Heel-striking and a brief history of the modern running shoe

Recently, Rick asked me to act as judge in a debate with a friend, who works in a store that sells running shoes, about heel-striking versus mid-foot landing.  

At first sight, it does seem rather amazing that the manufacturers of  running shoes continue to emphasize the virtues of cushioning and stabilization (to reduce pronation – the ‘natural’ tendency to roll from outside edge of the foot towards the medial edge as the longitudinal arch absorbs the energy of footfall)) five decades after Gordon Pirie trenchantly pointed out to Adi Dassler, the founder of Adidas, the reasons why the leading principle in running shoe design should be  ‘less is more’.   But the story has some interesting twists and turns.

Pirie argued that the arch of the human foot is well designed to absorb the stress of footfall provided the runner lands on the forefoot.  In chapter 3 of his book ‘Running Fast and Injury Free’ Pirie cites two observations to support his argument.  The first was the set of video recordings of 100 elite athletes at the 1972 Montreal Olympics, by Bill Toomey (winner of the decathlon gold medal in Mexico City in 1968).  According to Pirie, all 100 elite athletes filmed by Toomey were fore-foot strikers.  The second observation was more anecdotal: Pirie himself ran more recorded miles than any other human being (around 216,000 miles in 40 years) and suffered minimal injuries.  He attributes this to his forefoot running style. 

 

Zatopek and Dassler shoes

However, more recently video analyses reveal that a large number of elite and sub-elite are heel strikers.  What has changed?  I think the seeds were sown two decades before Montreal.  In Helsinki in 1952, Emil Zatopek won gold medals in the 5,000m, 10,000m and marathon, wearing Dassler shoes.  As far as I know, the shoes worn by Zatopek in Helsinki were in fact rather light-weight, though he is reputed to have trained in army boots.  However the more relevant fact is that at around that time, Dassler added the famous three stripes to Adidas shoes to stabilize the mid-foot.  As far I can see, that was the point at which engineering and marketing formed an alliance and abandoned the ‘less is more’ principle.  Fueled by Zatopek’s achievement, Adidas rapidly came to dominate the market.  Ultimately, the engineering led to more cushioned soles and marketing managers persuaded runners that cushioning and stability were crucial.  With heavy cushioning, it was no longer essential to land in a way that absorbed the energy of impact in the longitudinal arch of the foot, and eventually, many runners accepted heel striking as the norm.

In recent times, several schools of thought (most notably Pose and Chi) have resurrected Pirie’s ideas about efficient running, and there has been a resurgence of interest in minimalist shoes.  Nike, which grew from the foundation provided by Bill Bowerman’s famous waffle iron technique for fabricating a durable sole, and went on the eclipse Adidas, have recently capitalized on the minimalist trend with the Nike Frees.  Nonetheless, Nike are currently putting a lot of resources into promoting the Lunarglide, a lightweight shoe designed to combine cushioning and stability, and are targeting their marketing at female athletes.  Whatever the merits of the engineering, marketing has now made it almost impossible to draw any useful conclusions about how it is best to run from observations of elite and sub-elite athletes.

However, neither can we draw reliable conclusions from idealized accounts of ‘primitive’ tribesmen who are reported to achieve phenomenal long distance feats running barefoot or in rudimentary shoes.   Running a 10K in less than 27 minutes, or a marathon in just over two hours, are quite different from pursuing a wild animal for hour after hour across the African savanna or the North American prairies.  Drawing on arguments based on the evolution of the human foot to guide us about the most efficient way to run competitively might not be the best way to settle the question of how to run fast on road or track.

     

Short time on stance is crucial

One thing is fairly clear.  The fastest runners spend a short time on stance.  Studies by Peter Weyand and colleagues at Harvard University have demonstrated convincingly that the feature that distinguishes the fast runners from slower runners is a short time on stance (Journal of Applied Physiology, volume 89, pp 1991-1999, 2001).  A short time on stance necessarily entails a very strong push against the ground, resulting in powerful upwards propulsion, a long stride and relatively high cadence.

Schools of efficient running such as Pose also emphasize a short time on stance.  However, the theory of Pose promulgated by Dr Nicholas Romanov rather misleadingly  implies that the runner becomes airborne due to un-weighting of the foot as a result of gravitational torque, and a hamstring contraction that pulls the foot from the ground.  I believe that it is impossible to become airborne by this means.  A runner who spends 20% of the gait cycle on stance must necessarily exert an average downwards force on the ground that is 5 times body weight. 

Unfortunately, I do not know of any force-plate data that confirms that this is the case for a Pose runner.  I was a little disappointed when I attended a weekend Pose course with Dr Romanov, at Loughborough University (the home of Sport Science in the UK), and none of the Pose experts present showed any inclination to arrange a force-plate recording session.  Nonetheless, the Law of Conservation of Momentum requires that the impulse generated by ground reaction force must balance the downwards impulse generated by gravity acting on body weight, and hence the force exerted by the foot on the ground averaged over the entire gait cycle must be equal to body weight. 

A short time on stance not only ensures a powerful push against the ground, but also necessitates landing only a short distance in front of the centre of gravity, with the foot traveling backwards relative to the body’s centre of gravity at footfall.   This is most easily achieved with a forefoot or mid-foot landing.  Thus simple mechanical principles support Pirie’s argument for a forefoot landing.  However, it would be foolish to under-estimate the forces involved. 

 

Risks of minimalist shoes and forefoot landing

I was interested to note that Dallas Pose coach and stalwart of the PoseTech forum, Jack Becker, suffered a metatarsal stress fracture about two years ago.  While I have a great respect for Jack’s thoughtfulness, and I am personally grateful for advice that he once gave me regarding choice of shoes, I am inclined to think that his enthusiasm for minimalist Puma H-street shoes may have contributed to his stress fracture.  It is an interesting side-issue to note that Puma was founded Rudolf Dassler, brother of Adi – perhaps Rudolf took more note of Gordon Pirie’s opinions.  On balance, I am a little cautious about minimalist shoes, but certainly believe that cushioned heels, and heel striking, are undesirable.  It might be argued that it is better to train the intrinsic muscles of the feet to distribute the load along the arches of the foot rather than to allow these muscles to atrophy within heavily cushioned shoes.

