Biomechanical characteristics in the recovery phase after low back fatigue in passive and active tissues

Abstract

Previous studies have shown how abnormal low back conditions can increase the risk of low back pain (LBP) by using the flexion-relaxation phenomenon (FRP) to provide insights into the function of the lumbar spine around the full flexion. However, the characteristics of each abnormal condition during the recovery phase remain poorly understood. To expand our knowledge in this area, twelve subjects performed the following three protocols on three different days: (1) passive tissue elongation (PTE), (2) muscle fatigue (MF), and (3) its combination (PTE & MF). The lumbar angle at which FRP of the lumbar muscles is initiated (electromyography (EMG)-off point) and the full lumbar flexion angle were captured before and after the protocols and during the subsequent 40-min recovery period. Results showed no recovery in EMG-off point after PTE until 40 min of rest, but a rapid recovery in 5 min of rest after MF. The combined protocol did not exhibit any boosting effect by an interaction between muscle fatigue and stress-relaxation in passive tissues, but rather the trend closely mirrored the PTE recovery. However, the combined protocol demonstrated gradual recovery after 40-min resting time in both kinematic measures, although the EMG-off points and the full lumbar flexion angle were not fully recovered. These results suggest that the slow recovery of the viscoelastic tissues caused by the prolonged stooping of PTE and PTE&MF may lead to longer spinal instability than low back muscle fatigue. Relevance to industryFor workers performing various manual material handling tasks for up to 8 h daily, knowledge about the recovery phase from any abnormality can help develop an appropriate work-rest and job rotation schedule.