title: Tonic postural lean after-effects influenced by support surface stability and dynamics.
abstract: Tonic neuromuscular processes are evident during lean after-effects, which occur after prolonged standing on a fixed ramp. Postural processes underlying lean after-effects were examined here using dynamic surface conditions. Three tilt adaptation conditions were tested with eyes-closed (n=11). Tilt adaptation conditions involved standing for 120s on a fixed toes-up ramp (7°) or on a toes-up sinusoidally tilted surface (7° ± 3°), which was followed by 120s of standing on either a fixed horizontal surface or sway-referenced surface. All participants showed postural after-effects (p < .003). Specifically, standing on a fixed horizontal surface after sine-tilt adaptation, resulted in forward leaning which decayed over 120s back to baseline. Standing on a sway-referenced surface after tilt-adaptation, initially showed no lean after-effect, however over the course of the trial the center-of-pressure shifted backward (p < .02). This after-effect during sway-reference conditions was also evident in the sway-induced surface tilt, which increased in dorsiflexion (p < .002), rather than decaying back to baseline. Thus, adaptation occurs on a dynamically tilted surface, while reliability of the surface as a stable reference affects the return of the center-of-pressure and surface tilt to baseline. These findings relate to changes in flexor/extensor muscle tonic set-point which also occur following a prolonged voluntary isometric contraction.
authors.affiliation: Department of Physical Therapy, Temple University, 3307 N. Broad St., Philadelphia, PA 19140, USA. geoff.wright@temple.edu
authors.forename: W G
authors.initials: WG
authors.lastname: Wright
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D007699:Kinesthesis / Q000502:physiology*; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D009949:Orientation / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D011434:Proprioception / Q000502:physiology*; D012683:Sensory Deprivation / Q000502:physiology; D013028:Space Perception / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology; D055815:Young Adult