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 estimated 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 patterns resulted in a greater estimated active loading of the spine and, in turn, an increased risk of injury. SUMMARY OF BACKGROUND DATA: Epidemiologic studies highlight the increased risk of low back injury during repetitive lifting, implicating fatigue of muscles and/or passive tissues as causes of such injury. Increased trunk muscle activity or altered recruitment patterns resulting from fatigue in the primary trunk extensor muscles may indicate an increase in the active loading 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 rates, while their net muscular torque output and trunk muscular activity were measured. During each exertion, trunk torque, position, and velocity were controlled, so that any change in muscle activity could be attributed to fatigue. 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 repetition rate). RESULTS: Significant elevations were estimated for the latissimus dorsi and external oblique muscle forces in more than 70% of the endurance tests, whereas significant reductions in the erector spinae muscle force were predicted in 75% of the trials. The magnitude of the range of change of the erector spinae and latissimus dorsi muscle forces was dependent on the load level and repetition rate. The reduction in erector spinae forces offset the augmented force in the other muscles, because the net changes in compression and lateral shear forces on the spine were not significant, and the anteroposterior 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 muscular recruitment patterns. Therefore, future studies should focus on injury mechanisms that may occur as a result of a change in the viscoelastic passive tissue responses, muscular insufficiency, or a decline in neuromuscular control and coordination.