There have been very few studies that have directly compared the benefits and risks of fore-foot, mid-foot and heel striking.  Perhaps the best known is the study by Arendse and colleagues from Tim Noakes’ laboratory in Capetown (Medicine & Science in Sports & Exercise: Volume 36,  pp 272-277, 2004).  The fore-foot landing group was instructed by Nicholas Romanov.  The main finding reported in the published paper was significantly decreased stress on the knee joint in the fore-foot runners compared with the heel-strikers.   However, forces around the ankle were noted to be higher, and Ross Tucker, who assisted Dr Romanov, reports on the Science of Sport blog that calf and Achilles problems were common in the fore-foot group.

 (http://scienceofsport.blogspot.com/2007/09/running-technique-part-ii-scientific.html )

In fact since the publication of the Arendse study, many Pose coaches have reduced the previous emphasis on a ball-of-the foot landing with marked plantar flexion of the ankle.  At least some Pose coaches acknowledge that the heel should be allowed to touch the ground lightly, to relieve the strain on the plantar fascia and Achilles tendon.

 

Conclusion

A short time on stance is essential if you want to run really fast, and this is most easily achieved with a forefoot or mid-foot landing. However the ground reaction forces are necessarily large, and landing on the ball of the foot with ankle plantar flexed places a great strain on the feet, ankles and calf muscles.  At least during long races, it is probably best to let the heel lightly touch the ground, to minimize risk of injury to the plantar fascia and Achilles tendon and perhaps even, risk of metatarsal stress fracture due to bone fatigue resulting from repetitive impact.

The pros and cons of weight loss for runners

Both theory and practice indicate that the energy cost of running is proportional to body weight.  First the theory: the energy cost of running can be subdivided  in to three categories: energy required to do work against gravity; energy required to do work against horizontal ground reaction forces; and energy cost of internal muscle inefficiency. 

The cost of being airborne

We do work against gravity when we become airborne.  The energy required to lift the body is proportional to body weight. Some of this energy is recovered by converting the energy of impact into elastic energy at foot-fall, thereby allowing us to re-use that energy for upwards acceleration at lift-off from stance.  However only a proportion of the energy will be recovered.  Assuming that this proportion is approximately constant (an assumption that depends on not changing running style) the net energy cost of becoming airborne is proportional to body weight.

 The cost of being on stance

Although being airborne has a high energy cost, so does remaining on stance.  While the point of support is ahead of the centre of mass, we experience a braking force (due to the horizontal component of ground reaction) that reduces our momentum.  This braking force at any instant is determined by the angle of our leg and by the force transmitted along the length of the leg, which in turn in proportional to body weight.  Therefore the braking force will be proportional to body weight.  When the point of support is behind the centre of mass, the horizontal component of ground reaction  pushes us forwards.  When running at constant speed, the retarding impulse due to braking must be exactly balanced by the forward accelerating impulse in late stance.   We can capture some of the energy released by the braking force in early stance as elastic energy which helps provide the forwards impulse in the second half of stance, but due to inefficiency we cannot recover 100% of this energy.  Assuming no change in running style, an approximately fixed proportion of the energy will be lost.  So, the energy consumed in opposing horizontal ground reaction forces is also approximately proportional to body weight. 

Unfortunately, it costs energy to be airborne and it also costs energy to spend time on stance.  Both of these costs are proportional to body weight.  Incidentally, as discussed in my posts on efficient running style summarized in the page on the Dance with the Devil, the best way to minimize these costs is to increase cadence, because higher cadence reduces the cost of overcoming gravity  – though there is a limit due to internal inefficiency at very high cadence

Internal muscle inefficiency

The energy costs due to internal muscle inefficiency are less easy to estimate. The process of muscle contraction involves the pumping of various ions, especially calcium, sodium and potassium, across the membranes that separate the different compartments of a muscle fibre.  The molecular pumps are fuelled by the energy molecule ATP, which is regenerated by consumption of glucose. There are also other metabolic processes and frictional processes within the muscles and joints, and as a result the energy consumed by a muscle is greater than the external work done by the muscle.  However, it is probably a fairly good approximation to assume that over the usual range of output, the efficiency of muscles is approximately constant, so the energy wasted internally will be proportional to external work done.  As we have seen the external work done (against gravity and against horizontal ground reaction forces) is proportional to body weight.  Thus the loss due to internal inefficiency will also be approximately proportional to body weight, though variation in the internal regulators of metabolism (such as anabolic and catabolic hormones) might also influence the costs.

Theory compared with practice

Thus, theory predicts that averaged across many individuals, the energy cost of running is approximaltely proportional to body weight, though differences in factors such as efficiency of running style and hormone levels might result in two individuals with the same weight nonetheless having slightly different energy costs.  In fact the energy cost of running averaged across many people, based on actual measurement rather than theory is given by the formula:

Energy cost in  Kcal/min/Kg  = ( 0.0024 * speed² ) – ( 0.0104 * speed ) + 0.1408

where speed is measured in miles per hour ( http://swingleydev.com/misc/exercise.php ). 

This formula demonstrates that on average, energy cost per Kg is independent of weight (and hence total energy cost is proportional to weight) , but such formulae provide only a rough guide for the costs in each individual.

The conclusion from both theory and from practical evidence is that if you lose weight, you will require less energy per minute to maintain a given speed, so you can maintain a faster speed at any particular fraction of your total aerobic capacity.  Weight loss will lead to improved speed approximately in proportion to the weight loss – all other things being equal.

 

Balancing catabolism and anabolism

The crucial phrase is ‘all other things being equal’.  If the weight loss were to be so extreme that there were no fat reserves for use as fuel, this would result is reduced performance over distances for which fat is a valuable source of energy, such as the marathon and longer distances.  However, because fat is a very efficient fuel (providing lots of energy per gram of fat) it would be necessary to starve almost to death to deplete fat stores below the amount likely to be consumed in a marathon or ultra-marathon.  Nonetheless, even less severe weight loss can trigger hormonal changes (regulated by the hypothalamus) in order to conserve essential body issues, especially the brain, and it is likely that muscle protein would be sacrificed.  In other words, if the weight loss is sufficient to tip the balance from anabolism to catabolism, muscle protein will be broken down and muscle strength decreased.  Hence loss of speed would be expected.  

 

General conclusions

In practice, this means that an out-of-condition runner who has gained weight will almost certainly benefit from losing that weight.  However an athlete who has trained over a substantial period at a training volume just a little less than that required to produce the overtraining syndrome, has probably already reached the optimum balance between anabolism and catabolism and further weight loss would probably be harmful.    

