PURPOSE: An increased recruitment of type II muscle fibers has been suggested as a major cause of the slow component of O(2) uptake (VO(2)) kinetics. Furthermore, the rise in plasma ammonia (NH(3)) during high-intensity exercise, where a slow component is observed, has been associated with the activation of type II muscle fibers. Therefore, the purpose of this study was to examine the relationship between the VO(2) slow component, plasma NH3 concentration, and electromyography (EMG) responses during constant-load cycling. METHODS: Eight healthy adults (mean age +/- SEM: 21.4 +/- 1.0 yr) performed 7 min of heavy constant-load exercise. The breath-by-breath VO(2) response was characterized using a two-term exponential model. Plasma NH(3) concentration was measured at rest, following 3 min of unloaded cycling and at 3 and 7 min of constant-load exercise. Surface EMG activity of the right vastus lateralis muscle was measured during the final 10 s of every minute of exercise. RESULTS: The amplitude of the slow component was 561 +/- 52 mL.min(-1), and occurred 132 +/- 11 s following the onset of constant-load exercise. Plasma NH(3) concentration increased significantly from 3 to 7 min of constant-load exercise by 32.2 +/- 2.9 micromol.L(-1). The rise in plasma NH(3) concentration correlated significantly with the amplitude of the slow component (r = 0.79, P < 0.05). The mean power frequency of the EMG increased significantly while the integrated EMG/VO(2) ratio remained constant over the duration of the slow component. CONCLUSION: The rise in NH(3) concentration and the amplitude and spectral components of the EMG are consistent with a progressive increase in the recruitment of type II muscle fibers during the slow component phase of exercise.