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)

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6 Responses to “Running: a dance with the devil”

  1. Simon Says:

    Nice introduction – looking forward to the articles.

    One bit of data you may find of interest if you didn’t know already, the return of elastic energy stored from the landing was measured at around 60% in an elite compared to 90% return for Oscar Pistorius with his carbon fibre lower leg prosthetics. Oscar was banned from competing based on that advantage.

    Another area is the amount of time that elastic energy can be effectively stored and under what conditions (limb position, tension, muscular activity) – these factors need to be taken into account if elastic return is to be included in your dance steps. I’m sure you will have considered this but I thought I’d mention it just in case.

  2. canute1 Says:

    Simon,
    Thanks for your comment and thanks for the information about Pistorius.

    I would be pleased to hear any further thoughts you have about the the length of time elastic energy can be stored. It seems to me that once a muscle has been stretched, the stored elastic energy can be retained by a sustained isometric contraction which will itself use some energy, and after a certain length of time the cost of the sustained isometric contraction will exceed the energy recovered from elastic recoil.

  3. Simon Says:

    Canute,

    I’m certainly no expert in this field and am basing this on the few articles and comments that I have come across.

    Biomechanics resources often talk about the ‘stretch shortening cycle’ which some say produces very efficient return of stored energy though there appears to be a lot of debate still on the exact mechanism. The process basically requires an eccentric stretch of a muscle followed by concentric muscle contraction.
    I believe that it needs to happen very quickly to derive the maximum benefit (this can be seen anecdotally by jumping up and down and slowing the end of the squat – the more it is slowed the less the return). I also suspect that this is why Pose has the 1/1 framing.

    Here’s a paper that looks at SSC of the lower leg:
    http://jap.physiology.org/cgi/content/full/99/1/217

    This link has some interesting timings for muscles:
    http://www.unmc.edu/Physiology/Mann/mann14.html
    And the following chapter talks about reflex reaction.

    Chapter 15 of this book looks like it could be very useful, though I have not had time to read it:
    http://books.google.co.uk/books?id=8MxEuq7OEqQC&printsec=frontcover#PRA1-PA307,M1

    Happy hunting!

  4. canute1 Says:

    Simon, Thanks. I have not had time to fully digest all this material, but here are some initial thoughts. The first of your references (Ishikawa et al) is to a drop jump study. For me the interesting feature that distinguishes the muscle activity in the optimal jumps (i.e those producing the highest rebound) is increased quadriceps tendon tension, and also greater shortening of the vastus lateralis fasicles, at around 100 milliseconds post impact. This suggests that at least in the context of a drop jump, the time scale for effective recoil of the quads is around 100 millisec,

    I am not sure of the implications for running. I am inclined to think that during running the quad is the most relevant muscle with regard to the use of recoil energy to propel the body upwards, and therefore, these DJ data might be relevant .

    It is also noteworthy as strain level increased above the optimal, recoil was not effective. The authors do not offer a confident explanation, but raise the possibility of Golgi- tendon protective responses or even actin-myosin detachment. As we know time on stance gets shorter, mean and peak vGRF increase, so at very short time on stance, strain might increase to a point where these processes occur.

    So overall, despite my uncertainty about the relevance of these data to running (and about my interpretation of them) I think that they suggest that effective recoil might be expected with times on stance of 100-120 milliseconds. Furthermore, I wonder whether the less effective recoil at greater strain indicates that recoil might be less effective at very short times on stance.
    Canute

  5. Simon Says:

    Canute,
    After reading up some more I am still struggling to come up with anything very applicable to running. As you say, the drop jump study is interesting but not necessarily directly applicable to running.
    The intensity, rate of stretching and muscles involved all seem to effect how efficient the process is, as does the individual’s muscle make up and probably bone lengths too. This is seen in the study I linked to where the gastroc peaks at 50ms and the quad at 100ms; this is probably largely due to the range of motion of each structure (the gastroc has a smaller ROM than the quad).
    The conclusions that can be drawn is that it is likely there will be an optimal return of ME based on intensity and duration of the loading and unloading hence framing in running. Until a specific study looks at this though, it is impossible to identify. We can only assume that the world’s elite runners have tuned into this and that it is likely to be shorter rather than longer time on support based on how they run. As you have pointed out, short time on support has potential hazards for runners with non-elite genetics and conditioning though.

  6. canute1 Says:

    Simon,
    Thanks for your comments. As far as I can see from studies such as the Ishikawa study, the evidence indicates that the optimal time fro efficient recovery of elastic energy is in the range 50-120 ms. As you say, it depends on which muscle and the rate of loading – for viscoelastic materials stiffness depends on rate of loading.

    We need data from running specific movements – but even with running specific movements, it will depend on factors such as degree of knee flexion at impact and, in the case of gastrocnmeius, the movements that transfer load to the medial longitudinal arch.

    With regard to the time on stance for elite runners, as far as I am aware elite 5K-10K runners typically spend 50-80 milliseconds on stance. I am not aware of data for marathon runners, for whom efficiency is crucial. I will have to examine some videos of marathon runners.

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