If you want to achieve to your limit, it is probably best to monitor performance regularly while steadily increasing training volume.  When performance shows a tendency to decline despite increasing training volume, it is likely that the balance has shifted too far towards catabolism (break down of tissue), and you should drop back to a slightly lower training volume.  Pushing relentlessly onwards with increased volume despite decreasing performance will almost certainly result in a sustained period of staleness, in which your brain will not allow you to run at your best level.

Personal conclusions

It is also useful simply to monitor weight.  Experience has taught me that when I train regularly my weight stabilizes at around 62-63 Kg, irrespective of exactly how much running I do, at least while I remain below the limit where I feel perpetually tired. Thus I think my brain (and in particular, my hypothalamus) regulates hormonal function so that catabolism equals anabolism when my weight is around 62-63Kg. 

However, while 62-63 Kg appears to be my ‘equilibrium’ weight, is probable that my proportion of muscle to fat is not optimal at present.  Almost 40 years ago when I could run a marathon in 2 hours 25 minutes, my weight was also around 62 Kg.  Since then I have almost certainly lost muscle and increased fat. It is probable that I could improve my running performance by doing more resistance exercise.  Provided it is not excessive, resistance exercise promotes anabolic hormone release and would promote replacement of fat with muscle.  However, a large amount of resistance exercise resulting in bulking of fast twitch muscle fibres and a net gain in weight, without gain in aerobic capacity, would probably decrease my distance running performance on account of the increased energy cost of running with extra weight.

Spring and springs

It is less than three weeks to the spring equinox, and the duration of daylight now almost matches the duration of darkness.  Although snowdrops are still the predominant woodland flower, along the river bank the early buttercups are already in bloom.  But most importantly for me, the air temperature is rising.  After a frustrating few months in which the cold dry air has wreaked havoc with my attempts to do interval training, this morning I had only a slight feeling of constriction in the throat.  A few puffs into the peak flow meter revealed that my peak expiratory flow was 555 litres per minute, which is fairly good for a slightly built 63 year old standing 170 cm tall.  So I decided that it was high time for another attempt at 4×1Km intervals in the upper aerobic zone, though in light of recent my experiences I did take a precautionary puff of salbutamol before setting out.

 

After an easy few Km to warm up, I set out cautiously on the first loop around the 1Km circuit in Clifton Wood, aiming for a pace around 4:30 min/Km.  I had little recent experience to draw on to guide my pace setting, and was pleased to find that I had covered the distance in 4:24 with a mean heart rate of 138.  I was feeling quite comfortable so I increased the pace slightly in each successive interval.  My times for the four intervals were 4:24; 4:19; 4:14: 4:09 and mean heart rate recording were 138; 142; 144; 148.  In the final few hundred metres of the fourth interval I was probably in the anaerobic zone, but nonetheless still fairly comfortable.  My average pace was 4 sec/Km faster than in the only interval session I have managed to do since October, yet the mean heart rate was virtually identical.  It is re-assuring to find that my sessions on the elliptical cross-trainer in the winter months have maintained and perhaps even slightly enhanced my fitness for aerobic running.

 

After the cool down jog, my peak flow had decreased from 555 litres/min to 430 litres/min, so I had suffered a mild degree of broncho-constriction despite the salbutamol, but I am now reasonably optimistic that the worst of my winter respiratory problems is behind me for another year.

 

The only untoward event was a fall during the second Km.  I misjudged the height of a tree root protruding about 10 cm above the ground and caught it with my right big toe.  By the time I managed to plant my left foot, my centre of gravity was already too far forward  and I went sprawling face down.  My right knee absorbed most of the impact. I was on my feet and running again within a second or two, with a slight ache in my right knee, and a quite sharp pain radiating from the base of my right big toe.  Yesterdays news headlines regarding Paula Radcliffe’s recent broken toe flashed into my mind, but I decided it was probably only a strain of the metatarsophalangeal joint (at the base of the toe) and kept on running.  I took special care to keep my toes relaxed, in the manner recently recommended by Rick in his comment on my blog, to minimise stress on the toes.  The pain persisted for about 20 minutes but by the end of the cool-down jog, it had subsided to a very mild ache, so it appears that in fact the injury is only a mild strain.

 

Afterwards, I inspected the site of the fall.  The imprint of my right knee was clearly visible in the moist yet firm earth, but the most dramatic marker was the deep indentation created by my left foot.  It must have slammed into the ground with great force as I attempted to arrest the fall, but because my COG was already too far forward, the ground reaction force only served to create a destabilizing torque.  It was salutary to realize that my right foot must have still been traveling at an appreciable speed relative to the ground at the instant it snagged on the root.  At the stage in the gait cycle when the descending foot is only 10 cm from ground, it should be traveling backwards relative to the torso and at almost zero velocity relative to the ground.  So I probably need to improve the efficiency with which my hamstrings arrest the swinging leg.  On the other hand, the fact that I have only a mild strain of my toe joint rather than a more serious injury does suggest that my hamstrings had not failed too badly in their job.

 

Spira shoes

The springs that are the other theme this week are the ‘wavesprings’ embedded in the heels and under the forefoot of the Spira running shoes.  I think that there is little doubt that these shoes would reduce stress on the feet and also reduce the eccentric load on quads and calf muscles at footfall, thereby reducing the risk of DOMS and also possibly reducing long term damage to muscles.  Thus I am very tempted to try them, at least for training.

 

However the big issue is whether or not this is ethical.  One of the reasons why I prefer running to formula one car racing is that running requires little apart from one’s one natural speed, endurance, and mental strength.  Formula one racing no doubt requires greater skill and courage, but technology abolishes equality of opportunity in competition.  However, if we were to demand complete purity in running, we would have to return to the Greek ideal of nude, barefoot competition.  Without even considering the issue of whether or not a nude 63 year old would be a tolerable sight for onlookers, I have no doubt that running shoes are essential for me on account of the mild congenital deformity of my feet.  I need to spread the load that would otherwise be concentrated on the head of my downwards protruding second metatarsal.  Fortunately, despite the current enthusiasm on some quarters for barefoot running, no-one seriously challenges the ethics of using running shoes to protect the feet from injury.

 

In fact, despite the simplicity of running, we readily accept quite a lot of technology: shoes with spikes; support bras, etc.  Some of these technical items probably enhance performance in addition to minimizing risk of injury.  The question is where we draw the line.  In principle the answer is simple.  A sport is governed by arbitrary rules and participation in competition implies abiding by the rules set by the body governing that competition.

