Noise-enhanced postural stability in subjects with functional ankle instability

Objective: To examine the effects of stochastic resonance (SR) stimulation on the postural stability of subjects with functional ankle instability (FAI). Design: Experimental research design. Setting: Sports medicine research laboratory. Participants: 12 subjects with FAI who reported a history of recurrent ankle sprains and "giving way'' sensations at the ankle. Interventions: Subjects performed 20 s single-leg balance tests under SR stimulation at 0.05 mA and 0.01 mA and under control conditions. Testing order was randomised. Stimulators that delivered subsensory stimulation to ankle muscles and ligaments were worn. Subjects were blinded to the test conditions, as SR stimulation was subsensory and stimulators were turned off during the control condition. Main outcome measures: Anterior/posterior and medial/lateral centre-of-pressure velocities (COPVs) were combined to form a resultant vector (COPV-R). The COPV-R differences between the optimal SR stimulation and control conditions were analysed. Optimal SR stimulation was defined as the SR stimulation input intensity level (0.05 mA or 0.01 mA) that produced the greatest percentage improvement in postural stability compared with the control condition. Slower velocities indicated enhanced postural stability. Results: The optimal input intensity was 0.05 mA for nine subjects and 0.01 mA for the other three. The optimal SR stimulation significantly (p, 0.05) improved COPV-R compared with the control condition (6.60 (1.06) vs 7.20 (1.03) cm/s; mean (SD)). Conclusion: SR stimulation may enhance signal detection of sensorimotor signals associated with postural stability. This result has clinical relevance as improvements in postural instability associated with FAI may decrease ankle sprain injury.