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Abstract
Recent research has shown that systematic postural adjustments occur during periods of manual motor imagery (MI), but the timing (anticipatory or reactive) and directionality (against or in the direction of arm extension) of these postural motions relative to individual manual actions or imagery are not well understood. This study analyzed the anteroposterior hip and head motion of healthy young and older participants, while they imagined bilateral arm raises under self-initiated or environmentally triggered performance conditions. When MI was self-initiated, both age groups showed significant forward postural motion during the second prior to MI initiation. When MI (or physical arm movement) was environmentally triggered, however, older people did not show anticipatory forward postural motion, but did show compensatory backward head motion. These results suggest that manual MI is indeed accompanied by anticipatory postural motion, but this anticipation is attenuated in older people when they do not have control over the timing of manual movement onset.
Highlights
Goal-directed limb movement in upright humans is accompanied by concurrent postural control functions that prepare for and counteract the resulting perturbation to stance or gait (Massion 1992)
Bouisset and Zattara’s work (1987a, b, 1988) on uni- and bilateral arm raising demonstrated that anticipatory postural adjustments (APA) act in the direction opposite to the reaction forces generated by arm movement, and Cordo and Nashner (1982) showed that forward body sway that would result from a handle pull is counteracted by anticipatory gastrocnemius muscle activity producing backward sway
We first discuss the results for anticipatory postural motion (APM) and consider the case of compensatory postural motion (CPM)
Summary
Goal-directed limb movement in upright humans is accompanied by concurrent postural control functions that prepare for and counteract the resulting perturbation to stance or gait (Massion 1992). Studies on raising the arm while standing showed that leg muscles involved in postural control are the first to be activated (prior to the prime mover) (Belenkiy et al 1967), and a backward bending of the trunk compensates for a forward motion of the center of gravity (CG) caused by forward arm movement (Martin 1967). Bouisset and Zattara’s work (1987a, b, 1988) on uni- and bilateral arm raising demonstrated that anticipatory postural adjustments (APA) act in the direction opposite to the reaction forces generated by arm movement, and Cordo and Nashner (1982) showed that forward body sway that would result from a handle pull is counteracted by anticipatory gastrocnemius muscle activity producing backward sway. A key goal of APAs may be to regulate the CG (Bouisset and Zattara 1981, 1987b, 1988, 1990; Friedli et al 1988; Ramos and Stark 1990) or its projection on the ground (Mouchnino et al 1990; Rogers and Pai 1990)
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