Abstract  The purpose of this study was to investigate blood flow and its heterogeneity within and among knee muscles of five young (26 ± 6 years) and five old (77 ± 6 years) healthy men with similar physical activity levels when they performed two types of submaximal fatiguing isometric contractions that required either force or position control. Positron emission tomography (PET) and [15O]‐H2O were used to determine blood flow two minutes (beginning) and 12 min (end) after the start of the tasks. Young and old men had similar maximal forces and endurance times for the fatiguing tasks. Although muscle volumes were lower in the old subjects, total muscle blood flow was similar between the groups (young: 25.8 ± 12.6; old: 25.1 ± 15.4 ml min−1; age main effect, P = 0.77) since blood flow per unit mass of muscle in the exercising knee extensors was greater for the old men (12.5 ± 6.2 ml min−1 100 g−1) than for the young men (8.6 ± 3.6 ml min−1 100 g−1; age main effect, P = 0.001). Further, blood flow heterogeneity in the exercising knee extensors was significantly lower in the old (56 ± 27%) compared to the young (67 ± 34%) men. Taken together, despite smaller skeletal muscles, the intact neural drive to the muscle and the greater, less heterogeneous blood flow per gram of muscle indicates that old fit muscle achieves adequate exercise hyperaemia.