Which muscle is the primary plantar flexor with knee extended?

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Study for the Muscle Actions and Functions – Anatomy and Movement Test. Equip yourself with multiple choice questions, each with detailed explanations and hints. Prepare confidently for your exam!

Multiple Choice

Which muscle is the primary plantar flexor with knee extended?

Explanation:
When a muscle crosses two joints, its ability to produce movement depends on the position of each joint. The gastrocnemius crosses both the knee and the ankle, so with the knee straight, it is well placed to generate plantar flexion at the ankle. Its length and tension are favorable in that position, making it the dominant plantar flexor when the knee is extended. The soleus also plantarflexes the foot but doesn’t cross the knee, so its contribution remains steady regardless of knee angle. The tibialis posterior and fibularis longus assist with plantar flexion to some degree, but they are not the main power sources for this action.

When a muscle crosses two joints, its ability to produce movement depends on the position of each joint. The gastrocnemius crosses both the knee and the ankle, so with the knee straight, it is well placed to generate plantar flexion at the ankle. Its length and tension are favorable in that position, making it the dominant plantar flexor when the knee is extended. The soleus also plantarflexes the foot but doesn’t cross the knee, so its contribution remains steady regardless of knee angle. The tibialis posterior and fibularis longus assist with plantar flexion to some degree, but they are not the main power sources for this action.

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