Date of Award

Spring 1-1-2015

Document Type


Degree Name

Doctor of Philosophy (PhD)


Integrative Physiology

First Advisor

Roger M. Enoka

Second Advisor

Rodger Kram

Third Advisor

Robert S. Mazzeo

Fourth Advisor

François G. Meyer

Fifth Advisor

Kenneth P. Wright


In the context of human performance, the term fatigue describes a variety of phenomena that limit the ability of an individual to perform a given task. Fatigue is a symptom experienced by all humans; however, the magnitude of the effect depends on such factors as the specificity of the task being performed, age, handedness, and neurological health. This dissertation examined some of the mechanisms that contribute to both perceived fatigability and performance fatigability and how they are influenced by varying acute and chronic adaptations. The first study compared [18F]-FDG uptake by skeletal muscles and the amount of muscle activity as indicated by surface electromyography when young and old men performed two types of fatiguing contractions. The results indicated that young men were able to modulate muscle activation during the two tasks but old men were not. The second study compared changes in the modulation of sensory feedback in young, middle-aged, and old adults before and after practicing a task that involved the contraction of a pair of antagonistic muscles. The results highlighted the impact of fatigue on the ability to modulate afferent feedback and control muscle force across the three age groups and suggested that declines in nervous system function precede the decrease in motor function and these begin in middle age. The purpose of the third study was to compare activity of the same motor units in left-handed individuals during sustained contractions supporting two load types. This study showed that handedness does not influence the activity of single motor units during these types of fatiguing contractions. The fourth study investigated fatigue in individuals with multiple sclerosis. This study showed no differences in force declines following a set of fatiguing contractions when compared with control subjects, however, reported levels of fatigue were significantly greater for the multiple sclerosis group. This study also indicated that intellectual capacity to be protective against declines in neuromuscular function. The results of this dissertation underscore the concept that the physiological mechanisms limiting human performance during fatiguing contractions are influenced by factors such as the specificity of the task being performed, age, and neurological health.