Trunk muscle fatigue and its implications in EMG-assisted biomechanical modeling

• Muscular fatigue affects predicted tissue stress of EMG-assisted models.
• Gain value of EMG-assisted models needs to be modified as a function of fatigue.
• Change to gain factor is linearly related to the change in the EMG median frequency.

Muscle fatigue affects the underlying EMG-force relationship on which EMG-assisted biomechanical models rely. The aim of this study was to evaluate the impact of short duration muscle fatigue on the muscle gain value. Participants performed controlled, isometric trunk extension exertions at 10, 20, and 30 degrees of trunk flexion and controlled isokinetic trunk extension exertions at 5 and 15°/sec on five separate days. Fatigue of the lumbar extensors was generated by moderate-intensity, trunk extension exertions. Participants performed controlled test contractions at defined intervals throughout the fatiguing bout and the EMG activities of trunk muscles were collected. These EMG data were employed in a standard EMG-assisted model and fatigue-dependent trends in the gain factor were evaluated. The results of this study show a significant effect of fatigue on the gain factor. Further analysis revealed that by reducing the magnitude of the gain factor in proportion to the drop in median frequency of the EMG signal of the fatigued muscle, a more accurate estimate of muscle force can be calculated. Mean normalized error between predicted internal moments and the measured external moments improved from 17.5 to 9.6% error via the implementation of this modified gain factor.Relevance to industryBiomechanical models are used to quantify low back stress during occupational lifting tasks. Understanding how these models need to be modified under conditions of muscular fatigue will provide more accurate estimates of stress and can help reduce the incidence of low back injury.