An epidemiological study into the incidence of ankle sprain in elite and non-elite athletes was carried out to collect data on the incidence of sports injuries focusing specifically on ankle sprains in elite and non-elite athletes. Furthermore, to develop and validate a questionnaire to be used to collect data on the residual symptoms associated with a history of ankle sprain and functional instability. Ankle sprains accounted for 16% of all injuries and symptoms of functional instability were reported by 95% of athletes that reported sustaining an ankle sprain during the study. The questionnaire was then used to recruit subjects (19 subjects with a history of unilateral ankle sprain and functional instability and 19 healthy controls) for the subsequent experiments. Anteroposterior and medio-lateral postural sway in single-limb stance was examined using a KistierTM force platform. Each subject underwent twelve postural sway tests: three on each leg with eyes open and eyes closed. With eyes closed the injured ankle had significantly greater medial (p=O.001) and lateral (p=O.007) postural sway than the uninjured ankle. With eyes open the injured and uninjured ankles had similar postural control. With eyes open the injured ankle had significantly greater anterior (p=O.021, p=O.Oll) and posterior (p=O.019, p=O.018) postural sway than the dominant and non-dominant ankles respectively. With eyes closed the injured ankles had significantly greater medial (p=O.008, p=O.008) and lateral (p=O.014, p=O.015) postural sway than the dominant and non-dominant ankles respectively. The reaction time of peroneus longus, peroneus brevis, tibialis anterior and extensor digitorum longus to a non-pathological lateral ankle sprain mechanism was examined using a purpose built tilt platform. The platform had two moveable plates so that either ankle could be tilted spontaneously into combined plantarflexion and inversion. Electromyography was performed on each muscle and subjects had each ankle tilted six times. A computer-based onset detection method was developed to provide an objective method for identifying the onset of electromyography and tilt platform activity and calculating muscular reaction times. The injured ankle peroneus longus, peroneus brevis and tibialis anterior reaction times were significantly slower.