Creep and Fatigue Development in the Low Back in Static Flexion

Study Design. In vivo measurements of low back flexion posture and muscle activity before, during, and after static flexion. Objective. To identify the occurrence of creep and muscle fatigue development in the low back during static upper body deep flexion that resembled an above ground work posture. Summary of Background Data. Static lumbar flexion has been related to the development of low back disorders, and its injury mechanism has been focused on the changes in passive spinal tissues. Potential influences of muscle fatigue of extensor muscles have not been explored. Methods. A total of 20 asymptomatic subjects performed submaximal isometric trunk extension exertions and an isokinetic trunk flexion before and immediately after 5-minute static flexion while the trunk sagittal flexion angle and the myoelectric activities (electromyography [EMG]) of back extensor muscles were recorded simultaneously. Changes in the flexion-relaxation onset angle, maximum flexion angle, muscle activity level, and the median power frequency of EMG associated with the static flexion were evaluated. Results. Flexion-relaxation onset angle in isokinetic flexion and EMG amplitude of isometric extension were significantly greater after static flexion, indicating creep of spinal tissues in static flexion. Median power frequency of lumbar erector spinae EMG during isometric extension was significantly lower after static flexion, suggesting the development of muscle fatigue. Consistent but low level of EMG was observed together with sporadic muscle spasms during the static flexion period. Conclusion. Fatigue of low back extensor muscles may occur in static flexion due to prolonged passive stretching of the muscles. Low back extensor muscles are required to generate more active forces in weight holding or lifting after static flexion to compensate for the reduced contribution of creep deformed passive tissues in maintaining spinal stability and the posture. The degraded force generating capacity of the fatigued muscles can be a significant risk factor for low back pain.