Fatigue resistance of knee extensor muscles is higher during voluntary isometric contractions at short compared to longer muscle lengths. In the present study we hypothesized that this would be due to lower energy consumption at short muscle lengths. Ten healthy male subjects performed isometric contractions with the knee extensor muscles at a 30, 60 and 90° knee angle (full extension=0°). At each angle, muscle oxygen consumption (mVO₂) of the rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM) muscle was obtained with near infrared spectroscopy (NIRS). mVO₂ was measured during maximal isometric contractions and during contractions at 10, 30 and 50 % of maximal torque capacity. During all contractions blood flow to the muscle was occluded with a pressure cuff (450 mm Hg). mVO₂ significantly (p<0.05) increased with torque and at all torque levels and for each of the three muscles, mVO2 was significantly lower at 30° compared to 60° and 90° and mVO₂ was similar (p>0.05) at 60° and 90°. Across all torque levels, average (±SD) mVO₂ at the 30° angle for VM, RF and VL respectively was 70.0±10.4 %, 72.2±12.7 % and 75.9±8.0 % of the average mVO₂ obtained for each torque at 60 and 90°. In conclusion, oxygen consumption of the knee extensors was significantly lower during isometric contractions at the 30° than at the 60° and 90° knee angle, which probably contributes to the previously reported longer duration of sustained isometric contractions at relatively short muscle lengths.