This study examined the relationship between muscle recruitment patterns using surface electromyography (EMG) and the excess O2 uptake (ExV˙O2) that accompanies slow (SR, 8 W min-1) but not fast (FR, 64 W min-1) ramp increases in work rate (WR) during exercise on a cycle ergometer. Nine subjects (2 females) participated in this study (25 ± 2 years, ± S.E.M.). EMG was obtained from the vastus lateralis and medialis and analysed in the time (root mean square, RMS) and frequency (median power frequency, MDPF) domain. Results for each muscle were averaged to provide an overall response and expressed relative to a maximal voluntary contraction (%MVC). ΔV˙O2/ΔWR was calculated for exercise below (S1) and above (S2) the lactate threshold (LT) using linear regression. The increase in RMS relative to the increase in WR for exercise below the LT (ΔRMS/ΔWR-S1) was determined using linear regression. Due to non-linearities in RMS above the LT, ΔRMS/ΔWR-S2 is reported as the difference in RMS (ΔRMS) and the difference in WR (ΔWR) at end-exercise and the LT. SR was associated with a higher (P < 0.05) ΔV˙O2/ΔWR (S1, 9.3 ± 0.3 ml min-1 W-1; S2, 12.5 ± 0.6 ml min-1 W-1) than FR (S1, 8.5 ± 0.4 ml min-1 W-1; S2, 7.9 ± 0.4 ml min-1 W-1) but a similar ΔRMS/ΔWR-S1 (SR, 0.11 ± 0.01% W-1; FR, 0.10 ± 0.01 % W-1). ExV˙O2 was greater (P < 0.05) in SR (3.6 ± 0.7 l) than FR (-0.7 ± 0.4 l) but was not associated with a difference in either ΔRMS/ΔWR-S2 (SR, 0.14 ± 0.01% W-1; FR, 15 ± 0.02 % W-1) or MDPF (SR, 2.6 ± 5.9 %; FR, -15.4 ± 4.5 %). The close matching between power output and RMS during SR and FR suggests that the ExV˙O2 of heavy exercise is not associated with the recruitment of additional motor units since ExV˙O2 was observed during SR only. Compared to the progressive decrease in MDPF observed during FR, the MDPF remained relatively constant during SR suggesting that either (i) there was no appreciable recruitment of the less efficient type II muscle fibres, at least in addition to those recruited initially at the onset of exercise, or (ii) the decrease in MDPF associated with fatigue was offset by the addition of a higher frequency of type II fibres recruited to replace the fatigued motor units. Experimental Physiology (2002) 87.1, 91-100.