Which nerve root is associated with plantar flexion weakness?

Which nerve root is associated with plantar flexion weakness?

• A. L5
• B. L4
• C. C7
• D. S1

Answer: (D)

Plantar flexion relies on the gastrocnemius and soleus muscles, which are innervated by the tibial nerve arising from the lower sacral roots, mainly S1–S2. When plantar flexion is weak, it localizes to the S1 nerve root because S1 provides the primary innervation for these plantar flexors. Clinically, this is tied to the Achilles reflex, which is S1-mediated; a diminished or absent Achilles reflex supports an S1 involvement. Sensory changes may also appear in the S1 dermatomal distribution along the posterolateral foot. In contrast, weakness of plantar flexion is not typically from L5 or L4 (which more strongly involve dorsiflexion and knee/hip-related functions) and is not related to C7 (an upper-limb root). Therefore, the nerve root most associated with plantar flexion weakness is S1.

Plantar flexion relies on the gastrocnemius and soleus muscles, which are innervated by the tibial nerve arising from the lower sacral roots, mainly S1–S2. When plantar flexion is weak, it localizes to the S1 nerve root because S1 provides the primary innervation for these plantar flexors. Clinically, this is tied to the Achilles reflex, which is S1-mediated; a diminished or absent Achilles reflex supports an S1 involvement. Sensory changes may also appear in the S1 dermatomal distribution along the posterolateral foot.

In contrast, weakness of plantar flexion is not typically from L5 or L4 (which more strongly involve dorsiflexion and knee/hip-related functions) and is not related to C7 (an upper-limb root). Therefore, the nerve root most associated with plantar flexion weakness is S1.