 

However at this point, we face difficulties with the Spira shoe.  The IAAF rules allow that a running shoe might provide protection for the foot and enhance grip on the ground but must not provide unfair mechanical advantage.  The issue of unfair mechanical advantage is difficult to define.  The US athletics federation rules explicitly specify springs as an example of the type of device that might provide an unfair advantage.  Thus in the US, the rules might be interpreted as implicitly banning the Spira shoe, though until the evidence that the springs in the Spira show give an unfair advantage has been tested in court, it is not absolutely clear that Spira shoes are banned even in the US – though they are explicitly banned by the organizers of the Boston marathon.

 

One might argue that the IAAF should clarify the issue of whether or not the Spira does provide unfair advantage.  However, I suspect that there is a hidden wisdom in the IAAF’s reluctance to rule on the issue.  Even if the evidence clearly shows that the Spira confers an advantage (which I think is very likely) the word ‘unfair’ is less easily interpreted. 

 

In swimming, one of the technical advances that produced the greatest improvement in performance within recent decades was the introduction of goggles.  It is probable that goggles improve the ability to judge distance from the end of the pool as the swimmer prepares to turn.  On account of widespread availability and use of swim goggles, there is no clamor to outlaw them.  The recently introduces lazer swim suit has been more controversial, but now a large number of new Olympic records have been set by individuals wearing the suits, it would be scarcely practical to ban the lazer retrospectively.  Even if it were banned, no doubt other manufacturers would introduce suits designed to achieve similar benefits, so the controversy would be endless and might defy any resolution other than a return to nude swimming.

 

I suspect that explicitly banning the Spira would create a very diversive controversy, as it might be argued that other shoes already in use also provide an advantage.  Spira shoes are already widely available and are not terribly expensive.  If they become widely used within in the next few years we will probably accept them just as we currently allow spikes for track and cross-country events.  If the evidence indicates that they protect muscles from long term damage, I think that the small loss to the purity of our sport would be more than justified.

Why worry about the mechanics of running?

Does speculation about the theory of running make any difference to how fast you can run?  I have been observing and reading and talking and thinking about running mechanics since recommencing running again, about two years ago.  Has this made any difference to how fast I run?  The ultimate test is racing performance and I have run  too few races to draw any definite conclusions.  Furthermore, asthma has hindered my attempts to get fit, so even two years experience is scarcely enough to draw any definitive conclusions.  However, I think I can draw some practical conclusions that have the potential to improve my running.  These are my top 10 learning points:

1)      Short time on stance is more efficient because there is less braking but very short time on stance increases the risk of injury because ground reaction forces are greater.  So a very short time on stance makes sense for a sprinter but is more risky for a marathon runner.

2)      Landing with the foot only a short distance in front of the centre of gravity (COG) minimizes braking and facilitates short time on stance

3)      Holding the pelvis forwards facilitates landing a short distance in front of the COG.

4)      High cadence is more efficient because the amount of energy wasted in compensating for free fall under the influence of gravity is less when the distance is covered in a large number of smaller strides compared with fewer long strides – but beyond a certain point, high cadence becomes inefficient; probably the limit is determined by the optimum speed of the ratcheting interaction between the muscle proteins actin and myosin.  The question of whether or not this limit is fixed by your genes or alternatively might be improved with training is uncertain. 

5)      Landing with a rigidly extended knee increases risk of injury but landing with a very soft knee (low tension in quads) prevents a brisk recovery of impact energy via elastic recoil.  When speed is the highest priority, a fairly high degree of tension in quads is best. In longer races, less tension in the quads might be safer, but too little tension will result in wasted energy.

6)      The optimum point of contact between foot and ground at footfall depends on speed, and should be further forward under the ball of the foot at higher speeds. Except when sprinting, it is highly desirable to allow the heel to drop to the ground during stance to minimize risk of injury to the Achilles tendon and calf muscles

7)      Some of the benefits of training, such as strengthening of bones and connective tissues accumulate slowly over a period of many months or years; training at fast speed before building up the required strength creates high risk of injury.

8 )      Running requires a moderate degree of development of many muscle groups.  The muscular functions that are especially important to develop are: 

·        ability of the quads and calf muscles to capture impact energy at foot fall. This requires eccentric contraction.

·        ability of the hamstrings to arrest the forward motion of the swinging leg in late swing phase to as to allow the foot to fall only a little in front of the COG. This also requires an eccentric contraction.

·        strength of the hip abductors (eg glutes) to prevent the pelvis tilting to the unsupported side during stance.

·        Strength of the trunk muscles to allow a relaxed carriage of the pelvis in a forwards position.

9)      There is some evidence to suggest that running produces cumulative damage to muscles even in the absence of overt injury, so it is probably best to have mixed program that includes cross training.

10)  There is unlikely to be one running style that is best for all purposes. It is necessary to make choices and seek a balance that best meets one’s priorities  Perhaps the best illustrations of this are provided by the requirement of short time on stance and relatively high tension in quads at footfall when running fast, but due to the risk of injury, a somewhat longer time on stance and less tension in quads is preferable when running long distances.

If gravitational torque is a red herring, how do we run fast?

In his comment on my post yesterday, Ewen reported that he has a heart rate around 153 when race walking 1500m in 8 minutes, and a similar heart rate when running 1500m in about 5:40.  He assumes that in both instances he has a similar cadence of around 180 steps per minute, and therefore is able to compute his stride length as 1.04 metres when walking and 1.47 metres when running.  He concludes that walking is not a very efficient form of locomtion – at least at these speeds.  In fact, once we want to move at speed faster than about 7 Km per hour (8.5 min /Km) most people find that running is more efficient (i.e uses less energy per Km at a given speed). This is improved efficiency achieved by becoming airborne during each stride. 

 

If we want to accelerate from rest or from a jogging pace, we can unbalance ourselves by displacing our centre of gravity forward of our supporting foot.  As Jack Nirestein (or Nicholas Romanov) might point out, unbalancing ourselves in this way generates gravitational torque that produces a head forwards and down rotation, so that we naturally increase our speed.  However, gravitational torque is largely irrelevant to the distance runner, because we actually spend most our time running at near constant speed. 

 

Gravitational torque cannot provide forward propulsion at constant speed, because any torque producing a head forward and down rotation during part of the gait cycle must be counteracted by an oppositely directed torque at some other part of the gait cycle if we wish to remain upright.  In fact, allowing an appreciable gravitational torque to develop at any point in the gait cycle when running a constant speed is actually inefficient because the need to apply a force to reverse that torque results in waste of energy. 

