Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Doctor of Philosophy




Christie, Anita D.


Motor unit (MU) recruitment and firing rate (MUFR) modulation are the primary factors governing force control. Differences in the anatomical distributions of α-motoneurons, muscle properties, and perception of fatigue between males and females may cause sexually dimorphic MU firing behaviors. The purpose of this dissertation was to further examine external, anatomical, and methodological factors that may influence observations of sex-related differences in neuromuscular function. In chapter 2, sex differences in how mental fatigue may influence MU firing behaviors were examined. Mental fatigue did not significantly alter MU firing behavior, maximal force, or force tracing ability in either males or females. This study utilized various isometric contractions and found that females, but not males improved in their performance of tracing a novel, variable force tracing task. In chapter 3 the relationship between muscle pennation angle and MUFR was examined in the tibialis anterior of males and females. There were no significant sex-related differences in MUFR or pennation angle. Higher firing rates were associated with higher pennation angles only in males. Chapter 4 was an investigation of how the number of motor units in a recording impacts the calculation of mean MUFR in males and females. Mean MUFR of the entire active MU pool was estimated at submaximal and maximal contraction intensities through bootstrapping. The results from this study revealed that to achieve at least 50% probability of accurately reflecting the mean MUFR of the active pool, a minimum number of 4-7 MUs should be included in the calculation, depending on the contraction intensity in males and females. Overall, these data contribute to our understanding of sex-related differences in neuromuscular physiology and suggest that MU firing behavior is not significantly affected by mental fatigue or number of motor units recorded in males and females. However, pennation angle may influence MUFR differently in males and females.

Summary for Lay Audience

Muscular contraction is dependent on signals sent from the nervous system. These signals come from regions of the spinal cord and are directly influenced by signals sent from the brain. The spinal cord control of muscle has unique patterns determined, in part, by the qualities of the muscle it is connected to. Differences in how males and females respond to mentally fatiguing tasks may result in an alteration of signals sent to the spinal cord from the brain, and the characteristics of their muscles may influence the signals sent to the muscle from the spinal cord. Sex-related differences in the perception of fatigue and muscle characteristics suggest that these factors may result in different changes to the nervous system. Furthermore, limitations in the technology of recording spinal control patterns to muscle generally result in an inability to record the patterns in the entire muscle. This dissertation had three objectives. The first was to determine if there were sex differences in nervous control patterns after a mentally fatiguing task. The second was to determine if there was relationship between the nervous control patterns and the muscle fiber arrangement in males or females. The third objective was to use statistical methods to estimate how a common method of describing spinal control of muscle compares to an estimated mean used to represent the activity of the entire muscle. Nervous control patterns were unaffected by the perception of fatigue, and significantly related to muscle properties only in males. There were also no sex-related differences when it comes to estimating the activity of the entire muscle.