Glycogen depletion patterns in the muscle of standardbred trotters after exercise of varying intensities and durations.

The glycogen depletion patterns in the gluteus muscle of Standardbred horses were studied under different trotting intensities. After racing significant glycogen depletion was found in all Type I and IIA fibres and in a varying percentage of Type IIB fibres, depending on the individual horse. When horses performed exercise over a short distance (5 to 8 km) at three different speeds, glycogen depletion was difficult to detect for the faster speeds (10 and 8 m/sec) but notably involved a major proportion of Type I fibres at the slowest speed (6 m/sec). When exercise was prolonged over a greater distance a larger proportion of Type IIA fibres were depleted at faster speeds (8 m/sec) than at slower speeds over the same distance. However, when slow trotting was continued over even greater distances, a gradual increase in the percentage of glycogen depleted Type IIA fibres was noticed. Very significant glycogen depletion in all three fibre types was seen in all horses where a near maximal exercise tolerance test was performed 3 h after the longest and slowest exercise run. In a further study of glycogen depletion, during a near maximal exercise tolerance test on a graded treadmill with consecutively increasing velocity, depletion was seen in a proportion of all three fibre types with the highest proportion being Type I and the lowest Type IIB.