Modeling the mechanisms and the biomechanical manifestations of muscle fatigue: the good, the bad, and the in-between

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Repetitive motions are often accomplished during daily life. However, repetitively performing motor tasks has been identified as both a useful process to learn and develop expertise, and a risk factor for the development of musculoskeletal (MSK) injuries and disorders. An important aspect of repetitive movements is the potential consequence of fatigue, a phenomenon that affects both performance and perception. As part of this ongoing program, we were among the firsts to describe the whole-body consequences of repetitive motion-induced fatigue, uncovering patterns indicative of whole-body control strategies that take advantage of the musculoskeletal system's redundancy. We have also shown that different features of the motor task become more or less variable from one movement to the next, as fatigue develops. Finally, recent statistics show that women display a higher rate of MSK injuries, and that old adults stay longer in the workforce. We have advanced the understanding of sex and aging effects on mechanisms of repetitive motion-induced fatigue. However, much remains to be discovered in order to make workplaces accessible for people of both sexes and all ages. This research program has the following specific objectives: 1) quantify sex- and age-specific effects of fatigue on within-muscle activity patterns using a high-definition multi-electrode matrix; 2) quantify sex- and age-specific effects on full movement waveforms as fatigue develops using modern analytical techniques; 3) quantify sex- and age-specific effects on useful and harmful motor variability as fatigue develops; 4) use the knowledge gained in objectives 1-3 to design protocol based on wearable sensors to identify workers at risk of work-related fatigue and injuries. Male and female young and old adults research participants will perform experimental protocols of a repetitive unilateral arm movement task until reaching fatigue. We will continuously record the kinematic and muscle activity characteristics across the body (and within subsections of the trapezius muscle) as subjects will perform the task until the end. In a second session, the same task will be performed with a different performance objective (i.e. timing precision vs spatial precision of movement), to help determine effects of fatigue and sex on the balance of the movement variability characteristics that are useful vs harmful towards the task. Finally, a system of wearable sensors will be designed to track movement patterns on the field (workplace). Variability will be assessed with respect to the work performance objectives in order to determine the impact of personal factors such as work experience. This program will incorporate modern data analytical approaches to address fundamental, hypothesis-based research questions to help understand the control mechanisms of repetitive tasks, and to help guide approaches for field-based ergonomic interventions using emerging wearable sensing technology.