Referent control of the orientation of posture and movement in the gravitational field
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This study addresses the question of how posture and movement are oriented with respect to the direction of gravity. It is suggested that neural control levels coordinate spatial thresholds at which multiple muscles begin to be activated to specify a referent body orientation (RO) at which muscle activity is minimized. Under the influence of gravity, the body is deflected from the RO to an actual orientation (AO) until the emerging muscle activity and forces begin to balance gravitational forces and maintain body stability. We assumed that (1) during quiet standing on differently tilted surfaces, the same RO and thus AO can be maintained by adjusting activation thresholds of ankle muscles according to the surface tilt angle; (2) intentional forward body leaning results from monotonic ramp-and-hold shifts in the RO; (3) rhythmic oscillation of the RO about the ankle joints during standing results in body swaying. At certain sway phases, the AO and RO may transiently overlap, resulting in minima in the activity of multiple muscles across the body. EMG kinematic patterns of the 3 tasks were recorded and explained based on the RO concept that implies that these patterns emerge due to referent control without being pre-programmed. We also confirmed the predicted occurrence of minima in the activity of multiple muscles at specific body configurations during swaying. Results re-affirm previous rejections of model-based computational theories of motor control. The role of different descending systems in the referent control of posture and movement in the gravitational field is considered.
KeywordsBehavioural neuroscience Control variables Descending systems Vestibular Corticospinal Equilibrium-point
Study supported by NSERC (AGF). AAM received a doctoral fellowship from Heart and Stroke Foundation of Canada. MFL holds a Canada Research Chair in Motor Recovery and Rehabilitation. Authors wish to thank Dorothy Barthélemy and Anouk Lamontagne for help in data collection as well as the individuals who volunteered for the study.
The work was performed at the research centres of the Jewish Rehabilitation Hospital and the Institut de readaptation Gingras-Lindsay de Montreal, sites of the Center for Interdisciplinary Research in Rehabilitation, Montreal, Quebec. AGF and MFL conceived of the research design, AAM, SCH, SKS and AGF participated in data collection, AAM and NT performed data analysis. AGF, MFL and AAM interpreted the data and drafted the final version of the manuscript. All authors approved the final version of the manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.
Compliance with ethical standards
Conflict of interest
All authors state that they have no competing interests.
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