Archive for March, 2012

Natural running

March 29, 2012

The word ‘natural’ invokes images of wholesomeness but also carries a hint that we are in danger of being hoodwinked by a snake-oil merchant.  In contrast, the word ‘technological’ has overtones of something lacking wholesomeness.  Nonetheless, on the whole, I am glad I belong to a species with the brainpower to develop technology.   Many inventions created by human wit, ranging from reading glasses to electronic devices, extend the range of activities that are accessible to me and make life more interesting.  But when it comes to running, there is good reason to ask whether we have lost our natural skill as a result of growing up a modern technological society.

Humans are in fact remarkably good endurance runners.  Although many species can outpace us in a short sprint, few can maintain a steady pace for such long distances. On the other hand, a very large proportion of us get injured each year.  In a comprehensive review of studies of injury rates among distance runners, van Ghent and colleagues found that the incidence of lower extremity injuries reported in published studies ranges from 20% to 79% (Br J Sports Med 41: 469-480, 2007).

Persistence hunting

Accumulating evidence suggests that humans became good endurance runners because evolution favoured the development of anatomical (and perhaps biochemical) adaptations that enabled our forebears to engage in persistence hunting – in which the hunter pursues his quarry to the point of exhaustion – on the African savannah around two million years ago. (Bramble and Lieberman, Nature, 432, 345-352  2004).   We do not know whether or not early persistence hunters were also prone to injury, though evolution would scarcely have favoured those who were as prone to injury as modern-day runners. Perhaps only an elite few in the tribe were able to run without injury.  Among the few remaining peoples of the Kalahari desert who continue to practice persistence hunting today, the huntsman who engages in the long chase is an elite member of the hunting group.  Nonetheless, an ability to run far with few injuries is likely to have been a fairly common skill among our early ancestors.

Bare feet v shoes

So if we wish to minimize injury, it is probably worthwhile to ask how did our forebears run.  Perhaps the first point to note is that they would not have worn shoes (though it is also noteworthy that  modern-day persistence hunters in the Kalahari do wear shoes).  The most striking difference between barefoot and shod runners is the nature of the foot-strike.  Hasegawa and colleagues demonstrated that about 75% of runners wearing modern running shoes heel-strike (J Strength Cond Res. 21(3):888-93, 2007).  In contrast, Lieberman and colleagues have demonstrated that bare-foot runners are much more likely to land on the forefoot and then transfer a portion of the load to the heel whilst on stance.  Lieberman and colleagues have demonstrated this is a systematic study comparing American habitual barefoot runners with shod runners, and confirmed it in a less systematic observation of Africans who had grown up never wearing shoes (Nature. 463(7280):531-5, 2010.   Landing on the forefoot minimises the initial sharp increase in vertical ground reaction force that is seen with heel striking.

Lieberman is firm in pointing out that there is no strong evidence that minimising this sharp increase in ground reaction force leads to lower injury risk. However, in general, the repeated application of a rapidly rising large force is stressful and might be expected to lead to stress fracture.  So it is plausible that injury risk is greater when wearing shoes. This plausibility is confirmed by Kerrigan’s demonstration of greater torques at hip and knee during shod running (PM &R: The Journal of Injury, Function and Rehabilitation, Vol. 1, pp 1058-1063, 2009).  Thus it appears that Bill Bowerman’s first experiments with a waffle iron that led to the modern running shoe, were a faltering mis-step based on the mistaken idea that putting padding between the runners foot and the ground would increase safety and efficiency.

Getting airborne

I have discussed the question of running shoes and foot-strike in a previous post, and I will probably return to it again in the future.  However my main interest today is in the question of how our forebears were equipped to deal with the cardinal challenge of running: exerting a strong enough force to get airborne.  Getting airborne is the essence of running.  It allows us to minimise the inefficient braking that is an inevitable consequence of maintaining a stationary foot on the ground during the stance phase.  To minimise braking we must spend as small a portion of the gait cycle on stance as is possible.  We can do this by landing with the foot only a short distance in front of our centre of gravity (COG), but that necessarily entails the exertion of a large push against the ground.   If we are on stance for only a third of the gait cycle, the average push against the ground during stance must be three times body weight.

A substantial part of this push is generated via elastic recoil.  But in fact, measurements suggest that at most about 50% of the required energy can be generated by elastic recoil (Alexander, R.M. Energy-saving mechanisms in walking and running. J.Exp.Biol.160,55–69,1991).  So an equally substantial portion of the work must be done by an active push.   What evolutionary development allowed early member of the homo genus to achieve this crucial push?  A clue can be found by examining the anatomical differences between ourselves and our nearest primate relative, the chimpanzee.  Chimps, like other non-human primates, are not capable of endurance running.

