The effects of stance width and foot posture on lumbar muscle flexion-relaxation phenomenon

Characterizing the lumbar muscle flexion-relaxation phenomenon is a clinically relevant approach in understanding the neuromuscular alternations of low back pain patients. Previous studies have indicated that changes in stance posture could directly influence trunk kinematics and potentially change the lumbar tissue synergy. In this study, the effects of stance width and foot posture on the lumbar muscle relaxation responses during trunk flexion were investigated.

Thirteen volunteers performed trunk flexion using three different stance widths and 'toe-forward' or 'toe-out' foot postures (six conditions in total). Lumbar muscle electromyography was collected from the L3 and L4 level paraspinals; meanwhile three magnetic motion sensors were placed over the S1, T12, and C7 vertebrae to track lumbar and trunk kinematics. The lumbar angle at which muscle activity diminished to a near resting level was recorded. At the systemic level, the boundary where the internal moment started to shift from active to passive tissues was identified.

For the L3 paraspinals, the flexion relaxation lumbar angle reduced 1.3° with the increase of stance width. When changed from 'toe-forward' to 'toe-out' foot posture, the flexion relaxation lumbar angle reduced 1.4° and 1.1° for the L3 and L4 paraspinals respectively. However, the active and passive lumbar tissue load shifting boundary was not affected.

Findings of this study suggest that changes in stance width and foot posture altered the lumbar tissue load sharing mechanism. Therefore, in a clinical setting, it is critical to maintain consistent stance postures when examining the characteristics of lumbar tissue synergy.