The quadriceps muscles in the anterior thigh extend the knee and are innervated by the femoral nerve, which arises from the lumbar plexus. The L2, L3, and L4 nerve roots provide axons to the femoral nerve. The quadriceps muscles are primarily innervated by axons from L3 and L4 roots. The axons from the L2 root in the femoral nerve innervate the iliopsoas muscle, which flexes the leg at the hip. Knee extension weakness will typically present with difficulty rising from a seated position, climbing stairs, or with falls secondary to the knee buckling when walking. Careful examination of the anatomical pattern of the weakness helps identify the most likely location of the causative lesion. In some cases, clinical weakness is subtle and the pattern of weakness is more easily identified on needle electromyography (EMG) examination of these muscles.
Symmetric knee extension weakness is commonly seen in myopathies and can also be seen in neuromuscular junction disorders, particularly Lambert-Eaton myasthenic syndrome. However, unilateral knee weakness is primarily caused by neuropathic lesions. One exception to this is sporadic inclusion body myositis, which, unlike most myopathies, is frequently asymmetric. The pattern of muscle involvement in sporadic inclusion body myositis is also atypical for most myopathies, with a predilection for the deep finger flexor muscles of the anterior forearm in addition to the quadriceps. If finger flexor weakness is present in a patient with quadriceps weakness, consider inclusion body myositis.
In a patient with unilateral knee extension weakness, next assess for weakness with hip adduction, tested by asking the patient to squeeze the thighs together. The muscles that adduct the hip are innervated by the obturator nerve, which also arises from the lumbar plexus and receives axons from the L2, L3, and L4 nerve roots.
The combination of hip adduction weakness and knee extension weakness indicates either a L3/4 radiculopathy or lumbar plexopathy, with the former being much more common. A lumbar plexus lesion should be considered in a diabetic patient with weight loss and pain (diabetic amyotrophy). Patients with either lumbar plexopathy or L3/4 radiculopathy will typically also have numbness over the lateral thigh. Magnetic resonance imaging (MRI) of the lumbar spine without contrast should be the initial diagnostic test in patients with the combination of hip adduction weakness and knee extension weakness. MRI of the lumbar plexus and EMG can be considered if the lumbar spine MRI is nondiagnostic.
In the absence of clinical or EMG evidence of hip adduction weakness, unilateral knee extension weakness is likely due to femoral neuropathy. A lesion of the femoral nerve can be further localized based on whether the lesion is proximal or distal to the innervation of the iliopsoas muscle, which flexes the leg at the hip. The branch to the iliopsoas comes off above the inguinal ligament. Injury to the femoral nerve proximal to the inguinal ligament typically occurs in abdominal surgeries due to retraction. Injury to the femoral nerve at or distal to the inguinal ligament, which spares hip flexion, typically occurs secondary to a hematoma from catheterization of the femoral artery or vein or from prolonged flexion and external rotation of the leg as seen in pelvic surgery or labor and delivery.