Estimation of trunk muscle forces and spinal loads during fatiguing repetitive trunk exertions

Study Design. The effects of human trunk extensor muscle fatigue on the est imated trunk muscle forces and spinal loading were investigated during the performance of repetitive dynamic trunk extension. Objective. To evaluate if alterations in the trunk muscle recruitment patte rns resulted in a greater estimated active loading of the spine and, in tur n, an increased risk of injury. Summary of Background Data. Epidemiologic studies highlight the increased r isk of low back injury during repetitive lifting, implicating fatigue of mu scles and/or passive tissues as causes of such injury. Increased trunk musc le activity or altered recruitment patterns resulting from fatigue in the p rimary trunk extensor muscles may indicate an increase in the active loadin g of the spine, which could contribute to an increased risk of injury. Methods. Sixteen healthy study participants performed repetitive isokinetic trunk extension endurance tests at two load levels and two repetition rate , while their net muscular torque output and trunk muscular activity were m easured. During each exertion, trunk torque, position, and velocity were co ntrolled, so that any change in muscle activity could be attributed to fati gue. An electromyography-assisted model, adapted to accommodate the decline in maximum muscular tension generation resulting from fatigue, was used to estimate the 10 trunk muscle forces and spinal loading. Linear regression was used to quantify the rate of change in muscle force and spinal loading resulting from fatigue, while analysis of variance was used to determine if the rate of change was dependent on the task conditions (load and repetiti on rate). Results. Significant elevations were estimated for the latissimus dorsi and external oblique muscle forces in more than 70% of the endurance tests, wh ereas significant reductions in the erector spinae muscle force were predic ted in 75% of the trials. The magnitude of the range of change of the erect or spinae and latissimus dorsi muscle forces was dependent on the load leve l and repetition rate. The reduction in erector spinae forces offset the au gmented force in the other muscles, because the net changes in compression and lateral shear forces on the spine were not significant, and the anterop osterior shear was reduced. Conclusion. The results of the study do not suggest that an increase in the muscular loading of the spine occurs as a result of changing trunk muscula r recruitment patterns. Therefore, future studies should focus on injury me chanisms that may occur as a result of a change in the viscoelastic passive tissue responses, muscular insufficiency, or a decline in neuromuscular co ntrol and coordination.