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.