Development and validation of a fatigue-modified, EMG-assisted biomechanical model of the lumbar region

EMG-assisted biomechanical modeling is a well-established modeling technique for estimating muscle forces in biomechanical models of the lumbar spine. Fatigue, however, creates a problem in that fatigue alters the relationship between the EMG signal generated by the muscle and the amount of force being generated. The aim of this study was to evaluate the impact of fatigue on EMG-related components in an EMG-assisted biomechanical model: the gain factor (i.e. maximum muscle stress value in N/cm), force-length modulation factor, and force-velocity modulation factor. This is a particularly relevant research topic in that fatigue is considered a potential risk factor for musculoskeletal disorders, and being able to quantify muscle forces and joint reaction forces in these fatigued conditions would be helpful to understand the underlying risk factors in these types of exertion. The present study can inform and guide efforts in determining safety criteria in task design to decrease incidences of musculoskeletal disorders. This study was conducted in two phases: the isometric extension phase (1) and the isokinetic extension phase (2). Each was designed to provide the data necessary to evaluate the hypothesis that either the length-force modulation factor (Phase 1) or forcevelocity modulation factor (Phase 2) need to be dependent on the level of fatigue experienced by the extensor muscle of the lumbar region. Four subjects participated in each of these phases, performing trunk extension exertions at a level of 50% of their maximum force generation capacity to generate muscular fatigue in the trunk extensor muscles. In the isometric phase subjects performed controlled, isometric trunk extension test contraction exertions at 10, 20, and 30 angles