Associations between movement and properties of motor neurons and muscles
Movement is accomplished and controlled by the coordinated activity of motor neurons and the muscles they activate. This dissertation comprises four studies that examined the association between measures of movement performance and the properties of motor neurons and muscles. The first study simulated the recordings of electrical signals from muscles. The purpose was to determine how a property of muscle that reduces the amount of electrical signal received by devices used to record muscle activity influences our ability to detect the onset of muscle activity. The results provided guidelines on how to optimize methods for detecting the onset of muscle activity. The second study examined how previously established properties of motor neurons and muscle generalize to other muscles and evaluated the importance of these properties in controlling movement. This study compared the properties of motor neurons that activate a hand muscle with those that activate a leg muscle. The results showed that although the properties of motor neurons and muscle differed for the two muscles, the control of movement depends on both the variability in motor neuron activity and the mechanical properties of muscle. The third study examined a mechanism that potentially contributes to the decline in movement performance experienced by older adults. The study compared the distribution of activation forces for motor neurons in a hand muscle of young and old adults. The results indicated a similar distribution of motor neuron activation forces for young and old adults, despite older adults likely having fewer motor neurons. The fourth study investigated the association between a measure of movement performance and the length and orientation of muscle fibers as young and old adults lifted and lowered a light load at different speeds with the foot. The results showed that movement performance was similar for young and old adults, but was not associated with the length and orientation of muscle fibers. Movement performance was also similar when lifting and lowering the load, despite differences in muscle activity and muscle fiber length and orientation. Some associations were observed between movement performance and the properties of motor neurons and muscle.