The Neural Control of Force Production in Response to Directional Force Challenges
Abstract
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.