Repeated spinal flexion modulates the flexion-relaxation phenomenon

Objectives. To determine if repeated spinal flexion and loading modulate the deactivation of lumbar muscles near full flexion (flexion-relaxation). Design. Repeated measures experimental study of the effect of repetitive trunk flexion and added mass on the flexion-relaxation phenomenon. Background. Repeated flexion causes muscular fatigue, creep of passive tissues and diminished protective reflexes. However, flexion-relaxation has not been studied in repeated trunk flexion, and could be related to the increased risk of low-back disorders. Methods. Thirty healthy young subjects performed 100 trunk flexion movements between standing and full flexion. Erector spinae electromyography and lumbar spine flexion were measured during cycles 1-10 (no load), 11-20 (performed holding a mass in the hands), 81-90 (mass in the hands) and 91-100 (no load). The spinal flexion angle at myoelectric silence and full flexion were extracted from each movement cycle. Results. Twenty-three of the 30 subjects showed flexion-relaxation throughout the repeated trunk flexion. The flexion-relaxation and maximum flexion angles increased at the end of the experiment; the flexion-relaxation angle relative to the maximum flexion angle also increased. This effect depended on the load condition; the flexion-relaxation and maximum flexion angles showed a greater increase in the unloaded than loaded condition. Conclusions. The flexion-relaxation phenomenon was changed due to repeated trunk flexion. The increases in flexion-relaxation angle likely involve changes to the neuromuscular control system.