Why does a seated calf raise, with the knees bent to about 90°, emphasize the soleus more than the gastrocnemius?
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Answer: C
The gastrocnemius attaches only to the tibia, so knee position has no effect on it: The gastrocnemius starts above the knee on the femoral condyles. That is exactly why knee position affects it.
Knee flexion stretches the gastrocnemius to its optimal length, so the soleus is recruited first: Knee flexion shortens the gastrocnemius, not lengthens it. That puts it at a disadvantage rather than an optimal length.
Bent knees shorten the two-joint gastrocnemius, cutting its force, so the soleus works more: Correct. The gastrocnemius crosses both the knee and the ankle. Bending the knee slackens it (active insufficiency), so the one-joint soleus becomes the main plantar flexor.
The soleus is a two-joint muscle, so it becomes stronger whenever the knee is bent: The soleus is a one-joint muscle. It starts on the tibia and fibula and crosses only the ankle.
Two-joint muscles lose force when they are shortened across both joints (active insufficiency). Calves: straight knee = gastrocnemius bias; bent knee = soleus bias.