 

So how can we minimize the wasteful generation of gravitational torque?  Perhaps it might seem paradoxical but we do this by becoming airborne. If we spend an appreciable fraction of the gait cycle in the air, we minimize the effects of gravitational torque because gravity can only exert a torque when a part of us is anchored to the ground.  However becoming airborne also uses energy.  Due to elasticity of muscles and tendons at footfall we can recover some, but not all, of the energy spent lifting ourselves upwards against gravity.  So, if we wish to run we are faced with one of two alternatives: spending energy lifting ourselves upwards to become airborne only to see an substantial fraction of this energy dissipated as we fall back to earth, or allowing gravity to rotate us face forwards and downwards, only to have to then apply an opposite torque during the early part of the next stance phase, to correct this rotation.  Running is indeed a dance with a devil named gravity.  Hence the name of the series of articles on the mechanics of running that I wrote last year and have posted in the pages listed in the side bar of this blog: ‘Running: a dance with the devil’.

 

If gravitational torque is largely irrelevant when running at constant speed, what should we do if we wish to maintain a fast pace?  We might consider increasing cadence.  In fact the faster the cadence the less energy we waste on lifting our body against gravity because we fall less during a series of short fast steps than during a series of longer slow steps of the same total duration.  Hence, efficient runners tend to adopt a fast cadence (180 steps per minute or perhaps more) even at slow speeds.  Unfortunately, beyond a certain cadence, rapid turnover is no longer efficient.  I suspect this is because there is a limit to the maximum rate at which the actin and myosin protein filaments that form the contractile machinery within our muscle fibres can cycle through the sequence of engagement, ratcheting and release that occur when a muscle contracts.  So once we have reached maximum cadence, which might be 180 steps per minute or maybe 200 steps per minute for some people (eg Haile Gebresalassie) what can we do?  Most of us reach near maximum cadence at all speeds faster than a jog.  If we want to go any faster than a jog, we must increase stride length.

 

But this is where the task become a bit tricky.  If we reach out with our leading leg before footfall so that we strike the ground with the foot well in front of the centre of gravity, we suffer a braking force that places great stress on joints; dissipates our momentum; and places us in a position where gravity exerts a backwards torque.  So reaching out with the leading leg is both dangerous and inefficient.  As almost all experts on efficient running emphasize, at footfall the foot should be moving backwards relative to the body so it touches down at near zero speed relative to the ground, as near to directly underneath the centre of gravity as possible (though it must touch down at least slightly in front of the COG if we wish to remain upright unless we are running into a very strong headwind).

 

If we cannot afford to reach forwards, how can we increase stride length?  The answer is by propelling ourselves higher into the air at lift off.  In fact, most of the force that generates the upwards propulsion is actually delivered mid stance – during the latter part of stance the centre of gravity is already moving upwards as we remove the weight from the supporting foot.  Once we are airborne momentum carries us forwards, without need for forward propulsion except to overcome wind resistance.  So the answer to going faster than a jog is simply to push up more strongly in mid stance.  Probably about 50% of this push can be obtained from elastic recoil, but elasticity is far from 100% efficient, so we must achieve a substantial l part of the required work be achieved by active muscular contraction.  That is why we need to be fit to sustain a fast pace.  No amount of skillful technique can replace the need to be fit if we want to run far and fast.

 

So why bother with technique?  In fact many elite runners over the years have simply replied ; ‘don’t bother; just get fit’ or words to that effect.  I wasn’t an elite athlete or even a serious competitor.  I was actually far more interested in mountaineering during the years when I might have achieved my peak as a runner, but I did run marathons in less than two and a half hours without ever spending a minute thinking about technique.  Furthermore, I rarely suffered any injury.  I am inclined to think that was probably due to the fact that carrying a heavy pack up and down hills and mountains had given me a fairly rugged frame.

 

So why do I now have a blog with the words ‘Efficient Running’ in the title?  Because as I approach my mid 60’s I no longer have the natural bounce that used to propel me upwards on each stride with relative ease, and because I no longer have the rugged frame that allowed me to run 100 miles a week without thinking about injury, forty years ago.  I now need to pay more attention to just how it is done. 

 

I certainly do not yet know how it is best done, though I suspect that there is no single answer that will fit the needs of every individual, or indeed all of my own goals.  I appreciate the comments that people make and I am learning.  At this stage, I am fairly certain that merely increasing my cardiac output by aerobic training will not be enough.  I believe that I need to increase my muscular strength, but I also need to know how to apply that strength to best effect. 

 

I believe that I need to increase the power of eccentric contractions in my quads and calves to allow me to capture energy at footfall and return it with added push in mid-stance to initiate the next airborne phase.  I suspect that this should not be a conscious push.  Indeed, while I think the theory of Pose is misguided, I think that Nicholas Romanov’s emphasis on the subjective sense of a quick, light footfall is probably the best subjective experience to aim for, if one is to become airborne quickly enough.  However, because I would like to be still  running in my eighties, I am also concerned about the possibility that too much eccentric contraction (running fast and far too often) might produce lasting microscopic damage.  I am intrigued by the possibility that increased antioxidants in the diet might diminish this risk.

 

Although upward propulsion is paramount, getting airborne is not the only role of the legs muscles when running.  It is also necessary to get the swinging leg forward quickly so that it is in position to support the body at subsequent footfall.  This requires two actions. The first is acceleration of the leg forwards relative to the torso.  This is probably achieved best by a well coordinated simultaneous contraction of the hamstrings (to flex the knee) and the hip flexors, such as iliopsoas.  The second action is a neatly timed deceleration of the swinging leg relative to the torso, so that it drops to the ground just slightly in front of the centre of gravity, with the foot moving backwards relative to the torso and at near zero velocity relative to the ground.  This action requires an eccentric contraction of the hamstrings, together with a gentle tensioning of the quads to modulate the action.  The quad tensioning needs to be subtle, so that the leg is not too stiff on impact, if minimizing risk of injury is a priority.

 

While I am fairly confident with this outline of the muscular actions, there are many subtleties, such as contraction of the hip abductors (mainly glutes) in mid-stance to prevent the pelvis tilting down on the unsupported side.  Even more important is the question of the mental image required to achieve this series of muscular contractions with a degree of fine control that defies conscious direction.  So I still have much to learn.

