Mechanical stability, muscle strength and proprioception in the functionally unstable ankle.

Functional instability of the ankle is common following inversion sprain. Factors suggested as causes of this disability include mechanical instability of the talocrural joint, peroneal muscle weakness and motor incoordination due to impaired proprioception. This study documented physical examination characteristics of functionally unstable ankles relevant to these theories. Each ankle of 45 subjects with unilateral functional instability was examined. Mechanical stability was assessed by standard clinical instability tests. Evertor and inventor muscle strength was measured using the Cybex II dynamometer. The Uni-axial Balance Evaluator (UBE) was used to assess dynamic control of the ankle and was considered capable of detecting unilaterally impaired proprioception. Mechanical instability was frequently absent in the functionally unstable ankles tested. Evertor muscle strength was similar in the normal and functionally unstable ankles. UBE results were consistent with the theory of impaired proprioception contributing to functional instability, but the need for further research is emphasised.