Electronic Thesis and Dissertation Repository

Thesis Format



Master of Science




Christie, Anita D.


This study examined force and motor unit firing rate variability in the first dorsal interosseous (FDI) and the tibialis anterior (TA), focusing on sex-related differences and physical activity. Isometric contractions in the FDI and TA muscles in 12 males and 12 females were examined using electromyography and custom-built dynamometers. Physical activity was monitored using tri-axial accelerometers over seven days. Findings revealed that females exhibited higher CV (coefficient of variation) of force in the TA than FDI, while males displayed higher force variability in the FDI than the TA. Across all contraction intensities, the FDI showed higher firing rate variability than the TA, with no significant difference of muscle in CV of force. Moderate to strong relationships between activity levels and force variability in both FDI and TA at 10, 20 and 50% maximal voluntary contractions were observed. This study prompts further investigation into the neuromuscular control of force in upper and lower limbs, as well as interactions with sex and physical activity.

Summary for Lay Audience

Whether engaging in daily tasks or robust exercises, the nervous system and muscles work simultaneously to coordinate movements. The center of this process consists of motor units, where upon receiving a signal, the motor neuron sends electrical signals to the muscle fibers it controls, resulting in muscle contraction. We tend to perceive upper and lower limb muscles as more different than alike. This can be attributed to their differences in anatomical and physiological characteristics that allow them to execute a range of functions in the body. However, research remains inconclusive regarding whether significant differences exist in the fundamental processes through which upper and lower limb muscles perform their functions. The first dorsal interosseous (FDI) and tibialis anterior (TA) muscles have each been widely examined, but there has been limited research into the differences between them, especially regarding the neural control of force production. Enhancing our understanding of neuromuscular control entails exploring key factors like force and how muscle fibers are activated. As such, this research aims to investigate the differences in force variability and neural control between the FDI and TA. In accounting for potential influences of sex and physical activity levels on motor output variability in these muscle groups, we found that both males and females demonstrated higher motor unit firing rate variability in the FDI than TA. We also found that females had higher variability of force in the TA than in the FDI and females had a higher variability of force in the FDI than the TA. Additionally, across certain contraction intensities, levels of light, moderate, and vigorous physical activity were associated with force variability in the FDI and the TA. This study provides a comprehensive analysis of the control of force and motor unit firing rates in both the FDI and the TA, as well as their interactions with sex and physical activity.