Cadence, Heart, Lungs, Nirenstein, Pose and a Humorous but Horrible Video

Rick posted some interesting comments on my recent blog about elliptical cross training.  He pointed out that Haile Gebresalassie averages over 200 steps per minute, whereas I had recommended 180 (i.e. 90 left, 90 right).  Like most of the variables associated with running it is unlikely that there is one magic number that it best for all individuals at all speeds. 

 

A few simple principles of physics demonstrate that for a given pace, there is less up and down motion at higher cadence and probably less risk of injury. When falling under the influence of gravity, the body accelerates continuously. Hence the average speed of fall is greater when the body is airborne longer. The total height of falling is greater during a few long airborne periods than during a larger number of short airborne periods of the same total duration. The mathematics demonstrating this is given in the calculations page in the side bar of my blog.  Similarly the total amount of impact energy that must be absorbed is greater with when the strides are longer at a given speed. The repeated impact with the ground creates risk of repetitive stain injuries in runners, and under the assumption that the risk rises as the force of the impact increases, the risk of injury is almost certainly less with fast short steps that it is with slower cadence and longer strides.

 

However, if you want to maximize efficiency as well as safety the issues are a little more complex. There comes a point where short fast steps become inefficient. I think this point might be reached when length of time on stance becomes too short to allow optimum recovery of energy via elastic recoil of tissues, unless the stiffness of the leg at footfall is increased. The evidence from observing elite athletes suggests that the optimum is probably around 180 steps per second for the majority.  In general sprinters tend to have a slightly greater cadence, possibly because they land with a slightly stiffer leg which recoils more rapidly.  The increased risk of repetitive strain injury is less of a concern when you take about 30 steps in the entire race compared with several hundred thousand steps in a marathon.  However, if you are prepared to run the risk of tendon and joint injuries (especially Achilles tendon injury) then you might run faster marathon if you land with a stiffer leg and higher cadence.

 

At footfall, Haile Gebresalassie takes the weight fairly far forward on his foot, an action that is likely to maximize the storage of elastic energy in his calf muscles.  It may be that one of the several factors that have made him the world marathon record holder is the efficiency with which he recovers impact energy via elastic recoil.  However, I understand that he has required two operations to repair his Achilles tendon.  I personally am cautious about placing too much emphasis on recovery of impact energy via elastic recoil. I therefore aim for a relatively soft landing at a cadence around 180.

 

Rick also points out that by emphasizing loss of leg strength and elasticity as the cause of the decreased stride length that characterizes older runners, I  fail to take account of the decrease in pumping action of the heart and the decrease in efficiency of breathing that also occur with age.  He is absolutely right to emphasize the importance of loss of cardiac and respiratory function.  The reason I focus on leg muscle changes is that I think there maybe more scope for developing better strategies for delaying the loss of leg strength and elasticity.

 

 

With regard to cardiac efficiency, the majority of people suffer a decrease in maximum heart rate as they age.  One factor in this is probably the slow but inexorable decline of the sympathetic nervous system which produces adrenaline.  It is interesting to note that in his recent post ‘Bad, Mad, Glad’, Ewen reported a personal high maximum heart rate recording during the last lap of an exciting race in which he won a silver medal (see link in side bar).  I suspect his adrenaline levels were a bit higher than usual.  However, apart from increasing adrenaline levels, I do not know of anyway of increasing maximum heart rate. The other main cardiac measure is stroke volume. This does appear to respond to training and indeed increasing stroke volume is one of the main goals of aerobic training.  I regard resting heart rate as an indirect measure of stroke volume, and have been pleased that during my recent phase of elliptical training my resting heart rate has fallen from around 53 to around 43, implying that 43 beats is enough to supply the baseline oxygen and nutritional requirements of my body.  Maybe my metabolism has become more efficient, but I suspect a major factor is an increased stroke volume. 

 

The increase in required respiratory effort has also been something I have thought about but on account of my asthma, I have placed my main emphasis on minimizing the irritation of my bronchi and bronchioles.  However, I have noticed that even when I am not wheezing my respiratory effort in the upper aerobic zone is greater on the elliptical than when running.  I do not know why, but speculate that due relatively greater use of fast twitch fibres I am generating more lactic acid and therefore increasing blood acidity which might be expected to provide a greater drive to respiration.  Blowing off more carbon dioxide will decrease blood acidity and compensate for the increase in lactic acid.  If this is the case, then an added advantage of the elliptical is that I will be working my respiratory muscles harder and potentially minimizing the loss of respiratory muscle strength as I grow older. 

 

Finally Rick points out that he has been greatly helped by adopting Jack Nirenstein’s running technique.  Jack Nirenstein advocates a technique that has some similarity to the Pose technique advocated by Nicholas Romanov. The cardinal theoretical principle is the use of gravity to promote a fall forwards.  Perhaps the most interesting puzzle that I have conjured with in my recent attempts to understand running technique is the claim that gravity can provide net forward propulsion.  There are people who undoubtedly find that the Pose technique or the Nirenstein technique are helpful, but it simple consideration of the laws of physics suggests that the theoretical basis of these techniques cannot be correct. 

 

It is true that when the centre of mass of the body is in front of the point of support during stance, that gravity will exert a torque that causes an increase in rotaion in a head forwards and downwards direction.  This is very beneficial when accelerating and indeed it well illustrated by the exaggerated forward lean of a sprinter propelling him or herself from the blocks.  However, unless this head forwards and downwards rotation is counteracted by a forceful application of torque directed in the opposite direction, the runner will inevitable suffer a face down crash within a few strides.. This is an inevitable consequence of the law of conservation of angular momentum. 

 

The Pose coach, Cabletow, whom I regard as the person with the best intuitive grasp of running mechanics that I have ever met (I would venture to speculate that he has a more sound intuitive grasp that Nicholas Romanov himself), recently commented on the Fetch efficient running thread that my application of the principles of physics applies only to stick men.  Unfortunately that is not so.  One or two of the approximate models that I have proposed for the purpose of calculations are indeed approximations that apply to stick men, but the basic conservation laws of physics such as the law of conservation of energy and the laws of conservation of both linear and angular momentum apply to flexible human beings as much as to rigid sticks.. 

