The velocity at which blood lactate concentration ([LA]) of 4 mmol/L is reached (v(4)) is widely used to determine fitness, but there are few published data on using [LA] as a guide for the exercise speed for training in horses. In this study, the effect of 3 conditioning programs with [LA] guided exercise speeds on v(4), v(200) (speed at a heart rate of 200 beats/min), blood [LA], plasma FFA ([FFA]), and alanine concentrations ([alanine]), before and after exercise, as well as heart rate during exercise, of horses was examined. Six 2-yr-old Haflinger stallions underwent an initial treadmill-based standard exercise test (SET). A regression analysis [LA]-speed relationship was used to calculate v(1.5), v(2.5), and v(4). Horses were then randomly assigned to 1 of 3 conditioning programs according to a 3 x 6 Latin square design. During 6 wk, horses exercised on the treadmill every other day for 45 min at their calculated v(1.5) or v(2.5) or 25 min at their v(4). Each conditioning period (CDP) was followed by 5 wk without conditioning. At 2 and 9 d, and 5 wk, after the end of the CDP, the horses performed another SET to evaluate again the v(4) and v(200). Blood [LA], plasma [FFA], and [alanine] were measured before and after heart rate during exercise sessions 1, 11, and 21 in each CDP. None of the exercise programs had an effect on v(4) and v(200) (P > 0.05). The increase of the mean [LA] after exercise decreased during CDP (P < 0.05), and the increase of mean heart rate during exercise tended to decrease as well (P = 0.07). There was no difference among the conditioning programs. Plasma [FFA] before exercise was not influenced by the CDP (P > 0.05). The plasma [FFA] was always greater after exercise (P < 0.05), but there was no difference among conditioning programs. Overall, the increase was greatest after the 21st exercise session compared with the 1st and 11th exercise sessions (P < 0.05). The mean plasma [alanine] before exercise remained similar during all CDP (P > 0.05). Mean plasma [alanine] of the horses was increased after all exercise sessions measured (P < 0.05). There was no difference among conditioning programs (P > 0.05). It is concluded that conditioning with the exercise types used had small effects. This could have been because the exercise stress was too small, but also because the workload was not increased during the CDP.

Summary The effects of 3 different conditioning programmes on muscle glycogen concentration in horses were examined. Speed of exercise was selected according to the blood lactate values for each horse derived from a standardised exercise test before beginning a conditioning programme. Six 2‐year‐old Haflinger stallions were assigned randomly to one of 3 conditioning programmes according to a 6 × 3 latin square design: 45 min at their individual v1.5 or v2.5 and 25 min at v4. Each conditioning programme lasted 6 weeks (21 exercise sessions), followed by 5 weeks without conditioning (resting period). All exercise was carried out on a treadmill inclined at 17%. Muscle biopsies were taken 5 times from the gluteus medius muscle at 2 cm and 6 cm depth: before the start and in the middle of the conditioning period, then at Days 2, 9 and 35 after the last exercise session. It was found that glycogen concentration was not affected by conditioning until 9 days after finishing conditioning at v1.5 and v2.5 for 45 min (P<.05). By this time glycogen concentration in the muscle samples taken at 6 cm depth increased by 47 and 48%, respectively, and remained elevated until the end of the resting period. It was concluded that conditioning at lower intensity and for longer duration seemed to increase glycogen stores in the muscle while faster intensity but shorter duration exercise did not. To increase the likelihood of measuring effects of conditioning programmes on muscle variables, sampling should be done at different depths of a muscle and at several days after finishing a conditioning programme.

The effects of three different blood-guided conditioning programmes on ultrastructural and histochemical features of the gluteus medius muscle of 2-year-old sport horses were examined. Six non-trained Haflinger horses performed three consecutive conditioning programmes of varying lactate-guided intensities [velocities eliciting blood lactate concentrations of 1.5 (v1.5 ), 2.5 (v2.5 ) and 4 (v4 ) mm respectively] and durations (25 and 45 min). Each conditioning programme lasted 6 weeks and was followed by a 5-week resting period. Pre-, post- and deconditioning muscle biopsies were analysed. Although training and detraining adaptations were similar in nature, they varied significantly in magnitude among the three different conditioning programmes. Overall, the adaptations consisted in significant increases in size of mitochondria and myofibrils, as well as a hypertrophy of myofibrillar ATPase type IIA muscle fibres and a reduction in number of type IIx low-oxidative fibres. Together, these changes are compatible with a significant improvement in both muscle aerobic capacity and muscle strength. The use of v1.5 and v2.5 as the exercise intensities for 45 min elicited more significant adaptations in muscle, whereas conditioning horses at v4 for 25 min evoked minimal changes. Most of these muscular adaptations returned towards the pre-conditioning status after 5 weeks of inactivity. It is concluded that exercises of low or moderate intensities (in the range between v1.5 and v2.5 ) and long duration (45 min) are more effective for improving muscle features associated with stamina and power in sport horses than exercises of higher intensity (equivalent to v4 ) and shorter duration (25 min).

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