Does leaning help us run faster?

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

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

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

Lean might lead to increased speed by two mechanisms.

 

The impulse from horizontal ground reaction force.

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

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

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

 

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

 

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

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

 

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