Reasons for performing study: The influence of relevant exercise parameters such as intensity and duration of exercise on the muscular response to training has not been reported in Thoroughbreds. This knowledge may be of great value in practice.
Objective: To define the relative contribution of the intensity and duration of exercise on the myosin heavy chain (MHC) response to training in Thoroughbred racehorses.
Methods: Six 2-3-year-old Thoroughbreds, following a randomised 6 x 6 latin square design, performed 6 consecutive conditioning programmes of varying lactate-guided intensities and durations. Each programme lasted 22 days and consisted of 11 exercise sessions once a day, every second day on a 6% inclined treadmill followed by a 10 day resting period between consecutive programmes. Gluteus muscle biopsies (depth, 2 and 6 cm) were taken before, and after, each programme and electrophoretically analysed for MHCs.
Results: Overall, training increased the fraction of MHC IIA (mean +/- s.d., 41.1 +/- 3.5% to 43.4 +/- 4.42% P<0.001) and decreased that of MHC IIX isoform (from 50.2 +/- 4.4% to 47.4 +/- 5.6% P<0.001). Exercise intensity fixed effects on fast MHCs were highly significant (P<0.01), whereas exercise duration only had a marginal effect (P<0.1) on the IIA:IIX MHC ratio. On a per-individual basis, training impact on fast MHCs was only significant (P<0.05) in horses exercised for the longest duration at both intensities.
Conclusion: The short-term training-induced up-regulation of MHC IIA and down-regulation of MHC IIX in Thoroughbreds are more dependent on intensity than duration of exercise. However, protocols with exercises of moderate intensity and long duration can induce MHC changes similar to those promoted by programmes with exercises of higher intensities.
Potential relevance: 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.