title: Postural adjustments to support surface perturbations during reaching depend upon body-target reference frame.
abstract: We investigated whether target position relative to the body modifies the postural adjustments produced when reaching movements are perturbed by unexpected displacements of the support surface. Eleven healthy participants reached to a target located at their midline, acromion height and at 130% their outstretched arm length. They stood on two force plates mounted on a moveable platform, capable of delivering horizontal forward ramp-and-hold perturbations. Three types of trial were given: reach only (R), perturbations only (P) and reaching movements during which a perturbation was given at a random delay after reach onset (RP). The target could be mounted either on a frame suspended from the ceiling such that it remained world-fixed (exocentric target, RP/X) or at an equivalent position on the moving platform so that it moved with the body (egocentric target, RP/E). Arm and body 3D kinematics and muscle activity from the right tibialis anterior (rTA) and soleus (rSOL) muscles were recorded. Normalised rTA activity was significantly lower in RP than in P trials. Furthermore, long-latency rTA muscle activity was lower in RP/E than in RP/X conditions when perturbations were given during either the arm deceleration phase of reaching. The rSOL muscle activity was lowest for the RP/E (arm deceleration) condition. When balance is perturbed during reaching, the manner in which the target moves relative to the body determines the muscle activity produced in the lower-limb muscles. Furthermore, a target that moves with the body requires a different regulation of muscle activity compared with one that moves independently of the body.
authors.affiliation: Graduate Department of Rehabilitation Science, Faculty of Medicine, University of Toronto, Toronto, ON, Canada.
authors.forename: Alicia J
authors.initials: AJ
authors.lastname: Hilderley
authors.forename: Julia A
authors.initials: JA
authors.lastname: Leonard
authors.forename: Andrea
authors.initials: A
authors.lastname: Green
authors.forename: Ryan
authors.initials: R
authors.lastname: Ouckama
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Stapley
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D001696:Biomechanical Phenomena / Q000502:physiology; D004576:Electromyography; D005260:Female; D006801:Humans; D008297:Male; D009068:Movement / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology; D011187:Posture / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D055815:Young Adult