Acute effects of fatigue produced by a maintained 60% isometric loading on force production and relaxation characteristics of the leg extensor muscles were studied in male endurance (n = 9), power (n = 6) and strength athletes (n = 9). The initial non-fatigued isometric force-time curves differed considerably (p less than 0.05-0.001) between the groups so that the times of force production were the shortest and correspondingly the maximal rate of force production the greatest in the power athletes but the longest and the smallest in the endurance athletes. The endurance time of 70.7 +/- 32.9 s at the 60% fatiguing loading was in the endurance athletes longer (p less than 0.01) than in the power (30.6 +/- 7.1 s) and strength groups (31.7 +/- 5.5 s). The present fatiguing loading resulted in all athlete groups in significant (p less than 0.05-0.001) worsening in maximal force, in the times of force production and in the maximal rates of force production and relaxation. However, this worsening in the endurance athletes in maximal force (to 92.9 +/- 7.1%) as well as in the maximal rates of force production (to 79.2 +/- 20.8%) and relaxation (to 73.1 +/- 29.2%) were significantly (p less than 0.05-0.01) smaller than the corresponding decreases in the power athletes (to 64.3 +/- 8.0%, 74.8 +/- 7.4% and to 40.9 +/- 12%, respectively) and in the strength athletes (to 65.7 +/- 7.0%, 56.7 +/- 16.0% and to 34.8 +/- 6.7%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)