Eccentric muscle strength in functional ankle instability.

PURPOSE: To investigate muscle strength in subjects with unilateral functional ankle instability, with the specific aim of determining whether eccentric strength deficits exist. METHODS: Isokinetic invertor and evertor torques were assessed eccentrically and concentrically at speeds of 60 and 120 degrees.s(-1) in 16 subjects with unilateral ankle instability. Absolute peak torque values were compared between the injured and noninjured limbs to determine whether strength deficits existed. RESULTS: No eccentric or concentric evertor strength deficit was found in the injured limb. Interestingly, for the injured limb, a significant interaction effect was found whereby eccentric inversion strength was reduced. CONCLUSION: Functional ankle instability is not associated with deficits in evertor strength. Deficits in eccentric invertor strength were found, and this may contribute to the symptoms of functional ankle instability. Weak invertors may contribute to functional ankle instability because of a reduced ability to assist in controlling lateral displacement of the shank over the weight-bearing foot. Excess lateral postural sway outside of the base of support results in the foot's medial border rising from the floor once the limits of closed chain eversion is reached, which may result in the foot being forced into rapid inversion.