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

ObjectiveMaintenance 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.MethodsSeventeen 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.ResultsTTF 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.ConclusionConstraint 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.SignificanceResults 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.

► When maintaining the position of a segment against an external load, the time until the position cannot be maintained (time to task failure; TTF) is greater if potential for movement of the more proximal limb segments is restricted with supports.
► Shorter TTF in tasks with less proximal segment constraint is associated with greater activity of the prime mover muscle as well as antagonist muscles, and muscles of more proximal segments.
► Manipulations of the amount of imposed limb constraint may partly explain the differences in TTF that have been reported in studies of muscle contractions, which differ in load type (muscle contractions to a target force vs. muscle contractions to control a limb position).