Repetitive stress injuries are common in the workplace where workers perform repetitive tasks continuously throughout the day. Muscle fatigue may lead to injury either directly through muscle damage or indirectly through changes in coordination, development of muscle imbalances, kinematic and muscle activation variability, and/or movement instability. To better understand the role of muscle fatigue in changes in movement parameters, we studied how muscle fatigue and muscle imbalances affected the control of movement timing, variability, and stability during a repetitive upper extremity sawing task. Since muscle fatigue leads to delayed muscle and cognitive response times, we might expect the ability to maintain movement timing would decline with muscle fatigue. We compared timing errors pre- and post-fatigue as subjects performed this repetitive sawing task synchronized with a metronome using standard techniques and a goalequivalent manifold (GEM) approach. No differences in basic performance parameters were found. Significant decreases in the temporal correlations of the timing errors and velocities indicated that subjects made more frequent corrections to their movements post-fatigue. Muscle fatigue may lead to movement instability through a variety of mechanisms including delayed muscle response times and muscle imbalances. To measure movement stability, we must first define a state space that describes the movement. We compared a variety of different state space definitions and found that state spaces composed of angles and velocities with little redundant information provide the most consistent results. We then studied the affect of fatigue on the shoulder flexor muscles and general fatigue of the arm on movement stability. Subjects were able to maintain stability in spite of muscle fatigue, shoulder strength imbalance and decreased muscle cocontraction. Little is known about the time course for adaptations in response to fatigue. We studied the effect of muscle fatigue on movement coordination, kinematic variability and movement stability while subjects performed the same sawing task at two work heights. Increasing the height of the task caused subjects to make more adjustments to their movement patterns in response to muscle fatigue. Subjects also exhibited some increases in kinematic variability at the shoulder but no changes in movement stability. These findings suggest that people alter their kinematic patterns in response to fatigue possibly to maintain stability at the expense of increased variability. / text

The primary goal of human locomotion is to translate the body from point A to point B, but humans must have the variability and stability to adapt and recover from constraints they may encounter. The overarching aim of this dissertation was to investigate how constraints arising from external factors (i.e., military load carriage and speed) and internal factors (i.e., hip pain) affect kinematic variability and local dynamic stability of gait. In study 1, I focus on using traditional biomechanical measures to investigate if females and males use different gait mechanics when carrying military-relevant loads, as females and males are known to use different mechanics when walking with no load. In this study, I found that females and males do use different gait mechanics when walking with military-relevant loads. Females make kinematic adaptations at the ankle and knee while males make kinematic adaptations at the hip. The differences in adaptations between females and males may be related to females’ greater risk of injury when carrying load. In study 2, I used the same cohort to investigate how military-relevant loads affect the kinematic variability and local dynamic stability of gait. I found that kinematic variability and local dynamic stability were similarly affected by load. Participants had greater kinematic variability and decreased local dynamic stability when carrying loads, which may indicate an increased risk of falling while carrying load. I also found that local dynamic stability increased with increased walking speed at all loads in the mediolateral and anteroposterior directions. However, decreased stability was detected in the vertical direction, which may require increased energy expenditure. The results of this study indicate that walking faster with increased loads may be more stable, but less energy efficient. In study 3, I investigated the how kinematic variability and local dynamic stability were affected in individuals with hip pain and a history of developmental dysplasia. I found that kinematic variability and local dynamic stability were not similarly affected in these individuals. I found that kinematic variability was greater in individuals with hip pain compared to healthy controls, but there was no difference in local dynamic stability between groups. The overall finding of this dissertation is that the relationship between kinematic variability and local dynamic stability may be dependent on the factor investigated. / 2020-07-06T00:00:00Z