title: Changes in constraint of proximal segments effects time to task failure and activity of proximal muscles in knee position-control tasks.
abstract: OBJECTIVE
  Maintenance of a limb position against external load (position-control) fails earlier (time to task failure: TTF) than maintenance of identical force against rigid restraint (force-control). Although possibly explained by physiological differences between contractions, we investigated whether less constraint of movements in other planes and proximal segments (commonly less in position-control tasks) shortens TTF.
  
  
  METHODS
  Seventeen adults (32±7 years) contracted knee extensor muscles to task failure in a position-control task, with and without constraint of motion in other planes and proximal segments, and a force-control task with constraints. Electromyography of knee extensors, their antagonist and hip muscles was recorded with force/position.
  
  
  RESULTS
  TTF was shorter for position-control without (161±55 s) than with constraint (184±51 s). Despite identical constraint, TTF was shorter in position- than force-control (216±56 s). Muscle activity and position variability at failure was greater without constraint.
  
  
  CONCLUSIONS
  Constraint of motion of proximal segments and other planes increases position-control TTF with less muscle activity and variability. As TTF differed between force- and position-control, despite equivalent constraint, other factors contribute to shorter position-control TTF.
  
  
  CONCLUSIONS
  Results clarify that differences in the TTF between position- and force-control tasks are partly explained by unmatched restriction of motion in other planes and proximal segments.
authors.affiliation: The University of Queensland, Centre for Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane, Australia.
authors.forename: Peter C
authors.initials: PC
authors.lastname: Poortvliet
authors.forename: Kylie J
authors.initials: KJ
authors.lastname: Tucker
authors.forename: Paul W
authors.initials: PW
authors.lastname: Hodges
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D003627:Data Interpretation, Statistical; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D007717:Knee / Q000502:physiology*; D007866:Leg / Q000502:physiology; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D010812:Physical Stimulation; D011187:Posture / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*