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A herniated disk at the L5-S1 level primarily impacts the function of the S1 nerve root, which is crucial for the Achilles reflex, also known as the ankle jerk reflex. This reflex is tested by tapping the Achilles tendon located at the back of the ankle; a normal response involves plantar flexion of the foot.

The S1 nerve root contributes significantly to the motor and sensory innervation of the lower leg and foot. When a herniated disk occurs at this level, it can compress the S1 nerve root, leading to a diminished or absent Achilles reflex. This connection underscores the importance of the S1 nerve in mediating the reflex action of the calf muscles in response to Achilles tendon stimulation.

While the L5-S1 herniation may exert some influence on other reflexes, such as the plantar reflex, its most direct impact is on the Achilles reflex due to the specific anatomical and neurological pathways involved. Understanding the relationship between spinal nerve roots and their corresponding reflexes is crucial in diagnosing and managing conditions such as herniated disks in a clinical setting.