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POSTURAL AND MANUAL CONTROL DURING CONSTRAINED TASKS
The concurrent control of both standing and manual tasks are sophisticated since redundant, mechanically linked degrees of freedom (DOF) must be coordinated by a control strategy in a manner that affords completion of both tasks (Berret, Chiovetto, Nori, & Pozzo, 2011). In previous studies, a flexible control strategy was typically adopted and presented as the best behavior in the young adults in a task with only a manual task challenge (Kim et al., 2012) or postural task demand (Reisman et al., 2002). For the first study, we argued the flexible control strategy is the byproduct of experimental design with minimal challenge. When both manual and postural tasks are challenging, the motor system may adopt a less flexible control strategy to coordinate joint angles. We aimed in the first study to show that a less flexible control strategy can adapt to the challenges of a postural manual task in young adults. Twelve healthy participants (25 ± 4.2 years) performed a fitting task that required a small block to be transported, fitted, and held in a small or large opening for five seconds while standing on a narrow or wide surface. In addition to the uncontrolled manifold (UCM) analysis (variability spanned in the UCM space (Vucm), orthogonal space (Vort), and coordination metric (DVz) for hand and CoM control, we determined the hand and CoM standard deviation (SD) on 20 error-free trials (no block contact with the opening and no tilting of the surface). We found higher CoM and hand SD as well as invariant CoM and hand Vucm imposed by the narrow surface, which resulted in a reduction of joint-angle variability (less flexible control strategy) while holding a block in the small or large opening. The smaller CoM and hand SD, and greater Vucm, suggested a more flexible control strategy was adopted when standing on a wide surface and attempting the action of fitting the block to the small opening. The strength of the control strategy remained high across these conditions (high DVz). We concluded that a flexible control strategy is not a ubiquitous movement strategy in young adults (at both levels of coordinating joint angles and the variability of end effectors). We argued that the postural constraint (i.e., standing on the narrow surface) is the driving factor in the control strategy throughout a postural manual task. The immobilization of joints and muscle co-contraction were discussed that facilitated the postural task priority. The consequence of postural constraint (i.e., falling) appeared to increase the notion of postural control and explained our findings. Thus, in the first study, we inferred the consequences associated with the tasks (falling and losing precision) might induce higher priority for one task. The direct examination of the task prioritization was investigated in the second study.
In the second study, we examined task prioritization in a postural manual task. This specific paradigm was chosen because both manual and postural tasks can have consequences if they are not performed properly. In previous studies, posture is often considered to have priority over the concurrent performance of other tasks (Bloem et al., 2002). However, both postural and manual tasks can have consequences if they are executed poorly. The consequences of not performing a task appropriately can influence how the nervous systems prioritizes the individual component tasks. Typically, if one task, such as posture, is prioritized, other concurrent tasks’ performance can decline (Shumway et al., 1997). Additionally, task prioritization may have influenced the adoption of the control strategy observed in our previous study. The emergence of a less flexible control strategy may be associated with postural prioritization while standing on a narrow surface since safety and balance was important during this condition. In contrast, the flexible control strategy may have signaled manual prioritization while standing on a wide support surface and fitting a block to a small opening. In the second study, the main objective was to investigate how changing postural and manual task constraints determines task prioritization. Participants performed a postural manual task while standing on a wide or narrow surface and fitting a block to a small or large opening. We examined whether the postural or manual task was prioritized by calculating a dual-task cost (DTC) for the center of pressure (CoP) and hand variability. When participants were standing on the wide-support surface and fitting to the small opening, the hand and CoP Variability DTC were not significantly different, signifying no task priority. In contrast, higher hand Variability DTC than CoP Variability DTC when standing on the narrow surface in a condition with or without a manual challenge (fitting to either small or large opening) exhibited higher postural priority. Therefore, it appears that balance is prioritized over manual control when the postural task has consequences with higher hazard estimation.
Overall, my dissertation has extended a comprehensive understanding of the task-specific behavior of control strategy in the postural manual task, and how posture is prioritized when consequences of performing both postural and manual tasks are varied.