Differences between man and chimp

The most immediately apparent anatomical difference is man’s larger skull.  However, the larger skull is a feature of homo sapiens rather than early members of the homo genus.  Possibly we owe our large skull and brains  at least in part to a somewhat more subtle change at the lower end of the vertebral column that occurred earlier in homo evolution.  This subtle change, present in early members of the homo genus, such as homo erectus, is a substantial enlargement of the upper part of gluteus maximus.  Gluteus maximus is a hip flexor.  Although it acts with less mechanical advantage than the hamstrings, it is more massive   Could the enlargement of gluteus maximus have played a key role in the development of endurance running ability, thereby facilitating persistence hunting and providing the protein rich diet essential for the eventual development of homo sapiens’ large brain, over a million years later.

The roles of gluteus maximus

To address this question Lieberman and colleagues  examined the activity on gluteus maximus throughout the gait cycle, by recording the electrical signals from an electrode placed over the muscle, during both walking and running (Journal of Experimental Biology 209, 2143-2155, 2006.)  Their  first important  observation was that gluteus maximus is much more active during running than walking, consistent with it being an evolutionary development associated with the acquisition of capacity for endurance running.  During the running gait cycle, there are two main bursts of activity in gluteus maximus: one when the foot from the opposite side of the body is on stance and the other beginning shortly before the footfall of the foot on the same side as the muscle,  and continuing through early stance on that foot.  The activity when the opposite foot is on stance almost certainly reflects the action of arresting the forward motion of the swinging leg.   Interpretation of the role of the burst of activity in early stance on same-sided foot is more complex.  The magnitude of the activation increases with speed of running and is also correlated strongly with the velocity and timing of the forward pitch of the trunk that occurs at foot-strike.  Thus it is very likely that a major role of gluteus maximus is stabilizing the torso.

Mark Cucuzella’s resonant phrase ‘you can’t fire a cannon from a canoe’ powerfully expresses the importance of stabilization of the torso, but it also raises the question of what cannon is being fired.   Could gluteus maximus also contribute to generation of the vertical ground reaction force (vGRF) that launches the body forwards and upwards from stance?   Lieberman and colleagues  observed that the timing and magnitude of activity in gluteus maximus is also correlated with the timing and magnitude of contraction of another major hip extensor, biceps femoris, which is one of the hamstrings.    This suggests an active role in hip extension.  It is important to note this active hip extension is largely confined to the early part of the stance phase.  As the hip and knee are slightly flexed at that time, the main consequence of hip extension will be an increase in the downwards push against the ground.   Thus, this action would be expected to contribute to the initiation of the upward acceleration of the body commencing in mid-swing. Perhaps gluteus maximus also contributes to firing the cannon.

It is noteworthy that one of the early proponents of ‘natural’ running, Ken Mierke, recognised that combining contraction of gluteus maximus with the hamstrings would greatly increase the power of hip extension, thereby reducing fatigue of the relatively weak hamstrings and promoting endurance.  While I think that the essence of Ken’s proposal is sound, I would place a somewhat different emphasis on the effect of the hip extension.  Ken argues that the hip extension largely provides forward propulsion.  I do not think that fits well with the timing of the active contraction of either gluteus maximus or the hamstrings.  Even after allowing for the 40-50 millisecond delay between the electrical signal and the mechanical effect of a muscle contraction, the active contraction of gluteus maximus and the hamstrings is complete shortly after mid-stance.   I think that the main consequence of this powerful hip extension is to accelerate the body upwards thereby achieving a stance that is short – this is the key requirement for efficient running.

Other muscles also contribute, notably contraction of the gastrocnemius, which reaches its peak contraction a little later in stance.  This will generate a forward and upward GRF.  The upward component will add to the impulse that gets us airborne, while the forward component will help compensate for the braking that occurred in early stance.  Because the hamstrings cross both hip and knee, residual tension in the hamstrings in late stance might add to the upswing of the lower leg relative to the torso thereby facilitating the breaking of contact.  However it should be noted that the contribution of a hamstring to pulling the foot towards the torso cannot contribute to raising the centre of mass of the body (as is proposed in Pose theory).  That would be pulling oneself up by ones bootstraps.  The upwards acceleration of the mass of the body must be produced by a push against the ground.  (Added note: it should be acknowledged that Pose theory appears somewhat ambiguous regarding the mechanism by which the centre of mass is raised.  See the comments from Hans and Jeremy below.)

