Changes in constraint of proximal segments effects time to task failure and activity of proximal muscles in knee position-control tasks

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. Conclusion: 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. Significance: 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. (C) 2012 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.