Teach-in 2003 : Plantarflexor action
by Chris Kirtley, The Catholic University of America, Washington DC

The plantarflexors can produce two motions at the ankle when the foot is on the ground (closed-chain):
  1. Plantarflexion of the ankle and lifting of the body
  2. Posterior rotation of the shank

This table summarizes the power flows in each case:
Power Variable
Case #1
Case #2
Ankle Joint (muscle) Power (Mawa) Positive (concentric) Positive (concentric)
Active Foot Power Flow (Mawf) Positive (into foot) Zero
Passive Foot Power Flow (Fava) Negative (out of foot) Zero
(Ma = ankle moment, wa = angular velocity of ankle joint, wf = angular velocity of foot, Fa= reaction force at ankle, va = velocity of ankle joint)

It can be seen that the joint power has the same polarity in each case, but the power flows are non-zero in the first case and zero in the second.

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