might be more sensible to start with one foot 12-15 inches (30-40 cm) behind the other. I have previously posted (in the side bar of this blog) what I describe as the ‘swing drill’ to encourage the appropriate action for moving the leg forwards from one stance to the next when running. Today, I have modified this drill to recommend starting with one foot about 12-15 inches behind the other. I must emphasize that this suggested drill is experimental. I am still working on its development. It has not been tested on anyone apart from myself and cannot yet be recommended as a safe or useful procedure. Nonetheless, I would value any comments on this proposed drill.

More on the effects of gravitational torque

Earlier today I posted to say that my calculations indicated that the contribution of gravitational torque to running was minimal. Since then I have been engaged in an interesting discussion with Mike Stone (which you can see in the comments section at the bottom of the calculation page.) He initially challenged one of the mathematical steps in my calculation, but I think we have agreed that subject to the validity of the assumptions on which my calculations were made, that is OK. However, he is currently querying the assumptions.

My understanding of the situation is that the calculation I have done gives a reasonable good estimate of the effect of gravitational torque if the assumptions are correct. The most important of these assumptions is the assumption that the ankle, hip and shoulder remain in a straight line. This was one of the rules for good Pose running originally specified by Dr Romanov, but there is no doubt that in practice runners disobey this rule once the pace has increased beyond a fast jog. What actually happens is that the hip extends while the runner is on stance so that the leg on stance rotates through a larger angle than the torso. To do the calculation in a more realistic way, we need to treat the body as consisting of at least two elements joined by a hinge at the hips to allow for the fact that at speed, runners do flex (backwards) at the hips. This calculation could be done, but will require more complex mathematical procedures. The flexion results in the COG moving relative to the hips while the runner is on stance

So where have we got to: I still think that provided the line from the ankle to the COG at lift off from stance leans by no more than about 10 degrees, the contribution of gravitational torque to running is likely to be very small. However, there is no doubt that it would be best to do the calculation using a model in which the body is allowed to hinge at the hips. I hope that we can get this done in the not too distant future.