Influence of back muscle fatigue on dynamic lumbar spine stability and coordination variability of the thorax-pelvis during repetitive flexion-extension movements

Previous work has identified that individuals adopt different dynamic lumbar spine stability responses when experiencing back muscle fatigue, and that the neuromuscular system adjusts multi-joint coordination in response to fatigue. Therefore, this study was designed to determine whether distinct differences in coordination and coordination variability would be observed for those who stabilize, destabilize, or demonstrate no change in dynamic stability when their back muscles are fatigued. Thirty participants completed two repetitive trunk flexion-extension trials (Rested, Fatigued) during which lumbar flexion-extension dynamic stability, thoraxpelvis movement coordination, and coupling angle variability (CAV) were assessed. Dynamic stability was evaluated using maximum Lyapunov exponents (lambda max) with participants being allotted to stabilizer, destabilizer, or no change groups based on their stability response to fatigue. Each flexion-extension repetition was further segregated into two phases (flexion, extension) and vector coding analyses were implemented to determine thorax-pelvis coordination and CAV during each movement phase. Results demonstrated that when fatigued, -30% of individuals adopted more stable (lower lambda max) flexion-extension movements and greater CAV during the extension phase, -17% of individuals became less stable (higher lambda max) and exhibited decreased CAV during the extension phase, and the remaining -53% of individuals expressed no change in dynamic stability or CAV. Additionally, more in-phase coordination patterns were generally observed across all individuals when fatigued. Altogether, this study highlights the heterogeneous nature of lumbar spine movement behaviours within a healthy population in response to fatigue.