Comprehensive Osteopathic Medical Licensing Examination (COMLEX USA) Practice

Which muscle is primarily tested for S1 motor nerve root strength?

1. Quadriceps

2. Gastrocnemius

3. Gluteus medius

4. Tibialis anterior

The correct answer is: Gastrocnemius

The gastrocnemius muscle is primarily tested for S1 motor nerve root strength because it is innervated by the tibial nerve, which arises from the S1 nerve root and also a portion of S2. This muscle plays a crucial role in plantarflexion of the foot, especially evident during activities like standing on tiptoes or walking on the balls of the feet. When assessing motor strength in the context of nerve root integrity, testing the strength of the gastrocnemius can provide insights into the function of the S1 nerve root. Specifically, weakness in this muscle may indicate a problem with the S1 nerve root, which can be assessed through physical maneuvers that activate this muscle's function. The other muscles listed are associated with different nerve roots: the quadriceps is primarily linked to the L3-L4 regions (innervated by the femoral nerve), the gluteus medius is primarily associated with the L4-L5-S1 nerve roots (innervated by the superior gluteal nerve), and the tibialis anterior is innervated by the L4-L5 nerve roots (innervated by the deep fibular nerve). Thus, they do not specifically test the S1 reflex pathway