Other evolutionary developments

Development of gluteus maximus was not the only anatomical change occurring early in the evolution of the genus homo.  Freeing up of the tethering of head to shoulders that limits the independent rotation of upper torso and head in other primates, allows us to produce the counter rotation of the torso necessary to balance the swinging leg, while maintaining the head upright and forward-facing.  In addition, the development of a longer Achilles tendon that occurred at some point along the evolutionary path from our even earlier ancestor, australopithecus, to early homo species, is likely to have enhanced the efficiency of capture of impact energy as elastic energy.   But in my opinion, it was the development of gluteus maximus that was the decisive development that allowed us to get airborne efficiently.

Minimizing risk of injury

While these speculations might explain how our forebears came to be efficient endurance runners, it still leaves us with the question of how we might avoid injury in the face the inevitably large vertical ground reaction forces generated by the powerful push.  I think that Kerrigan’s  demonstration of greater torques at hip and knee during shod running is a key observation.  This suggests that the orientation of the foot on the ground during the period around mid-stance when vGRF is at its peak is likely to play a major part in determining how much torque is produced.  Drills that help develop a sharp contraction of gluteus maximus that is well coordinated with the down swing of the contralateral arm will ensure that the non-conscious brain is well appraised of just when the peak vGRF will occur.  In addition, an appropriate  sharing of weight between forefoot and heel at mid-stance facilitated by  shoes that are light enough to allow a good perception of the distribution of ground reaction forces might allow the non-conscious motor control system in our brain  to coordinate the application of the push in a way that minimises potentially damaging torque at the knee and hip.

Conclusion

We have grown to adulthood spending hours each day sitting at a desk or in an automobile seat, and even longer periods with our feet encased in rigid shoes.  If we are to run naturally, in a style similar to that which allowed our early homo ancestors to master the art endurance running, perhaps we should focus on re-training our bodies so that our non-conscious brains can once again integrate the sensory signals from the joints in our arms and legs, with those from the numerous sensory nerve terminals in our feet, to coordinate the delivery a powerful, well-timed but fairly safe push against the ground to get us airborne.

Training to increase sprinting speed

March 15, 2012

The issues raised by Klas in his comments on my recent post on Usain Bolt’s sprinting style have led me to wonder just what it is that determines peak sprinting speed and what a runner might do to increase sprinting speed.

The key relevant scientific study is the investigation of 33 physically active adults (aged between 18 and 36) of varying sprinting ability, published by Peter Weyand and colleagues from Harvard University in Journal of Applied Physiology (J Appl Physiol, 89: 1991–1999, 2000).  They measured characteristics such as cadence, time on stance, swing time and ground reaction force observed across a range of speeds up each individual’s top sprinting speed.  The range of top speeds extended from 6.2 metre/sec up to 11.1 m/sec.  They observed that the faster sprinters exerted a stronger push on the ground while on stance and concluded ‘runners reach faster top speeds not by repositioning their limbs more rapidly in the air, but by applying greater support forces to the ground’.

I agree with their conclusion, but closer inspection of their data leads me to a slight modification that might have important implications for how a runner should train to increase speed.

Limb repositioning time

First let us consider the time taken to reposition the swinging leg from its position behind the centre of gravity (COG) at lift-off from stance, to a position a little ahead of the COG at foot-fall.  This is the swing time.  It embraces two airborne intervals and a period of stance on the other leg.  Perhaps surprisingly, the swing time at top speed varies very little between runners of markedly different sprinting ability.  The average swing time of the 33 runners was 0.38 seconds with only weak evidence that faster runners have a shorter swing time.  For comparison, the average swing time of the three medal winners in the male 100m at the 1996 Olympics was 0.33 sec.  However, there is little evidence of a consistent trend across the range of sprinting ability.  For example, the slowest of the 33 individuals studies by Weyand had a swing time of 0.34 sec despite running only a little faster than half the speed of the fastest runners.

Although faster runners spend less time on stance, because their speed is greater, the foot gets left further behind during stance. Typically, a slow runner has to move the foot forward by about 85 cm relative to the COG during the swing, while the fastest runners have to move the foot forwards by about 105 cm.  Thus, the faster runners do swing their foot forwards a little faster. For an elite sprinter it is worthwhile expending some effort on improving swing dynamics, for example by flexing the knee to create a short lever arm at mid-swing.  However, this is only fine tuning – perhaps it might make the difference between a gold medal and fourth place, but it is not likely to produce the magnitude of improvement that might encourage a recreational distance runner to choose to become a sprinter instead.

