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 that had been pretrained for 2 mo performed six consecutive conditioning programs of varying lactate-guided intensities [velocities eliciting blood lactate concentrations of 2.5 mmol/l (v2.5) and 4 mmol/l (v4), respectively] and durations (5, 15, 25 min). Pre- and posttraining 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 that 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 are that exercises for 15 to 25 min/day at velocities between v2.5 and v4 can improve in the short term (3 wk) the muscular stamina in thoroughbreds. However, exercises of 5-15 min at v4 are necessary to enhance muscular features related to strength (hypertrophy).

The objective of the study was to examine the validity of v(4) [velocity run under the defined conditions inducing 4 mmol/L of blood lactate concentration ([LA])] and v(200) (velocity run under the defined conditions inducing a heart rate of 200 beats/min) to differentiate performance level among Standardbred racehorses. For this purpose, 19 Standardbred trotting racehorses with differing racing time records in 2 training yards were submitted to a standardized exercise test to determine their v(4) and v(200) (6 horses of one yard only). The test consisted of 4 or more consecutive intervals depending on when the blood [LA] of a horse increased above 4 mmol/L. Speed and time trotted in each interval as well as time between consecutive intervals were the same for horses of a training yard. The blood [LA] measured after each interval was plotted exponentially against running speed to derive v(4) from the blood lactate-running speed relationship, and the mean heart rate during the intervals was plotted linearly against running speed to derive v(200) from the heart rate-running speed relationship. The correlation coefficient between v(4) and the racing time record was 0.77 and 0.75 for horses in racing yard A and B, respectively. There was no correlation between v(200) and the racing time record. Therefore, v(4) is a valid indicator of performance level of Standardbred trotting racehorses; however, v(200) may not be or to a lesser extent.

At the International Conference on Equine Exercise Physiology (ICEEP7), about 70 people attended the workshop on workload and conditioning guided by the authors. Most of the audience were involved in Thoroughbred or Standardbred racing, and only a limited number of people were mainly involved in FEI equestrian disciplines (sport horses). The workshop and this review article address the measurement of workload and conditioning of the Thoroughbred racehorse. It was proposed that workload could be quantified using a few selected parameters commonly recorded in the racing industries, such as velocity and distance, to generate a cumulative workload index. The review of conditioning focuses on the Thoroughbred racehorse and examines what can be modified with training, how training programmes should be designed based upon scientific methods and how training programmes should be routinely designed in current practice. It would appear that, in general, the methods used in practice for training Thoroughbred racehorses are quite similar to those used in a set of recent scientific studies, particularly in young (2-3-years-old) Thoroughbreds. Nevertheless, both the length of the training programme and the total amount of exercise are usually shorter/lower than ideal in order to maximize physiological adaptations within the animal's body. In planning the training programme, it is very important to recognize that different adaptations occur at different rates, and this will affect the relative amount of training that should be applied to achieve specific adaptations.

The speed producing the maximal lactate steady state (maxLASS) is supposed to be the optimal speed to condition for endurance. The maxLASS was defined as the maximal speed at which the blood lactate concentration ([LA]) between the 5th and the 25th min of continuous exercise did not increase by more than 1 mmol/L. According to the aerobic-anaerobic lactate threshold concept determined in humans, maxLASS corresponds to v(4) [speed in a standardized exercise test (SET) shown to produce an [LA] of 4 mmol/L; generalized to v(i) for the speed producing an [LA] of i mmol/L]. Four Thoroughbreds were submitted to a treadmill-based SET to determine their blood lactate-running speed (BLRS) relationship and calculate the individual v(1.5), v(2), v(2.5), v(3), and v(4) values (velocities run under defined conditions inducing 1.5, 2, 2.5, 3, and 4 mmol/L of blood LA). Afterward, horses ran on the treadmill for 40 min at their v(1.5), v(2), and v(2.5) every 3 d. Another 14 horses were submitted to SET in the field to determine their BLRS relationships and to calculate their v(2). The day after the SET, these horses ran once between 15 and 30 min at their v(2). In the horses that ran on the treadmill, maxLASS only occurred when running at their v(1.5). Blood [LA] did not increase by more than 1 mmol/L between the 10th min and the end of exercise for all the horses that ran in the field at their v(2.) These data indicate that maxLASS of horses is not greater than v(2) and therefore less than in running humans.

Exercises for up to 25 min/day at velocities eliciting a blood lactate concentration between 2.5 and 4 mmol/l every second day for 3 weeks improve stamina in Thoroughbreds.