This study examined the magnitude of the oxygen uptake slow component (VO(2) SC) during heavy exercise when preceded by heavy knee extension (KE) exercise. Nine males (26.6 +/- 1.7 years, +/-SE) performed repeated bouts of heavy exercise, each lasting 6 min with 6 min of recovery. Cycling-cycling trials (CYC(1), CYC(2)) involved step transitions to a workrate corresponding to 50% of the difference between peak VO(2) and the lactate threshold (Delta 50%). During bilateral KE-cycling trails (KE, CYC(3)), KE was performed at an intensity requiring twofold greater muscle activation relative to CYC(1) followed by a cycling transition to Delta 50%. VO(2) was measured breath-by-breath and was modeled using three exponentials to determinate the amplitudes (A (2)', A (3)') and time constants (tau (2), tau (3)) of the primary phase and VO(2) SC. Electromyography (EMG) recorded from the vastus lateralis and medialis was averaged and reported relative to maximal voluntary contraction (%MVC). EMG was higher (p < 0.05) during KE (37.6 +/- 8.1 %MVC) than CYC(1) (20.8 +/- 1.9 %MVC), CYC(2) (21.6 +/- 5.7 %MVC) and CYC(3) (19.8 +/- 6.3 %MVC). The amplitude of the VO(2) SC was lower (p < 0.05) in CYC(2) (197 +/- 120 ml min(-1)) and CYC(3) (163 +/- 51 ml min(-1)) compared to CYC(1) (325 +/- 126 ml min(-1)). No difference in VO(2) SC was observed between CYC(2) and CYC(3). Although the activation of additional motor units during KE exercise reduced the amplitude of the VO(2) SC, the decrease was similar to that observed following heavy cycling exercise. Thus, the activation of motor units in excess of those required for the activity does not alter the VO(2) response during a subsequent bout of exercise.