Sex Comparisons of External, Anatomical and Methodological Factors on Motor Unit Firing Behavior
Abstract
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.