MUSCLE MECHANICS - Biomechanics and Motor Control of Human Movement - page 242

Figure 9.15 Time history of muscle moment-velocity during stance phase of walking.
Dorsiflexor moment is shown as a dashed line; plantarflexor moment is a solid line.
Stance begins with negative work, then alternating positive and negative, and finally a
major positive work burst later in push-off, ending at toe-off.
the muscle moment. Thus, using in vivo data, the traditional force-velocity
curve becomes a moment-angular velocity curve. Figure 9.15 is the resultant
plot for the period of time from heel contact to toe-off.
It can be seen from this time course of moment and angular velocity that
this common movement is, in fact, quite complex. Contrary to what might
be implied by force-velocity curves, a muscle does not operate along any
simple curve, but actually goes through a complex combination of force
and velocity changes. Initially, the dorsiflexor muscles are on as the foot
plantarflexes between the time of heel contact and flat foot (when the ground
reaction force lies behind the ankle joint). Negative work is being done by
the dorsiflexors as they lower the foot to the ground. After flat foot, the
plantarflexors dominate and create a moment that tends to slow down the
leg as it rotates over the foot, which is now fixed on the floor. Again, this is
negative work. A few frames after, heel-off positive work begins, as indicated
by the simultaneous plantarflexor muscle moment and plantarflexor velocity.
This period is the active push-off phase, when most of the “new” energy is
put back into the body.