Effects of hamstring flexibility and trunk flexion posture on creep deformation of viscoelastic lumbar tissues: An exploratory study.

This study explores theeffects of hamstring flexibility on the creep deformation of viscoelastic lumbar tissues and its interaction with trunk flexion angles and exposure-recovery schedules. Sixteen participants, divided into two groups (low-flexible, high-flexible) based on hamstring flexibility, performed four 30-minute protocols with alternating intervals of trunk flexion (exposure) and upright standing (recovery). The protocols included combinations of two trunk flexion postures (maximal, submaximal) and two exposure-recovery schedules (3:6 min schedule, 1:2 min schedule) tested on four separate days. Before and after the protocol, trunk flexion-extension motions were employed to capture the changes in lumbar flexion angles of flexion-relaxation (EMG-off) in lumbar paraspinals, denoting lumbar spinal creep. The results revealed no significant main effect of individual hamstring flexibility but showed a significant interaction between flexibility and trunk flexion posture on the EMG-off lumbar flexion angles. The low-flexible group exhibited significantly greater EMG-off angles in the maximal posture (Δ2.4 on average) compared to the submaximal posture (Δ0.7), while the high-flexible group showed no effect. The analysis of lumbopelvic posture during sustained trunk flexion postures supported that altered pelvic angles as a function of individual hamstring flexibility may affect passive tissue loading at, or near, maximal flexion postures. These results suggest that reduced hamstring flexibility can play a significant role in spinal tissue creep with different trunk flexion postures. This work is a secondary analysis of a dataset previously published by Kang and Mirka (2025a), designed to address a distinct research question related to hamstring flexibility.