SKELETAL-MUSCLE ADAPTATIONS TO PROLONGED TRAINING, OVERTRAINING AND DETRAINING IN HORSES

Thirteen standard-bred horses were trained intensively for 34 weeks an d detrained for 6 weeks to study skeletal muscle adaptations to prolon ged training, overtraining and detraining, Training included endurance (phase 1, 7 weeks), high-intensity (phase 2, 9 weeks) and overload tr aining (OLT) (phase 3, 18 weeks). During phase 3, horses were divided into two groups, OLT and control (C), with OLT horses performing great er intensities and durations of exercise than C horses. Overtraining w as evident in OLT horses after week 31 and was defined as a significan t reduction in treadmill run time in response to a standardised exerci se test (P<0.05). Relationships between peripheral (skeletal muscle) a nd whole body (maximum O-2 uptake, (V) over dot O-2,O- max, treadmill run time) adaptations to training were determined. Prolonged training resulted in significant adaptations in morphological characteristics o f skeletal muscle but the adaptations were limited and largely complet ed by 16 weeks of training. Fibre area increased in all fibres while t he number of capillaries per fibre increased and the diffusional index (area per capillary) decreased. Mitochondrial volume density continue d to increase throughout 34 weeks of training and paralleled increases in (V) over dot O-2,O-max and treadmill run time. Significant correla tions were noted between mitochondrial volume and (V) over dot O-2,O- max (R=0.71), run time and (V) over dot O-2,O- max (R=0.83) and mitoch ondrial volume and run time (R=0.57). We conclude that many of adaptiv e responses of muscle fibre area and capillarity occur in the initial training period but that markers of oxidative capacity of muscle indic ate progressive increases in aerobic capacity with increases in traini ng load. The lack of differences between C and OLT groups indicated th at there may be an upper limit to the ability of training stimulus to evoke skeletal muscle adaptive responses. There was no effect of overt raining or detraining on any of the adaptive responses measured.