The purpose was to compare the time to failure and muscle activation patterns for a sustained isometric submaximal contraction with the dorsiflexor muscles when the foot was restrained to a force transducer (force task) compared with supporting an equivalent inertial load and unrestrained (position task). Fifteen men and women (mean ± SD; 21.1 ± 1.4 years) performed the force and position tasks at 20% maximal voluntary contraction (MVC) force until task failure. MVC force performed before the force- and position tasks was similar (333 ± 71 N vs. 334 ± 65 N) but the time to task failure was briefer for the position task (10.0 ± 6.2 min vs 21.3 ± 17.8 min, P <0.05). The rate of increase in agonist RMS EMG, EMG bursting activity, rating of perceived exertion, fluctuations in motor output, mean arterial pressure and heart rate during the fatiguing contraction was greater for the position task. EMG activity of the vastus lateralis (lower leg stabilizer) and medial gastrocnemius (antagonist) increased more rapidly during the position task but co-activation ratios (agonist vs antagonist) were similar during the two tasks. Thus, the difference in time to failure for the two tasks with the dorsiflexor muscles involved a greater level of neural activity and rate of motor unit recruitment during the position task, but did not involve a difference in co-activation. These findings have implications for rehabilitation and ergonomics in minimizing fatigue during prolonged activation of the dorsiflexor muscles.