 

Some time ago there was a humorous but horrible illustration of the law of conservation of angular momentum in one of those humorous but horrible television programs that show home videos of domestic accidents and other forms of everyday mayhem.  The video clip (which was also published on You Tube but I am afraid I do not have the link) showed a man descending down a slide holding an infant in his arms.  At the bottom of the slide, the man landed on his feet and stepped forwards as he began to rotate to the upright position.  However, because the combined centre of gravity of the man-infant combination was so far forward, he could not get his foot forward quickly enough to plant it in front of his centre of gravity at footfall.  With each step, the man-infant combo accumulated angular momentum-in a head forward and downward direction (unfortunately also an infant forward and downward direction).  They continued to accelerate forwards but also to rotate out of control.  After about 11 steps they suffered the inevitable face-down crash.  The video clip did no make it clear whether or not the infant was hurt, but I hope not.

 

The forward lean proposed by Pose and by Nirenstein is good for acceleration but must be counteracted by a force that exerts an opposing torque at some point in the gait cycle if a face down crash is to be avoided.  Thus, as far as I can see the theoretical basis of both Pose and the Nirenstein methods is fundamentally flawed.  As I have stated here on my blog before, I believe that the Pose method has some very good features.  I am prepared to accept the same of the Nirenstein method, though I have not attempted to practice it.  In particular by taking the focus away from a powerful push off and claiming that gravity provides net forward propulsion, these two methods maximize relaxation while running and minimize the risk of dangerous over-striding.  However, if the goal is to maximize speed without compromising safety, I think one of the goals is to work out how best to develop and apply a powerful push off without over-striding.   I suspect that a major part of the answer is a well timed deployment of hamstrings and other hip extensors in mid-airborne phase to arrest the forward trajectory of the leg relative to the trunk

 

So I am grateful to Rick for his interesting comments.  For me they illustrate the issues that have been at the heart of a lot of my ponderings in the past year or so.

Push or pull?

Yesterday Ewen raised the issue of getting the foot off the ground quickly to minimise time on stance. I certainly agree that getting the foot off the ground quickly is an essential strategy. The main debate is how is it done. Does it require a push or a pull?.

The Pose school emphasizes a pull, via hamstring contraction – however an isolated hamstring pull might bring the foot nearer to the buttock (or buttock nearer to foot) – but cannot lift the centre of gravity (COG) any more than you can lift yourself off the ground by pulling on your own bootstraps – so lifting the COG must involve a push.

Lifting the COG is an inescapable necessity because once airborne, the body must inevitably fall. In part, the energy to lift the COG in the next step can be provided by capturing the energy of the fall and recovering it via elastic recoil, but unless the capture and recovery of the energy of the falling body at the end of the airborne phase is 100% efficient, there must be some additional push.

Some elite runners (such as Sebastian Coe) consciously focussed on this push. Maybe if you want to run really fast, it is useful to focus on pushing, but I am more inclined to focus on the pull and let the push look after itself. This is because a conscious push might easily result in a tendency to contract the quadriceps, and this would be counter-productive because in early swing the knee needs to flex to allow the foot to be carried through reasonably high to minimise the length of the swinging leg. So the conscious emphasis should be on lifting the foot smartly from the ground, and letting elastic recoil, aided by little bit of automatic push by the calf muscles lift the body .

The next question is how to execute the pull. A pure hamstring contraction is inadequate because that would not only bring the heel towards the buttock, but also extend the leg backwards at a time when it is crucial to get the leg swinging forwards rapidly. So it is essential to activate hip flexors at the same time – I suspect that iliopsoas is the most useful muscle for this. However, I do not find it helpful to try to micromanage each muscle; it is better to imagine the required direction of travel of the foot. So I simply focus on rapidly lifting the foot and bringing it smartly forwards – but the forward propulsion of the leg relative to torso must be short and sweet, or there will be a risk of over-striding.

Listening to the body

The aspect of running that requires greatest judgment is deciding when to persist despite tiredness or pain. The safe simple answer is never run through pain. However at least for a person approaching his mid-sixties, this apparently simple answer is not really simple. It is rare that there is no trace of discomfort in muscles or joints, and judging when to persist and when to desist is often tricky.

In accord with my plan to increase intensity a bit, this week I planned 8×400m at 8/10 of maximum effort on Thursday and 6×1Km at 4:20 per Km (estimated 5K pace) on Saturday.

Thursday’s 8×400m session went well. I ran on the somewhat overgrown grass of an abandoned sports field, and was pleased to find that I could run with a feeling of fluency. I did not wear a watch and have little idea of my pace. It felt like a 58-60 second pace used to feel in my youth, which probably means the actual pace was about 85 sec per 400m. The only niggle was a slight tension in my right calf that developed during the 7^th repetition. I usually run with a forefoot landing, which is gentle on the knees but places more strain on the calf muscles. (See http://www.sportsscientists.com/2007/09/running-technique-part-ii-scientific.html for the scientific evidence). So for the eighth repetition I adopted a mid-foot landing and concentrated especially on lifting my foot from stance using hip flexors and hamstrings, while avoiding any trace of pushing off using calf muscles. The tension in the right calf caused no further trouble. I arrived home feeling satisfied that I had achieved my target and had avoided injury.

On Friday I had slight stiffness in quads and hams, but no pain in my calf. I did 3 sets of 20 body-weight calf raises standing on one leg, on right and left leg, with only a barely perceptible trace of discomfort in my right calf.

One Saturday I set out to do the 6x 1Km on the trail in Clifton Wood, as planned. The weather has continued mainly fine in the past week and the trail was fairly firm underfoot, apart from two muddy patches. A deepening layer of autumn leaves covered the ground, obscuring the tree roots, and the dappled sunlight filtering through the trees compounded the difficulty of identifying treacherous obstacles. However I love running in the woods, and despite my multiple musculo-skeletal weaknesses, I have only ever once seriously twisted an ankle – that was when carrying my young son on my back down a steep and stony hillside many years ago. I think I have fairly tough ankle ligaments as a result of lots of hill walking in younger adult life. So I decided that the delights of running though autumnal woodland justified the risk of ankle injury.

The first three repetitions went well. The times were 4:13; 4:18 and 4:14 per Km and my average heart rate recordings were 140, 143, and 142. At the halfway point on the fourth repetition, the tension in my right calf returned. As on Thursday, I adopted a mid-foot landing, but within a further 200m it was clear that the tension was increasing. What I had not taken into account in that this part of the path rises gently uphill over an irregular root-ribbed surface, and re-distributing the load onto my heel in mid-stance to alleviate tension in the Achilles tendon and calf was less easy than it had been on a level grass surface on Thursday. I slackened the pace for the remaining 300m. I was pleased that my time for the fourth Km was 4:16 and average HR was 143. However, gentle stretching of gastrocnemius and then of soleus revealed that there was a definite tear of soleus – the deeper of the two calf muscles that is mainly active when knee is flexed. Thus the features of my current style that I have developed to protect my vulnerable knees- forefoot landing on a slightly flexed knee, combined with the demands of running uphill on an irregular surface, had been my undoing. I abandoned the session and am now sitting with an ice pack over my lower calf – though unfortunately soleus is deep and less accessible to the benefits of ice. So, my plan to increase intensity has produced pleasing evidence that I can still run fairly fluently at moderate pace, but I am now nursing an injured muscle.

