TRAINING-INDUCED ALTERATIONS IN MUSCLE GLYCOGEN UTILIZATION IN FIBER-SPECIFIC TYPES DURING PROLONGED EXERCISE

Using the glycogen depletion technique, we have examined utilization of specific fibre types during prolonged submaximal exercise to investigate the recruitment pattern employed by the central nervous system to sustain force generation in the face of a progressive glycogen depletion. Six male subjects (VO2 max, 52.8 ± 2.5 mL·kg-1·min-1,X̄ ± SE) cycled at 59% of pretraining VO2 max (the same absolute power output) for 99.5 ± 6 min on two occasions, before training and after 10-12 days of intensive training, involving 2 h of cycling per day. Prior to the training, glycogen concentration during exercise in the type I and type IIA fibres of the vastus lateralis muscle as measured by microphotometric techniques was progressively reduced during exercise. The pattern of depletion in both of these fibre types was parallel and showed an early marked depletion amounting to 51 (p < 0.05) and 35% (p < 0.05) in the type I and type IIA fibres, respectively, during the first 15 min of exercise. At the end of exercise, glycogen levels in type I and type IIA fibres were reduced to 9 and 44% of initial levels, respectively. In contrast, glycogen concentration in type IIB fibres was not significantly (p < 0.05) altered throughout the exercise. Following training, a pronounced glycogen sparing occurred that was conspicuous in only the type I and type IIA fibres, which was most pronounced during the first 15 min of the exercise. Similar to pretraining, glycogen concentrations in type IIB fibres were unaffected by either exercise or training. These results support the hypothesis that the muscle fibre recruitment patterns are established early in exercise and that even in the face of extensive glycogen loss observed late in exercise in the lower threshold type I and type IIA fibres, the higher threshold type IIB fibres are not recruited.