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 maximum force that can be produced by muscle is reduced. This finding compromises 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 biomechanical model of spinal loading was developed and validated for use during fatiguing repetitive trunk extension exertions. A time-varying maximum muscle stress was included to model the effect of a change in the maximum force-producing capacity of the erector spinae muscle. Sixteen men performed submaximal 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 varied during four testing sessions. Using trunk muscle electromyography and the measured torque as input, the model predicted significant linear reductions in the maximum muscle stress in 78% of the endurance tests, which resulted 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 erector spinae force would have been predicted to increase in 73% of the tests. The magnitude of the change in predicted erector spinae maximum muscle stress and force depended on the exertion level and repetition rate. This model will allow researchers to assess the effects of changes in recruitment patterns of trunk muscles during dynamic trunk extension on the estimated spinal loading of the lumbar spine.