Low Back Muscle Fatigue Measurements of Cyclic and Prolonged Stooped Work

Abstract

Fatigue experiments were performed on human subjects to study muscle fatigue measurements of the erector spinae muscles during cyclic and prolonged static stooped bending using electromyography (EMG). Nineteen subjects participated in the study. The median muscle firing frequency (fm) data of the erector spinae muscles were measured noninvasively and continuously during simulated cyclic and prolonged stooped work. In the prolonged condition, the subject conducted an isometric stooped work at 4:1 work to rest ratio; whereas in the cyclic condition, the subject conducted an isometric stooped work at 4:1 work-to-rest ratio; each followed by a recovery period. To quantify the rate of fatigue, a fatigue index was defined as the change in the fm with respect to muscle activation time. All measurements were normalized to each subject’s individual erector spinae maximum voluntary contraction using the Biering-Sørenson Muscle Endurance test for lower back endurance as control measurement. The results indicated that the use of EMG showed no significant differences in the muscle fatigue between cyclic and prolonged stooped conditions. However, when EMG results for both are compared to the control conditions, similar fatigue responses are shown. This may be an indication that both cyclic and prolonged stooped work conditions are equally detrimental to the spine, which calls for effective interventions to limit their effects on workers who commonly perform these types of tasks.