The lumbar spine, or lower back, consists of five large vertebral segments (L1 through L5). These segments are the largest in the spinal column because they bear the majority of the body’s weight. The L4 and L5 segments are located at the base of the spine, connecting the upper body to the pelvis.
A pair of spinal nerves, the L4 and L5 nerve roots, exit the spinal column near these vertebrae. Each nerve root is a bundle of motor and sensory fibers that control specific muscles and provide sensation to distinct areas of the lower limbs. High mechanical stress at this junction makes the L4 and L5 levels frequent sites of structural changes that can compress these nerves, leading to pain and functional deficits in the leg and foot.
Functions Controlled by the L4 Nerve Root
The L4 nerve root primarily controls motor function for knee extension and ankle dorsiflexion (lifting the front of the foot). It innervates the quadriceps femoris muscle group, which straightens the knee, and contributes to the function of the tibialis anterior muscle.
Physicians test the L4 nerve by checking the patellar tendon reflex (knee jerk). A diminished or absent response suggests an issue at this level. The L4 dermatome supplies sensation to the front of the thigh and the inner (medial) side of the lower leg, extending to the inner ankle.
Functions Controlled by the L5 Nerve Root
The L5 nerve root controls muscles responsible for lifting the foot and toes. Its primary motor function is innervating the extensor hallucis longus muscle, which extends the big toe upward. L5 also works with L4 to innervate the tibialis anterior muscle, driving ankle dorsiflexion.
L5 also innervates the hip abductor muscles (gluteus medius and gluteus minimus). The sensory fibers provide sensation to the outer side of the lower leg, the top (dorsum) of the foot, and the space between the first and second toes. Since no single deep tendon reflex is attributed to L5, motor and sensory testing is crucial for diagnosis.
Common Causes of L4 and L5 Nerve Root Compression
Radiculopathy (nerve root compression) is primarily caused by structural changes. Due to biomechanical stress, the L4-L5 and L5-S1 intervertebral discs are the most common sites for disc herniation. When the inner disc material protrudes, it typically presses on the nerve root traveling down to the next level.
A herniation at L4-L5 usually compresses the traversing L5 nerve root. Less commonly, a far lateral herniation at L4-L5 can compress the L4 nerve root as it exits the foramen. Another common cause is spinal stenosis, which is the narrowing of the spinal canal or neural foramina. This narrowing often results from age-related overgrowth of facet joints or thickening of spinal ligaments.
Spondylolisthesis, where one vertebra slips forward over the one below it, also frequently affects the L4-L5 segment. This slippage misaligns bony structures, narrowing the openings where nerve roots exit. Degenerative spondylolisthesis at L4-L5 is common in older adults and reduces the available space for the L4 and L5 roots.
Translating Nerve Compression into Observable Symptoms
L4 nerve root compression causes primary motor weakness, manifesting as difficulty extending the knee, leading to instability and trouble climbing stairs. A diminished patellar reflex is a physical sign of L4 involvement. Patients often report numbness or a pins-and-needles sensation (sciatica) running down the inner side of the leg, following the L4 pathway.
L5 compression presents with weakness, particularly the inability to lift the front of the foot and the big toe. This dorsiflexion weakness causes “foot drop,” where the person must lift the knee high to avoid tripping. Radiating pain and numbness follow the L5 dermatome, traveling down the side of the leg and across the top of the foot to the toes.