Eleven subjects performed a series of 30-s work bouts on a cycle ergometer at power outputs ranging from 20-120% of the work capacity (Wcap) achieved during an incremental cycle to exhaustion and estimated the intensity of several sensations (leg effort, muscle tension, muscle discomfort, muscle pain, and breathing discomfort) by using Borg's category-ratio scale (range 0-10 units). Leg effort was perceived as "just noticeable" at 31 +/- 15% Wcap, muscle tension was just noticeable at 31 +/- 16% Wcap, muscle discomfort was just noticeable at 47 +/- 21% Wcap, breathing discomfort was just noticeable at 52 +/- 19% Wcap, and muscle pain was just noticeable at 58 +/- 33% Wcap. The intensity of all sensations increased in a positively accelerating manner with increases in power output (P < 0.001). Above 60% Wcap, the intensity of leg effort and muscle tension exceeded the intensity of muscle pain (P < 0.01), and above 100% Wcap the intensity of muscle discomfort also exceeded the intensity of muscle pain (P < 0.01). At 120% Wcap, leg effort, muscle tension, and muscle discomfort were rated between "severe" and "very severe" (6.1 +/- 2.2, 6.4 +/- 2.0, and 5.6 +/- 2.1 Borg units, respectively), whereas muscle pain and breathing discomfort were rated between "moderate" and "somewhat severe" (3.6 +/- 2.1 and 3.3 +/- 1.9 Borg units, respectively). These results suggest that subjects have a perception of muscle pain during muscular work that is distinct from perceptions of leg effort, muscle tension, and muscle discomfort.