People frequently perform daily activities that involve multitasking. For example, we maintain posture while holding a cup of coffee or a cell phone. It is not uncommon to experience a perturbation during such a task performance. In this dissertation, we study control of vertical posture in the presence of postural instability and holding an object in one hand. Successful completion of these tasks depends on efficient interaction of two components of the task: maintenance of vertical posture (postural component) and stabilization of the object (focal component). While each component was studied extensively in the past, investigating the interaction between the postural and focal components received much lesser attention in the prior literature. As such, a series of studies were conducted in order to examine the interplay between the postural and focal components.
In the first study (chapter 3), we investigated the effect of postural asymmetry induced by performing a secondary motor task on control of vertical posture. Participants (N=9) were asked to stand on the force platform and hold an object in one hand that induced body asymmetry. In addition, the subjects stood with feet apart and feet together (narrow base of support) and were subjected to perturbationa applied to their shoulders. In this study and following studies the side of holding an object will be referred as a target side and the other side of body will be referred as a contralateral side. Reciprocal activation of muscles on the target side and co-contraction of muscles on the contralateral side were seen when standing in asymmetrical stance and being subjected to external perturbation. Decreased magnitudes of muscle activation were seen in the APA phase while standing with narrow base of support.
In the second study (chapter 4), we investigated motor control perspectives of coordinating maintenance of posture and application of grip force when holding an object and being perturbed. In this study, participants (N=10) were asked to stand on the force platform holding an instrumented cup while being perturbed at the shoulders. Gripping task demands were manipulated by positioning a slippery cap on top of the cup. Onsets of grip force were seen before the onsets of the center of pressure (COP) displacement and initiation of the movements of the cup during the anticipatory postural adjustments (APAs) phase of postural control, while the onsets of maximum grip force preceded the maximum COP displacement. When the task demands increased by holding an instrumented cup with the slippery cap, participants tended to generate grip force earlier and of a smaller magnitude. Moreover, the COP displacement in the APA phase when holding the cup with the slippery cap was smaller as compared to the holding the cup only.
In the third study (chapter 5), we studied APAs and CPAs people use to maintain balance while standing on a sliding board that was either unlocked (and as such unstable) or locked (and as such stable) and performing voluntary arm movements. Nine subjects were enrolled in the study. Larger EMG integrals were seen in the muscles of the lower extremity in both APA and CPA phases of postural control when standing on the unstable surface (unlocked sliding board). No significant difference was observed in the trunk muscles. Larger maximum COP displacement was seen when participants stood on the stable surface (locked sliding board). The results also demonstrated that the CNS modified activation of the lower extremity muscles rather than trunk muscles when standing on a free to move sliding board and performing bilateral arm flexion.
In the forth study (chapter 6), we studied the role of a secondary task (holding an object) in maintenance of vertical posture in the presence of postural instability. Participants (N=12) were asked to stand on a free to move sliding board while holding an instrumented cup (with and without a slippery cap) and being perturbed. Reciprocal activation of muscles of the shank on both target and contralateral sides and co-contraction of muscles on both sides of the trunk was seen in the APAs phase. Decreased magnitudes of muscle activation in the target side were seen when standing and holding a cup with the slippery cap. The results of the study support the suggestion that the CNS employs dual-process for controlling postural and motor components when balance is jeopardized
