Adaptation of postural orientation to changes in surface inclination |
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Authors: | JoAnn Kluzik Robert J Peterka Fay B Horak |
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Institution: | (1) Neurological Sciences Institute, Oregon Health & Science University, 505 NW 185th Avenue, Beaverton, OR 97006-3499, USA;(2) Department of Neurology, Johns Hopkins School of Medicine, 707 N. Broadway, Baltimore, MD 21205, USA |
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Abstract: | We previously showed that standing on an inclined surface resulted in an after-effect of leaning in many healthy, blindfolded
subjects when they returned to standing on a horizontal surface (Kluzik et al. in Exp Brain Res 162:474–489, 2005). The direction of leaning depended on the direction of prior surface inclination, always in a direction that preserved the
relative alignment between the body and the support surface. For example, subjects leaned forward after they stood on a toes-up-inclined
surface. In the present study, we investigated how the amplitude of surface inclination affected postural muscle activity,
joint position, body segment orientation, and body center of mass (CoM) and foot center of pressure (CoP) locations before,
during, and after subjects stood on an inclined surface. We asked whether the mechanism that underlies the lean after-effect
involves regulation of local postural variables, such as the position of the ankle joint or the level of muscle activity,
or whether instead, the mechanism involves regulation of global, whole-body postural variables that can only be determined
by multisensory processing, such as orientation of the trunk or the body’s CoM. In one experiment, we found that varying the
amplitude of a toes-up surface inclination between 2.5° and 10° had a systematic, linear, effect on the post-incline orientation
of the trunk and head, but did not systematically affect the post-incline orientation of the legs, position of the ankle joint,
the level of EMG activity, or the location of the CoP. In a second experiment, we found that preventing the legs from leaning
in the post-incline period did not abolish leaning of the upper body. These findings suggest that (1) the body-to-support-surface
relationship is an important reference for the CNS internal representation of postural orientation which is subject to adaptive
modification and (2) the adaptive mechanism underlying the post-incline after-effect of leaning acts at the level of global,
whole-body postural variables. |
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Keywords: | Adaptation After-effect Posture Human |
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