The gastrocnemius is more effective as a plantar flexor when the knee is: fully extended
The gastrocnemius with the soleus, is the main plantar flexor of the ankle joint. The muscle is also a powerful knee flexor. It is not able to exert full power at both joints simultaneously, for example when the knee is flexed, gastrocnemius is unable to generate as much force at the ankle.
As the front leg in gait is moving through towards swing phase, the tibia is slowed down by eccentric lengthening of the gastrocnemius and a number of other calf muscles. This slowing along with the momentum of the body creates knee extension as it pulls back on the distal femur as ankle dorsiflexion is decelerated
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