In daily living activities, performance of spatially accurate manual movements in upright stance depends on postural stability. In the present investigation, we aimed to evaluate the effect of the required manual steadiness (task constraint) on the regulation of dynamic postural control. A single group of young participants ( n = 20) were evaluated in the performance of a dual posturo-manual task of balancing on a platform oscillating in sinusoidal translations at 0.4-Hz (low) or 1-Hz (high) frequencies while stabilizing a cylinder on a handheld tray. Manual task constraint was manipulated by comparing the conditions of keeping the cylinder stationary on its flat or round side, corresponding to low and high manual task constraints, respectively. Results showed that in the low oscillation frequency the high manual task constraint led to lower oscillation amplitudes of the head, center of mass, and tray, in addition to higher relative phase values between ankle/hip-shoulder oscillatory rotations and between center of mass/center of pressure-feet oscillations as compared with values observed in the low manual task constraint. Further analyses showed that the high manual task constraint also affected variables related to both postural (increased amplitudes of center of pressure oscillation) and manual (increased amplitude of shoulder rotations) task components in the high oscillation frequency. These results suggest that control of a dynamic posturo-manual task is modulated in distinct parameters to attend the required manual steadiness in a complex and flexible way. NEW & NOTEWORTHY We evaluated dynamic postural control on a platform oscillating in sinusoidal translations at different frequencies while performing a manual task with low or high steadiness constraints. Results showed that high manual task constraint led to modulation of metric and coordination variables associated with greater postural stability. Our findings suggest that motor control is regulated in an integrative mode at the posturo-manual task level, with reciprocal interplay between the postural and manual components.
Previous studies have indicated that increased steadiness constraints given by a manual task leads to gains of postural stability in quiet and perturbed standing. The aim of the present study was to evaluate the effects of manual task constraints on the regulation of dynamic postural control according to the frequency of sinusoidal translations of the base of support. Twenty university students performed a dual posturomanual task consisting of maintaining a cylinder on a handheld tray as stable as possible while maintaining the dynamic upright balance on a platform oscillating in sinusoidal translations at 0.4 Hz (low) or 1 Hz (high) frequencies during intervals of 1 min. The effects of manual task were studied through the manipulation of task constraint conditions set by positioning the cylinder supported on its round or flat side, corresponding respectively to high (HC) and low (LC) manual task constraints. Results showed that HC led to reduced oscillation amplitudes of the head, center of mass, and tray at 0.4 Hz, but not at 1 Hz. The coordination variables indicated that at 0.4 Hz, HC led to increased relative phase values between ankle-shoulder and hip-shoulder rotations, as also between center of mass-feet and center of pressure-feet oscillations in the slow oscillation frequency. Further analyses showed that the high manual task constraint also affected variables related to both postural and manual task components at 1 Hz, with increased center of pressure and shoulder rotation amplitudes of oscillation. These results suggest that dynamic postural control is regulated through different parameters in a flexible and integrated way in order to attend the stability demands imposed by a posturomanual task.
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