The S1 segment of the spine includes the first sacral vertebra (S1) and the S1 spinal nerve root. This area is located at the base of the spine, connecting the lumbar region and the pelvis. The S1 level supports the body’s weight and hosts a nerve root that controls major functions in the lower limb. The health of this segment directly influences movement, sensation, and strength in the legs and feet.
The S1 Vertebra and the Sacrum
The S1 vertebra is the uppermost component of the sacrum, a large, shield-shaped bone situated at the posterior base of the spine. The sacrum is formed by the complete fusion of five individual sacral vertebrae (S1 through S5), a process that typically finishes by the mid-twenties.
The S1 segment connects with the fifth lumbar vertebra (L5) above it at the lumbosacral junction. Laterally, the S1 vertebra features large, wing-like projections called the alae, which articulate with the hip bones to form the sacroiliac joints.
The S1 nerve root exits the spinal canal through openings known as the sacral foramina. This exit point is important for understanding problems that can arise at this level.
Functional Role of the S1 Nerve Root
The S1 nerve root is a mixed nerve, carrying both motor (movement) and sensory (feeling) information to and from the lower extremities. As a major contributor to the sciatic nerve, S1 controls several powerful muscle groups in the leg. Its primary motor function is ankle plantarflexion, the action of pointing the toes and foot downward. The nerve also contributes to the strength of the hamstring muscles and the gluteus maximus, a major hip extensor.
In terms of sensation, the S1 nerve root supplies a specific area of skin known as the S1 dermatome. This sensory pathway covers the back of the thigh and calf, wraps around the outer side of the foot, and extends into the outer toes. A change in sensation, such as numbness or tingling in this precise pattern, points directly to a problem at the S1 level.
The most common way to assess the health of the S1 nerve root is by testing the Achilles tendon reflex, often called the ankle jerk reflex. A healthcare provider taps the Achilles tendon just above the heel, which should cause the foot to reflexively point downward. A diminished or absent reflex indicates reduced S1 nerve function.
Recognizing Problems at the S1 Level
Irritation or compression of the S1 nerve root is termed S1 radiculopathy, which commonly results in sciatica. This compression is often caused by a herniated disc at the L5-S1 level, where the disc material presses on the nerve root. Other common causes include spinal stenosis (narrowing of the spinal canal) or a vertebral slip called spondylolisthesis.
The primary symptom of S1 radiculopathy is pain that radiates from the low back or buttocks down the back of the leg. This pain can be described as sharp, shooting, or burning, and it follows the path of the affected nerve. The pain is often worsened by movements such as coughing, sneezing, or prolonged sitting.
Specific signs that identify S1 involvement include numbness or a pins-and-needles sensation in the S1 dermatome, particularly along the outer edge and sole of the foot. Weakness is also a recognizable symptom, manifesting as difficulty pushing off the ground when walking or an inability to stand on the toes. This weakness in plantarflexion can sometimes cause a subtle change in a person’s gait.