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SummaryBlood lactate measurements are used widely in horses to assess athletic performance, but there are few published data on using lactate as a guide for endurance improvement. The velocity at which blood lactate concentration ([LA]) of 4 mmol/l is reached (v 4 ) is widely used to determine fitness. In an earlier study of v 4 , exercise at low intensities for longer duration was more effective at improving endurance than shorter, higher intensities. However, the prescription was unchanged during the study (6 weeks). We hypothesised that, to produce greater improvement in v 4 in the same time frame, it is necessary to adjust conditioning regimes more frequently. Six horses underwent an initial treadmill-based standard exercise test (SET). A regression analysis [LA]-speed relationship was used to calculate v 2 . Horses were then conditioned for 45 min at their calculated v 2 3 times/week (6% incline). Every 2 weeks the horses performed an SET to evaluate the v 2 conditioning protocol which ended with SET 4. Mean v 4 increased 17% from SET 1 to SET 4 (P = 0.003; 5.8 ± 0.3 to 6.8 ± 0.4 m/s). This study demonstrates that a lactate-guided conditioning programme can significantly enhance endurance performance over a 6-week time peroid when the conditioning protocol is adjusted every 2 weeks based on v 4 improvement.

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Horses fatigued on a treadmill and during the natural course of an endurance ride responded differently, biomechanically. On the treadmill, where speed is constrained, stride frequencies decreased and stride lengths increased. During one endurance ride, stride frequencies were the same, although speeds were substantially reduced. Limb length was shorter in fatigued endurance horses. It remains to be determined if these changes in mechanics are advantageous or disadvantageous in terms of energetics or injury. Further examination of endurance rides is also warranted.