Ankle instability caused by prolonged peroneal reaction time.

The reaction of 15 functionally unstable ankles to sudden inversion was described by monitoring muscle activity, joint motion, and alternation of the body center of pressure. The results were compared with those of 15 stable controls. Stable and unstable subjects showed a similar reaction pattern to sudden inversion: first, a peripheral reflex action, namely, a contraction of the peronei counteracting the ankle inverting momentum, and, then, a centrally elicited pattern, namely, a flexion of the hip, knee, and ankle relieving the vertical pressure on the ankle and producing ankle eversion. Unstable subjects did not show a defect in their central processing of afferent input. In contrast, a prolonged reaction time (median 84 msec compared with 69 msec in stable subjects) suggested a partial deafferentation of the reflex stabilization of the ankle and substantiated the theory of a proprioceptive deficit being responsible for ankle instability.