A Multi-Faceted Approach to Understanding the Effects of Fatigue on Muscle and Kinematic Variability

Abstract

An in vivo human model of muscular and kinematic indeterminacy, the shoulder joint offers potential insight on how the central nervous system optimizes several competing biophysical variables. Muscle fatigue is a condition that impacts several such biophysical variables and can instigate load-sharing; centrally mediated muscle control changes utilized to manage several optimality variables like effort, metabolic cost, and joint load. However, considerable variability in fatigue-mediated shoulder kinematic and muscle activity changes has challenged the ability to make clear inferences on fatigue-mediated shoulder control changes. The goal of this dissertation was to quantify fatigue-mediated shoulder control changes from a multifaceted perspective, in hopes that comprehensive analyses and novel methodologies would further our understanding of the mechanisms that drive centrally mediated shoulder control. Of specific interest was the intent to identify factors which may explain some of the fatigue-mediated shoulder variability that remains unclear. Chapter 3 of this dissertation used coactivation ratios of the scapular stabilizers to describe muscle control changes following a shoulder fatigue task. Shoulder kinematic and coactivation responses to fatigue were variable, yet results indicated that 20-40% of individuals may increase their risk of subacromial impingement syndrome due to fatigue. Chapter 4 of this dissertation investigated how individuals adapt their fatigue-mediated muscular and kinematic responses when completing the same fatigue task from chapter 3 a second time. Participants demonstrated more aggregate kinematics on day 2, while consolidating a more mechanically efficient posture that may minimize serratus anterior fatigue exposure. Chapter 5 harnessed an optimal control biomechanical shoulder model to predict shoulder kinematics changes associated with isolated scapular stabilizer muscle weakness. This study identified a key role of serratus anterior for stabilizing arm elevation, and identified shoulder muscle synergies which may compensate for serratus anterior weakness. Chapter 6 sought to identify how fatigue-mediated changes in muscle elastic modulus may affect muscle control strategies. However, muscle stiffness appeared to be sensitive to the type of exercise/fatigue stimulus which was unforeseen and may be an important consideration for fatigue-related variability. This dissertation concludes by summarizing the integrated findings of chapters 3-6 and proposing new considerations for fatigue-mediated shoulder muscle control.