Impaired trunk and ankle stability in subjects with functional ankle instability

To examine differences between subjects with and without functional ankle instability (FAI) for measures of trunk and ankle stability. Twelve healthy individuals and 12 individuals with FAI participated. Subjects were assessed for self-rated disability, time to stabilization (TTS), and muscle reflex responses to sudden trunk perturbation. TTS results were calculated using an unbounded third-order polynomial. Trunk reflexes to a sudden unloading task were tested during flexion and extension movements. Loads were 65 N for males and 40 N for females. ANOVA procedures were used to compare TTS times, latency times, and trunk displacement data between groups. Regression analyses were used to determine the relationship between TTS and trunk latency times. Subjects with FAI had worse perceptions of their ankle disability but had the same vertical jump height. TTS times were delayed in the FAI group (6.0 T 2.8 vs 2.9 T 1.0 s; F(1,22) = 12.7, P = 0.002). Trunk muscle onsets were delayed in FAI subjects in both flexion (F(1,22) = 7.6, P = 0.01) and extension (F(1,22) = 4.5, P = 0.04). Regression and analysis identified x-axis TTS times as significantly associated with extension latency times (r2 = 0.19, P = 0.043). This study has provided evidence for proximal nervous system adaptations associated with FAI. Delayed trunk reflexes have been shown to predispose individuals to developing low back pain. A cause or effect relationship between trunk and ankle instability has not been established here.