Dynamic Postural Stability in Copers and Individuals with Chronic Ankle Instability

Abstract

Ankle sprains are the most common orthopedic injury in the United States and result in chronic ankle instability (AI) for 40−75% of patients. Interestingly, 25−60% can maintain high level activities (copers) and are thought to have a coping mechanism that limits the impairments seen in AI patients. Understanding the differences between copers and AI patients may elucidate the mechanisms of AI. PURPOSE: To determine if dynamic postural stability varies among copers, AI patients, and healthy controls. METHODS: Dynamic postural stability scores were assessed in 72 subjects (24 in each group) during a single leg hop stabilization protocol. Subjects started 70cm from a force plate and jumped off both legs to touch an overhead marker placed at 50% of their maximum vertical jump height before landing on the force plate with a single leg. Force data were collected at 200 Hz and normalized to the subject's body weight. Directional and composite scores were calculated using modified dynamic postural stability index formulas. These indices are created from the standard deviation fluctuations around a zero point. The medial/lateral stability index and anterior/ posterior stability index assess the standard deviation fluctuations from zero along the frontal (X) and sagittal (Y) axes of the force plate. The vertical stability index assesses the fluctuation from the subject's body weight along the Z-axis of the force plate. RESULTS: Group and limb means were analyzed with a 3 x 2 MANOVA that revealed a significant group effect [F(8,272)=6.226, p<0.001]. Further analysis revealed that copers (0.122±0.002) and AI patients (0.119±0.002) performed worse than healthy controls (0.106±0.002). Composite scores also revealed a reduction in performance for copers (0.296±0.003) and AI patients (0.292±0.003) compared to controls (0.282±0.003). However, in the sagittal plane copers (0.047±0.001) were significantly worse than AI (0.042±0.001) and controls (0.042±0.001). CONCLUSIONS: The results indicate that frontal plane dynamic postural stability is altered after acute ankle injury and that the copers' decreased performance in the sagittal plane may represent the hypothesized coping mechanism. Supported in part by the College of Health and Human Performance Opportunity Fund.