A complete lesion of the femoral nerve as it leaves the lumbar plexus primarily affects which motion?

A complete lesion of the femoral nerve as it leaves the lumbar plexus primarily affects knee extension. The femoral nerve innervates several important muscles, including the quadriceps femoris, which is the primary extensor of the knee. Damage to this nerve would lead to significant weakness or paralysis of the quadriceps, thereby impeding the ability to extend the knee effectively.

Although other movements may be affected to a lesser degree due to the nerve's involvement in other muscle groups, the most notable and direct impact will be on knee extension. The femoral nerve also plays a role in hip flexion through innervation of the iliopsoas muscle, but the most pronounced and observable functional deficit following a complete lesion will be the inability to extend the knee. Understanding the specific functions of the femoral nerve and its innervated muscles is crucial in evaluating the impact of nerve lesions on motion.