Once spinal shock wears off, the natural reflex which is present in everyone reappears. Spasticity is an exaggeration of the normal reflexes that occur when the body is stimulated in certain ways. In an abled bodied person, a stimulus to the skin is sensed, and a sensory signal is sent to the reflex arch where it travels to the brain via the spinal cord, the brain then assesses the stimulant, and if the stimulant is thought not to be dangerous, an inhibitory signal is set down the spinal cord, and cancels the reflex from moving the muscle.

In a person with a spinal cord injury this inhibitory signal is blocked by the structural damage in the cord, and the natural reflex is allowed to continue resulting in a contraction of the muscle.

Muscle spasms can occur in a person with a spinal cord injury any time the body is stimulated below the level of injury. This is usually noticeable when a muscle is stretched, or there is a painful stimulant below the level of injury. Because of the injury to the spinal cord, these sensations can trigger the reflex resulting in the muscle to contract or spasm.