Kevin M.,
At the time of heel off, the center of gravity is over the metatarsal
phalangeal joints (MPJ'S) and is descending. (the peak is at midstance).
Plantarflexion is a gait determinant to decelerate the descending of the
center of gravity. Also since the center of gravity is over the MPJ's
(which is also the fulcrum for this second class
lever[http://www.usoe.k12.ut.us/curr/science/sciber00/8th/machines/sciber/lever2.htm])
there is no tension required to lift the heel.
Respectfully,
Stanley
>Kevin M said:
> The plantar flexors, aided by the windlass, lift the COM in the
> gravitational field. The reason I put emphasis on the windlass mechanism
> is because the plantar flexors will plantar flex the foot even if the COM
> is NOT raised. In short, the windlass is the key to generating the
> potential energy. (Gravitational potential energy is a product of weight
> and vertical displacement.) Flexing the hips independently does little
> to change COM position. But a mass as great as a leg moving vertically
> generates enough momentum to ease ! the burden on the planter flexors and
> the windlass. In short, the magnitude of vertical displacement of the
> COM will be the sum of the forces created by hip flexion and plantar
> flexion acting around a lever created by the windlass mechanism. The
> implication is that a maximum force will still yield little in the way of
> gravitational potential energy if the lever arm is not sufficiently
> long. Add to this the timing issues already mentioned, and proper
> formation of the windlass becomes obvious for the MOST efficient
> gait. However, as I mentioned in the earlier post, fail-safe mechanisms
> will activate to keep the organism ambulating until its limit for
> compensation is reached.
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