An electromyography-assisted model to estimate trunk muscle forces during fatiguing repetitive trunk exertions

During submaximal shortening muscle contraction, fatigue characteristically results in an increase in measured surface electromyography, whereas the m aximum force that can be produced by muscle is reduced. This finding compro mises researchers' ability to estimate muscle stress in a joint system such as the spine, which is composed of more muscles than degrees of freedom of the joint. A three-dimensional, electromyography-assisted, dynamic biomech anical model of spinal loading was developed and validated for use during f atiguing repetitive trunk extension exertions. A time-varying maximum muscl e stress was included to model the effect of a change in the maximum force- producing capacity of the erector spinae muscle. Sixteen men performed subm aximal isokinetic trunk extension endurance tests at 15 degrees per second. The exertion level (35% and 70% of their maximum dynamic extension torque) and repetition rate (5 and 10 repetitions per minute) of the tests were va ried during four testing sessions. Using trunk muscle electromyography and the measured torque as input, the model predicted significant linear reduct ions in the maximum muscle stress in 78% of the endurance tests, which resu lted in an estimated decrease in erector spinae force in 75% of the tests. Conversely, if the maximum muscle stress was assumed to be constant, the er ector spinae force would have been predicted to increase in 73% of the test s. The magnitude of the change in predicted erector spinae maximum muscle s tress and force depended on the exertion level and repetition rate. This mo del will allow researchers to assess the effects of changes in recruitment patterns of trunk muscles during dynamic trunk extension on the estimated s pinal loading of the lumbar spine.