Fatigue's effect on eversion force sense in individuals with and without functional ankle instability.

CONTEXT Force sense (FS), the proprioceptive ability to detect muscle-force generation, has been shown to be impaired in individuals with functional ankle instability (FAI). Fatigue can also impair FS in healthy individuals, but it is unknown how fatigue affects FS in individuals with FAI. OBJECTIVE To assess the effect of fatigue on ankle-eversion force-sense error in individuals with and without FAI. DESIGN Case control with repeated measures. SETTING Sports medicine research laboratory. PARTICIPANTS 32 individuals with FAI and 32 individuals with no ankle sprains or instability in their lifetime. FAI subjects had a history of ≥1 lateral ankle sprain and giving-way ≥1 episode per month. INTERVENTIONS Three eversion FS trials were captured per load (10% and 30% of maximal voluntary isometric contraction) using a load cell before and after a concentric eversion fatigue protocol. MAIN OUTCOME MEASURES Trial error was the difference between the target and reproduction forces. Constant error (CE), absolute error (AE), and variable error (VE) were calculated from 3 trial errors. A Group × Fatigue × Load repeated-measures ANOVA was performed for each error. RESULTS There were no significant 3-way interactions or 2-way interactions involving group (all P > .05). CE and AE had a significant 2-way interaction between load and fatigue (CE: F1,62 = 8.704, P = .004; AE: F1,62 = 4.024, P = .049), and VE had a significant main effect for fatigue (F1,62 = 5.130, P = .027), all of which indicated increased FS error with fatigue at 10% load. However, at 30% load only VE increased with fatigue. The FAI group had greater error as measured by AE (F1,62 = 4.571, P = .036) but not CE or VE (P > .05). CONCLUSIONS Greater AE indicates that FAI individuals are less accurate in their force production. Fatigue impaired force sense in all subjects equally. These deficits provide evidence of impaired proprioception with fatigue and in individuals with FAI.