Lumbar spinal creep as a function of individual flexibility

This study aimed to explore the effects of lumbar flexibility and hamstring flexibility on the creep deformation of viscoelastic lumbar tissues. Sixteen human subjects, classified into two groups (GROUP: high flexibility, low flexibility) for each test, were asked to perform four 30-minute protocols combining two trunk flexion postures (POSTURE: Near-full, Full) and two load cycle duration (CYCLE: Short-cycle, Long-cycle). Before and after the protocols, the subjects were asked to perform controlled trunk flexion-extension motions to capture the change in peak lumbar flexion angle and the change in lumbar flexion angle at which erector spinae muscles achieve flexion-relaxation. Two ANOVA models were employed, once for the 2×2×2 design with lumbar flexibility and once for the 2×2×2 design with hamstring flexibility. Analysis of lumbar flexibility and hamstring flexibility individually revealed significant interactions between GROUP and POSTURE on the lumbar spinal creep. The simple effects analysis, sliced by GROUP, showed that low-flexible subjects exhibited significantly greater lumbar spinal creep responses in the Full condition compared to the Near-full condition, while high-flexible subjects were unaffected by POSTURE. The analysis of lumbar flexibility also revealed a significant interaction between GROUP and CYCLE on the lumbar spinal creep. The simple effects analysis, sliced by GROUP, showed that high-flexible subjects exhibited significantly greater lumbar spinal creep responses in the Short-cycle condition than in the Long-cycle condition, while low-flexible subjects were unaffected by CYCLE. These results suggest individual flexibility can play an important role in passive lumbar tissue loading with different trunk flexion postures and load cycle durations.