Relationship between Hamstring Muscle Tension and Muscle Activation and Flexion-Relaxation Phenomenon During Trunk Flexion and Extension Movements

Objective: This study aimed to examine the relationship between hamstring muscle tension, muscle activation patterns, and flexion-relaxation during trunk flexion and extension. Design: Cross-sectional study. Methods: Twenty-five healthy adults were recruited and stratified into two groups based on hamstring tension; flexible (n = 13) and rigid (n = 12). Surface electromyography was used to quantify muscle activation in the trunk, hamstring, and gastrocnemius muscles during trunk flexion and extension. The primary outcome measure was the level of hamstring activation immediately after trunk flexion, which served as an indicator of flexion-relaxation. Results: Participants with elevated hamstring tension demonstrated significantly greater hamstring muscle activation immediately after trunk flexion (p < 0.05). Hamstring muscle activation and flexion-relaxation had a significant correlation, with significant differences in the flexion-relaxation ratio between both groups (p < 0.05). Conclusions: Hamstring tension was significantly correlated with muscle activation and flexion-relaxation during trunk flexion and extension. Hamstring tension may contribute to the development and persistence of lower back pain and offer deeper insights into the biomechanical mechanisms underlying back pain.