It is interesting to wonder why swing time at top speed varies so little between elite sprinters and non-athletes.  It appears that most of the gain a  faster sprinter derives from increased ability to reposition the foot rapidly relative to the COG is required to compensate for the modest increase in the range of the swing required at higher speed.  It appears to be impossible to get swing time appreciably below a third of a second.  Although the swinging leg is not merely a passive pendulum it is hard to drive it much faster than its natural swinging rate

Time on stance

The strongest predictor of top sprinting speed is ability to get off stance rapidly.  In Weyand’s study, the slowest sprinters spent 0.135 sec on stance while the fastest spent about 0.09 sec on stance.  Furthermore, there was a very consistent trend for decreasing time on stance to predict faster top speed, across the full range of sprinting ability. The correlation between stance time and top speed was 0.76.

Shorter time on stance is associated with stronger push against the ground.  The average vertical ground reaction force (vGRF) during stance increased from 1.9 times body weight to 2.4 times body weight, although the relationship was not quite so consistent across the range of top speeds.  The correlation between average push and top speed was 0.62.  Thus the average vGRF while on stance was not quite such a reliable predictor of top speed as stance time.

It is of interest to note that because stance time decreases as strength of push increases, the impulse delivered (product of force by time for which the force acts) varies relatively little between the slower sprinters and the fastest.  The vertical impulse was 2.49 newton-sec at a top speed of 6.2 m/sec and 2.25 newton-sec at a top speed of 11.1 m/sec. As the vertical impulse determines how much upward momentum is imparted to the body, it determines how high the COG is elevated between mid-stance and mid-flight. .The peak elevation of the COG was marginally lower in the fastest spinters.  The precise gain in elevation from a given impulse depends on the shape of the relationship between force and time while on stance. . For a forefoot runer it is approximaltey sinusoidal and in this case, the range of vertical oscillation of the COG was 5 cm at 6.2 m/sec and 4.3 cm at 11.1 m/sec.

Estimated values for slowest and fastest runners based on linear trends across the group of 33 runners. *The calculation of peak vGRF and elevation assumes a sinusoidal variation of vGRF with time during stance – typical of a forefoot runner

Conclusion

These observations indicate that if one wants to sprint faster, one should aim to increase push and decrease time on stance.  Although these two variables are related, in fact the decrease in time on stance is a stronger predictor of peak speed than the magnitude of the push.  This is not surprising because decreased time on stance directly reduces braking, which leads not only to increased fuel efficiency, as discussed in my post on 16th January, but also to more efficient utilization of peak power.

It is necessary to have strong leg muscles to get off stance quickly, so it is worthwhile training so as to increase leg strength.  As eccentric contraction is required, plyometrics are potentially helpful. However, the fact that the ability to get off stance quickly is the strongest predictor of top speed, suggests that one requires not only adequate strength but also good coordination of the muscles so as to capture impact energy as elastic energy and then release that energy in a smoothly coordinated way.  This conclusion is similar to that reached on the basis of considering the style of Usain Bolt.  If I want to increase my sprint speed I should focus not only on increasing my strength, but also my coordination.

I suspect that genes and development during infancy play a large part in determining how quickly a person can get off stance.  Nonetheless, the fact that top speed decreases with age demonstrates that top speed is not fixed, and suggests that a training program aimed at producing changes opposite to those produced by aging might produce an increase in sprinting speed.

How might I increase my coordination?  Plyometrics are likely to increase coordination in addition to increasing strength, though they are risky, and should be performed in moderation.  A more direct focus on coordination might be worthwhile.  Coordination depends on proprioception  (the ability to sense  where ones limbs are) and the ability to send messages from the central nervous system to the muscles with the appropriate  precise timing.  I believe that drills such as ‘change of stance’ are likely to be an effective way to achieve this

Does Usain Bolt run Pose style?

March 11, 2012

The contrast between the muscular torso, arms and legs of a sprinter compared with the slight frame and skinny legs of a marathon runner tell us that the requirements for effective sprinting are not the same as for efficient long distance running.  Nonetheless, as I have grown older I am acutely aware of my loss of speed and am eager to do something to arrest this decline.  During my recent examination of the implications of Newton’s equations of motion for the mechanics for efficient running, I have pondered what these equations tell us about sprint technique.  The equations demonstrate that a high cadence and a short time on stance facilitated by a relatively large vGRF generated by a strong push, are key elements of efficient fuel consumption.  Although efficient fuel consumption is not as important for a sprinter as for a distance runner, observation of elite sprinters demonstrates that high cadence and short time on stance are also key features of fast sprinting.

