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):
-
Plantarflexion of the ankle and lifting of the body
-
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?
Email your answers to [n/a]
What
we said on the CGA list
Back to Teach-in page