The mechanisms underlying the VO₂ slow component during supra lactate threshold (LT) exercise are poorly understood. Evidence suggests that the VO₂ slow component may be caused by progressive muscle recruitment during exercise. We, therefore, examined whether leg muscle activation patterns [from the transverse relaxation time (T2) of magnetic resonance images] were associated with supra-LT VO₂ kinetic parameters. Eleven subjects performed 6-min cycle ergometry at moderate (80% LT), heavy (70% between LT and critical power; CP), and very heavy (7% above CP) intensities with breath-by-breath pulmonary VO₂ measurement. T2 in 10 leg muscles was evaluated at rest, and after 3 and 6 minutes of exercise. During moderate exercise 9 muscles achieved a steady state T2 by 3-min; only in the vastus medialis did T2 increase further after 6-min. During heavy exercise T2 in the entire vastus group increased between minutes 3 and 6, which was supplemented by adductor magnus and gracilis during very heavy exercise. The VO₂ slow component increased with increasing exercise intensity (being functionally zero during moderate exercise). The distribution of T2 was more diverse as supra-LT exercise progressed: T2 variance (ms) increased from 3.6±0.2 to 6.5±1.7 between 3-min and 6-min of heavy exercise and from 5.5±0.8 to 12.3±5.4 in very heavy exercise (rest = 3.1±0.6). The T2 distribution was significantly correlated with the magnitude of the VO₂ slow component (P<0.05). These data are consistent with the notion that the VO₂ slow component is an expression of progressive muscle recruitment during supra-LT exercise.