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.

Questions
• What makes the effect of the plantarflexor contraction differ in the two cases?
• Why does the foot lift in the first case but stay on the ground in the second?
• Where does the power go in the second case?