| dc.description.abstract | Recent ergonomic research suggests that individuals with low motor variability (repeaters) are at higher risk of developing work-related musculoskeletal disorders than individuals with high motor variability (replacers) when performing repetitive tasks. Importantly, the repeaters-replacers hypothesis is dependent on the fundamental condition that motor variability is genuinely an individual trait, which is currently unknown. Therefore, this dissertation aimed to examine several measures of whole-body kinematic variability under different task constraints during lifting, during fatigue development in lifting and in different occupational tasks to evaluate kinematic variability as an individual trait.
Healthy females and males were recruited from the student population for two experimental sessions to perform self-paced repetitive lifting, carrying and simulated sawing tasks. The lifting task was performed four times under different task constraints of foot movement (restricted by instruction versus no restriction) and load weight (low versus high). For these six tasks, the total number of repetitions of each task was limited to 105 repetitions to avoid inducing excessive fatigue. The unrestricted high load lifting task was repeated in a prolonged protocol until volitional fatigue or up to a maximum of 1 hour. Whole-body joint angles and crate trajectories were obtained using optoelectronic motion capture. Kinematic variability was quantified using three different measures, a linear measure of joint angle mean point-by-point standard deviation, nonlinear continuous relative phase (CRP) variability of joint angle couplings, and nonlinear task-relevant and task-irrelevant variability derived from joint angles and crate trajectories. In addition, rate of perceived exertion was assessed as an indicator of fatigue.
In repetitive lifting under different constraints, individual variability demonstrated strong consistency independent of variability measures. However, across individuals, variability increased in response to removing the foot movement restriction when assessed using linear and nonlinear measures while task-relevant and task-irrelevant variability did not show any differences. When individuals were ranked on variability, strong consistency across measures was also demonstrated although CRP measures appeared to capture a slightly different construct than the other measures. In different repetitive tasks of lifting, carrying and simulated sawing, only moderate consistency was found in linear individual variability. Across individuals, linear variability was affected by task type where the order from highest to lowest variability was carrying, lifting and sawing, respectively. When unrestricted high load lifting was compared to three phases during prolonged unrestricted high load lifting, individual variability demonstrated strong consistency independent of (non)linear measures. In addition, across individuals no changes in variability were observed with different fatigue states. Variability during unrestricted high load lifting was associated with some indicators of fatigue.
This work reveals strong evidence for kinematic variability as an individual trait across investigated task constraints, variability measures, and fatigue development in lifting; however, variability could be task specific. Based on the effects of foot movement and task type on kinematic variability, variability increased when more degrees of freedom were allowed. Also, during lifting kinematic variability showed different responses to task constraint depending on variability measure. However, kinematic variability was related to some fatigue measures. The findings of this dissertation provide insight into kinematic variability as an individual trait in repetitive occupational tasks and therefore contribute to an essential aspect of the repeaters-replacers hypothesis. If kinematic variability can be related to risk of work-related musculoskeletal disorders, risk of injury could be prevented or lowered by altering individuals’ variability through training or workplace interventions assuming it is possible to convert a repeater into a replacer. | en |
