Endurance time of a submaximal sustained contraction is longer when the muscle is fatigued in a shortened position. The aim of the present study was to compare central and peripheral mechanisms of fatigue after an isometric contraction of the knee extensor muscles performed at 20% maximal voluntary contraction (MVC) at two knee angles (35°, short length vs. 75°, long length, 0°= full extension) until exhaustion. Eleven men (24 ± 4 yr) attended two experimental randomized sessions. Endurance time was greater at 35° compared with 75° (974 ± 457 s vs. 398 ± 144 s, P<0.001) despite a similar reduction in knee extensors MVC (-28.4 ±16.0%, P<0.001 vs. -27.6 ±18.8%, respectively, P<0.001). Voluntary activation level was similarly depressed after the fatiguing contraction performed at the two muscle lengths (-19 ±16.7% at 35°, P<0.01 vs. -13.7 ±14.5% at 75°, P<0.01). Following the fatiguing contraction, peak twitch potentiation was observed only at the short length (+31.8 ±17.6% at 35°, P<0.01 vs. +6.4 ±21.3% at 75°, P>0.05), whereas M-wave properties were similarly altered for the two angles. These results suggest that (1) central fatigue at task failure for a sustained isometric contraction was not dependent on the muscle length, and (2) the longer endurance time of a sustained isometric contraction performed at a shortened length is related to potentiation. It is suggested that the greater endurance time of a sustained isometric contraction observed at 35° is related to the occurrence of potentiation at this short length, as central fatigue is similar at task failure for both tasks.