Which nerve injury is characterized by weakness of all muscles below the knee?

Which nerve injury is characterized by weakness of all muscles below the knee?

• A. Sciatic nerve injury
• B. Common peroneal nerve injury
• C. Femoral nerve injury
• D. Obturator nerve injury

Answer: (A)

The key idea is how nerve supply to the leg muscles is organized along the nerve branches. The sciatic nerve travels down the back of the thigh and, just above the knee, splits into the tibial nerve and the common peroneal (fibular) nerve. These two branches together provide motor innervation to virtually all muscles below the knee. If the sciatic nerve is injured proximal to that split, both tibial and common peroneal pathways are affected, so every muscle below the knee loses its nerve supply and weakness occurs across the entire region. In contrast, injuries to the individual branches produce more selective deficits: the common peroneal nerve mainly weakens the muscles that dorsiflex the foot and evert the ankle (and sensory changes on the dorsum of the foot), while sparing many plantarflexors and other functions still supplied by the tibial nerve. The femoral nerve affects muscles in the anterior thigh, and the obturator nerve affects the medial thigh adductors, not the entire group of muscles below the knee. Thus, weakness of all muscles below the knee best fits a proximal sciatic nerve injury before it branches.

The key idea is how nerve supply to the leg muscles is organized along the nerve branches. The sciatic nerve travels down the back of the thigh and, just above the knee, splits into the tibial nerve and the common peroneal (fibular) nerve. These two branches together provide motor innervation to virtually all muscles below the knee. If the sciatic nerve is injured proximal to that split, both tibial and common peroneal pathways are affected, so every muscle below the knee loses its nerve supply and weakness occurs across the entire region.

In contrast, injuries to the individual branches produce more selective deficits: the common peroneal nerve mainly weakens the muscles that dorsiflex the foot and evert the ankle (and sensory changes on the dorsum of the foot), while sparing many plantarflexors and other functions still supplied by the tibial nerve. The femoral nerve affects muscles in the anterior thigh, and the obturator nerve affects the medial thigh adductors, not the entire group of muscles below the knee. Thus, weakness of all muscles below the knee best fits a proximal sciatic nerve injury before it branches.