How can we achieve a short time on stance? Anyone who has followed my blog for a while will probably know that I am sceptical about the claims of Dr Romanov’s Pose technique, but I am not inherently anti-Pose.  For more than eight years I have been fascinated by Pose on account of the fact that it appears to facilitate a short time on stance. I have read widely about it, talked to many Pose coaches and even attended a two-day Pose clinic conducted by Dr Romanov, in an attempt to sort out the science from the pseudo-science.  Despite the fact that Pose theory is based on questionable physics, observation of masters of the Pose technique reveals that they can achieve a very rapid lift-off from stance.  During the two-day Pose clinic the observation that impressed me most was the way in which Pose coach, Jon Port, reacted when Dr Romanov gave him a sharp sideways push on his shoulder while he was standing poised on one leg.  Instead of falling sideways, Jon managed to remain upright by getting airborne before his body had a chance to pivot sideways around his point of support.

Therefore, I have been rather intrigued by Dr Romanov’s article on the Post Tech website in which he appears to claim that Usain Bolt runs Pose style.  In an analysis of Bolt’s technique exhibited during the 100m World Championship in Berlin in 2009. Dr Romanov claims he is not “pushing off” but is “waiting”, “allowing” gravitational torque to provide the angular acceleration of the GCM’.   I do not think Dr Romanov’s description of Bolt ‘waiting’ on stance while he allows gravitational torque to provide acceleration of his centre of mass is credible.  There is no way that waiting for gravity to act, without an active push, could get him moving forwards and upwards with the required speed.  Nonetheless, could it be that Bolt’s legendary relaxed manner reflects a mental state similar to that which enables a good Pose runner to get airborne quickly without conscious awareness of a push?

My attempts to identify the features of Pose that promote a short time of stance have led me to conclude that it is achieved by two related features.  Pose drills such as ‘change of stance’ promote rapid flexion of the hip accompanied by flexion of the knee.  In addition, I believe the conscious focus on rapid lift off advocated by Pose can lead to tensioning of the major muscles of the leg at point of impact thereby facilitating efficient capture and recovery of impact energy via elastic recoil.  The combination of efficient recovery of impact energy via elastic recoil and rapid flexion of hip and knee creates a mental focus that promotes a short time on stance and an associated large vGRF.  Does Bolt achieve his powerful drive from stance by this mental focus, or does he consciously focus on a powerful push?

Tim Huntley, who writes a blog about his goal of running a fast 400m, recently posted an article in which he asks whether or not Pose is the way to go.  The responses make a very interesting debate.  Brian McKenzie replied ‘Yes, the Pose method is the only way we really run’.  In contrast, Tom Tellez, former coach of Carl Lewis, was very dismissive, saying  ‘Running action such as reaching and pulling with the hamstrings has been scientifically proven not to produce the most efficient movement. ’ Tellez quotes Peter Weyand’s evidence that  faster running speeds are achieved with greater ground forces, not more rapid leg movements (see Journal of Applied  Physiology, vol 89: pages 1991–1999, 2000)

Tim emailed Dr Romanov who replied in typically vague Pose style: ‘Sprinting or any running is the product of gravity, shaped and moulded by this universal field of the force.  The cadence and efforts of a sprinter are governed by the angle of falling.”   Tim also posted a link to a U-tube clip in which Bolt describes his own understanding of what he does.  ‘After the acceleration phase the goal is to: ‘Keep driving, driving, driving.. …. After completing the drive: ‘Get tall, knees up, dorsiflex, get your toes up, plant, push again’

Bolt’s own emphasis on driving and pushing are somewhat at odds with Dr Romanov’s  claim that he is not pushing off.  Could it be that when he runs he lets his natural instincts take-over, and what he says on the video is merely an attempt to put into words something that is too primeval for words.  I think this is very unlikely.  As Tim Huntley reports, Bolt’s coach Glen Mills makes it clear that Bolt’s style is not the product of some natural primeval intuition.  According to Mills, when he started working with Bolt ‘one of the things that stood out like a sore thumb was his poor mechanics.   We set about doing drills, then we took videos of his workouts and broke them down on the screen in slow motion to show him exactly what he was doing.’

So I think the evidence is fairly clear that Bolt achieves his powerful drive from stance as a result of a physical and mental process that focuses explicitly on a powerful push.  However, I believe that a conscious focus on pushing is only likely to be successful if you have finely tuned bodily awareness, together with rapid reactions to the sensations generated by ground contact.  Without such awareness and rapidity of reaction, it is likely that a conscious focus on pushing will result in too long a delay on stance.  Therefore, in my own attempts to arrest the decline in my speed, I practice Pose drills such as ‘change of stance’ and when running, I focus on rapid lift off from stance rather than pushing.  I would not recommend Pose for a runner with serious hopes of achieving elite status, but for a recreational runner, it has some worthwhile features.


Follow

Get every new post delivered to your Inbox.

Join 65 other followers