The Dance with the Devil: putting the steps together

Preamble

In the last three posts, I have attempted to describe what happens in the various stages of the gait cycle. However, the cycle is one integrated sequence, so this post will focus on how it all fits together, and including some detail about the torso and arms

Torso

Gordon Pirie recommends upright torso; Pose recommends a forward lean maintaining a straight-line from point of support via hips to shoulders, at mid-stance. The theory behind the Pose lean is based on what I believe to be false biomechanics. The proposal by Dr Romanov that gravitational torque can generate forward propulsion even when running at constant velocity is tempting, but violates the law of conservation of angular momentum. Subjectively, the lean can feel helpful – but I think that is a misleading perception based on the experience of starting from a stationary position. There is no doubt that a lean promotes acceleration that is helpful for a sprinter driving from the blocks, but acceleration is only a very minor part of longer distance running. So whose advice is more helpful: Pirie or Romanov? In a previous post I came down favouring Romanov, but that was after a session running into a strong wind. Maybe lean helps when you need continuing reinforcement of the horizontal drive on account of wind, but I am now inclined to think that Pirie’s advice is best on a level surface when the wind is not too strong.

The reason I think an upright torso is best is that it promotes a greater eccentric stretch of the hip flexors during late stance, and this will facilitate hip flexion after lift off, thereby bringing the leg forwards to overtake the torso by mid-swing. Upright torso also likely to promote effective deceleration of the leg by hamstrings and gluteus maximus in late stance, and good coordination of hamstrings and quads to achieve the required flexion of hips and knee at footfall. Both the quads and hamstrings cross hip and knee, and appear to have evolved so that despite being mutual antagonists, simultaneous contraction of both can produce well coordinated movements at both joints when the torso is upright.

Similarly, keeping the hips forward (i.e. avoiding ‘sitting in the bucket’) promotes more efficient hip flexor stretch in late stance which helps get the legs forward quickly; and promotes good coordination of the hip and knee at footfall.

Arms

When the neurosurgeon Wilder Penfield used electrodes applied directly to the brain to stimulate muscle contraction, as part of pre-surgical exploration of brain function in patients needing surgery for epilepsy at the Montreal Neurological Institute in the 1930’s, he demonstrated that a much larger area of the motor cortex in the brain is devoted to controlling the upper limb than the lower limb. This fits with the observation that most people are more dextrous with their hands than their feet. However, the upper and lower limbs automatically work in synchrony when running. So it is plausible that conscious focus on what we do with the upper limb will be more effective than focussing on the legs and feet. In particular, focus on the backward movement of the arm on the side of the leg that is beginning to swing in early swing phase is likely to help bring the leg forward in the optimum direction at the beginning of swing. Even more importantly, making sure that the subsequent forward swing of that arm does not go too far forward will promote the required (non-conscious) braking of the swing leg in late swing and minimize risk of over-striding. The harness proposed by Jack Cady of Stride Mechanics achieves this. This arm swing should be purposeful and controlled but not too tense to avoid wasteful isometric contraction in the shoulders. I find it helpful to form a lightly held ring-shape by resting the index finger of each hand against the adjacent thumb. I think this fairly delicate action encourages a controlled but relaxed arm. This recommendation for hand posture is attributed to Emil Zatopek, the great distance runner of 1950’s, who was famous for his contorted neck and facial features while running, but nonetheless, managed to maintain remarkable relaxation of his limbs (see Wikipedia entry for Emil Zatopek).

Rotation about the long axis of the body

As the hip extends in late stance, the hip rotates externally, thereby producing eccentric stretching of the internal rotators. At lift off, the hip rotates internally bring the leg around and forwards, thereby lengthening the stride and ensuring that the foot is in the midline by foot-fall. This rotation is facilitated by a balancing rotation of the upper torso produced by arm swing.

Integration

Assembling the main features from the previous postings, together with these principles regarding the upper body leads to the following integrated picture:

1) Cadence should be high (e.g. in the range 180-200 strides per minute) to minimise work required to overcome gravity in the airborne phase.

2) Time on stance should be short, though there is a balance between peak mechanical efficiency achieved with a time on stance around 50-60 milliseconds) and minimization of risk of tissue damage (maybe best achieved at around 100-120 milliseconds on stance. Short time on stance minimizes braking in early stance

3) Torso should be held near to upright, with perhaps a slight forward lean if needed to counter wind resistance.

4) Arms should swing in a relaxed but controlled manner, avoiding swinging too far forwards.

5) At foot fall, the hip and knee should be slightly flexed and the ankle near neutral, but with very sight plantar flexion to that the initial point of contact is on the outside edge just forward of mid-sole. As speed increases, the degree of flexion of the hip and knee should decrease making the leg stiffer, leading to a more rapid recoil and shorter time on stance. However, this will increase stress on musculo-skeletal tissues. It should be noted that many elite athletes actually land on the heel. This will result in an even stiffer leg, which may enhance mechanical efficiency, but the risks of over-striding and of musculo-skeletal damage are likely to be higher.

6) In mid-stance, contraction of the hip abductors prevents the hip dropping on the unsupported side, allowing the leg to swing freely and avoiding sideways slanting of the torso.

7) In late stance, recoil aided by contraction of the quadriceps will generate the vertical Ground Reaction Force that provides the impulse required to lift the body. Extension of the hip will preload the hip flexors.

8 ) A conscious pull using hamstring promotes a well-timed lift-off, and an associated concentric contraction of hip flexors in early swing brings the leg forwards to overtake the torso by the time the other leg is at mid-stance.

9) Rotation about the vertical axis of the body, produced by synchronised concentric contraction of the hip rotators with arm swing, will help open up the stride, and ensure that the support foot lands on the midline.

10) In late swing, gluteus maximus and hamstrings decelerate the leg so that horizontal velocity relative to the ground is near zero at footfall.

The next section of the Dance with the Devil will tackle the issue of the perceptions that allow us to achieve these actions effectively, and the mental state that prepares us for peak performance.

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.