This study examined the effects of the intensity and duration of exercise on the nature and magnitude of training adaptations in muscle of adolescent (2-3-yr old) racehorses. Six Thoroughbreds which had been pre-trained for 2 months performed 6 consecutive conditioning programs of varying lactate-guided intensities (v2.5 and v4) and durations (5, 15, 25 min). Pre- and post-training gluteus muscle biopsies were analyzed for myosin heavy chain content, fiber-type composition, fiber size, capillarization, and fiber histochemical oxidative and glycolytic capabilities. Although training adaptations were similar in nature, they varied greatly in magnitude among the different training protocols. Overall, the use of v4 as the exercise intensity for 25 min elicited the most consistent training adaptations in muscle, whereas the minimal training stimulus which evoked any significant change was identified with exercises of 15 min at v2.5. Within this range, muscular adaptations showed significant trends to be proportional to the exercise load of specific training programs. Taken together, these data suggest that muscular adaptations to training in horses occur on a continuum that is based on the exercise intensity and duration of training. The practical implications of this study is that exercises for 15 to 25 min/d at velocities eliciting blood lactate concentrations between 2.5 and 4 mmol/L can improve in the short-term (3 wks) the muscular stamina in Thoroughbreds. But exercises of 5 to 15 min at v4 are necessary to enhance muscular features related strength (hypertrophy).