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±7years) 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±55s) than with constraint (184±51s). Despite identical constraint, TTF was shorter in position- than force-control (216±56s). 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.