Doctor of Philosophy
Musculoskeletal Health Research
Garland, S. Jayne
Maintenance of upright standing posture is fundamental to human mobility and requires muscular control of the body’s centre of mass over the base of support. Force production, necessary for standing, is achieved by neural activation of populations of motor units (MUs). The possibility of preferentially activating different MUs within or across synergists has been debated in the motor control literature. Accordingly, the overall objective of this dissertation was to examine the neural control of standing balance in response to different directional force challenges. High Density Surface-Electromyography (HD-sEMG) was used to observe the location of muscle activity, and signal decomposition techniques were utilized to understand single MU recruitment and firing characteristics. Study one aimed to determine if there was regionally-specific muscle activity in response to directional perturbations. Participants in unilateral stance were exposed to externally induced perturbations in different directions. Findings supported that there is task-dependent regionally specific muscle activation, with higher activity opposing the direction of the perturbations. Study two aimed to evaluate the recruitment location of single MUs during a multidirectional leaning task. Participants leaned in five directions, and the HD-sEMG signals were decomposed into trains of single MU action potentials. Further, using a MU tracking analysis, subpopulations of MUs were identified as being common or unique across leaning directions. It was found that MUs common in the leaning directions were recruited in similar locations, whereas MUs that were uniquely recruited in leaning directions were found in different muscle locations. Study three investigated the firing behaviours between the two MU subpopulations. There were different firing behaviours between the two MU subpopulations, supporting that differential firing between distinct subpopulations of MUs was utilized to meet force requirements. Finally, study four investigated whether this regional recruitment and differential firing behaviour strategy was utilized in older adults. Notable changes in the older adult population included attenuated modulation of firing characteristics, increased intermittent firings, and recruitment of MUs in the proximo-distal direction of the muscle only. Taken together, these studies suggest that the central nervous system has the ability to control subpopulations of MUs differently, and this ability is affected as humans age.
Summary for Lay Audience
Maintaining an upright standing posture is essential for human mobility, and it requires muscular control of the body. This control is achieved through activation of different groups of motor units (MUs); a MU is defined as the motor nerve and all the muscle fibres it innervates. Scientists have been debating if the brain can control specific MUs for different tasks for over five decades. This dissertation contains four studies that investigate how the brain controls standing when people are pulled or lean in different directions. To accomplish this, an innovative tool called high-density surface electromyography (HD-sEMG) recorded muscle and MU activity during different standing tasks.
The first study found that when people are pulled in different directions, different muscle locations get more active, depending on which way they are pulled. The second study looked at where the MUs were located when people leaned in different directions. It was found that a group of MUs in the same muscle location were activated in multiple leaning directions. A different group of MUs were in different muscle locations in only specific leaning directions. The third study looked at how active the groups of MUs were, compared to each other. It was found that the different groups of MUs had specific activity that was dependent on the direction that the participant leaned in. Finally, the fourth study looked at whether older people used these groups of MUs differently than young people. It was found that older people have a decreased ability to control the groups of MUs separately.
In conclusion, this research shows that our brain can control different groups of MUs to help us balance when we are standing. However, as we get older, our ability to use these different groups of MUs changes which may have implications on the ability to balance correctly.
Cohen, Joshua W., "The Neural Control of Force Production in Response to Directional Force Challenges" (2023). Electronic Thesis and Dissertation Repository. 9313.
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