Differential effects of muscle fatigue on dynamic spine stability: Implications for injury risk.

This study was designed to assess the utility of using a measure of dynamic spine stability in an unfatigued, rested state as a predictor of dynamic spine stability in a challenged, fatigued state. Participants completed three trials (Day 1: Rested, Fatigued; Day 2: Recovery) during which the dynamic stability of the spine was assessed over 30 repeated flexion/extension motions using maximum finite-cycle Lyapunov exponents. Multiple sets of dynamic trunk extensions were performed to fatigue the trunk extensor muscles. Across the sample population, an increase in dynamic spine stability when fatigued was observed, as well as a moderate correlation between the level of dynamic stability when rested and a stabilizing response when fatigued. Further analysis of the data on a person-by-person basis revealed three distinct responses in which participants either stabilized, destabilized or had no change in dynamic spine stability when fatigued. Therefore, the mean response of the sample population did not adequately represent the true, meaningful response of individuals within the population. These results illustrate the importance of considering individualized responses when examining dynamic stability measures, and provide preliminary evidence that suggests that individual injury risk cannot be completely captured by measures taken in an unchallenged, rested state.