When the foot is on stance, the body becomes unbalanced due to two effects:

1) Inertia due to the forward linear velocity of the body will cause the body to lean forwards.

2) As the body begins to lean, the downwards force of gravity is now acting at an angle to the axis of the body, and will exert a torque that increases the speed of rotation (that is, an increase in angular momentum)

According to the theory of the Pose technique developed by Dr Romanov (http://www.posetech.com) this gravitational torque provides useful forwards propulsion. However, as discussed in my blog on 2 Jan, if this forward acceleration is not corrected at some point in the gait cycle by a torque acting in the opposite direction, the angular momentum will continue to increase with each stride. If the effect is of substantial magnitude, a face down crash would be expected after a few strides. On Jan 2, I speculated that it might be necessary to land in front of the COG to avoid this problem.

However, an alternative possibility is that the effect of gravitational torque is negligibly small and can be ignored. On the calculation page, I have posted a calculation of the magnitude of the increase in angular momentum during a single stride, for a runner travelling at 5 metres/sec (corresponding to a marathon time of about 2 hours 21 min). The calculation is the second of the calculations presented on the calculation page in the side bar of this blog.

The increase in angular momentum during a single stride is 0.2 per cent of the angular momentum arising from inertia associated with forward linear motion. This calculation assumed that the runner remains on stance long enough to increase lean by 6 degrees. If the time on stance is long enough to increase lean by 10 degrees, the increase in angular momentum will be about 0.4 per cent per stride. This suggests that it is mainly linear momentum that keeps us going (provided we move our legs forward quickly enough) and that gravitational torque will have a minor effect compared with other factors such as wind resistance. Therefore, we need not be too concerned about the need to reverse this torque at some other stage in the gait cycle.

While this is re-assuring, it appears to me to raise concerns about the role of gravitational torque in the theory of the Pose technique.

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This entry was posted on January 6, 2008 at 1:46 pm and is filed under Running Mechanics. You can follow any responses to this entry through the RSS 2.0 feed.
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