Effect of constant load training on skeletal muscle histochemistry of thoroughbred horses

Skeletal muscle adaptations to training of differing intensities were examined in 10 thoroughbred horses that underwent six weeks of treadmill training followed by six weeks of detraining. The horses were randomly assigned to either a slow group exercised at 40 per cent maximum oxygen uptake (VO2max) or a fast group at 80 per cent VO2max. Resting muscle biopsies were taken before training, after six weeks of training and after six weeks of detraining, from m gluteus medius and m biceps femoris. Muscle was analysed histochemically for fibre type composition (myosin ATPase) and capillary supply (PAS amylase). Cross sectional area and lesser fibre diameter were measured by planimetry and image analysis. No alterations were found in the proportions of different muscle fibre types during training or detraining. Capillary density increased by 54 per cent in m biceps femoris of the fast group during training and decreased to the pretraining level following detraining. Few changes in fibre size occurred as a result of training and detraining. It was not possible to draw conclusions as to the effects of detraining because of the small number of training induced changes. The results suggest that for major adaptations in skeletal muscle, an increasing exercise intensity throughout training may be more significant than the degree of exercise load, when exercise intensity is submaximal.