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).
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.
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.
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.