Effects of cycle strategy and fibre composition on muscle glycogen depletion pattern and subsequent running economy

PURPOSE: In this study, the effects of variable and constant-intensity cycling on muscle glycogen depletion patterns and subsequent running economy was examined. 60 minutes of cycling at a constant power (CON) or variable intensity (VAR) followed by a treadmill run to determine running economy was completed by nine male triathletes (Vo(2)max = 67.7 (4.9 ml) kg(-) min(-1)). During CON, there was greater glycogen depletion in the type I fibres compared with type II (0.08 (0.04) vs 0.02 (0.01) optical density (OD) units; p<0.05), while during VAR, there was greater glycogen depletion in the type II fibres compared with type I (0.06 (0.03) vs 0.03 (0.02) OD; p<0.05). The variation in muscle glycogen depletion patterns was not associated with the detriment in running economy, which was not significantly different between conditions (52.1 vs 52.8 ml kg(-1) min(-1)). There was a strong correlation between total muscle glycogen depletion and the change in running Vo(2) (r = 0.73, p<0.05) when the data from both trials were combined. There was also a negative correlation between type I fibre percentage and glycogen depletion within type II fibres during CON (r = -0.85, p<0.05). The results demonstrate that the decrease in running economy, subsequent to 60 minutes of cycling, is not affected by the cycling strategy employed. While different glycogen depletion patterns in the type I and II fibres were observed between conditions, total glycogen depletion may be more important to subsequent running economy. The percentage of type I fibres was associated with the glycogen depletion pattern during constant load, but not variable-intensity exercise.