Purpose: This study examined the effects of variable- and constant-intensity cycling on muscle glycogen depletion patterns and subsequent running economy.
Methods: Nine male triathletes (VO2max = 67.7 ± 4.9 mL∙kg-1∙min-1) completed 60 min of cycling at a constant power (CON) or variable intensity (VAR) followed by a treadmill run to determine running economy.
Results: During CON, there was greater glycogen depletion in the type I fibers compared to type II (0.08 ± 0.04 vs. 0.02 ± 0.01 optical density units (OD); P<0.05), while during VAR there was greater glycogen depletion in the type II fibers compared to 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 VO2 (r = 0.73; P<0.05) when the data from both trials were combined. There was also a negative correlation between type I fiber percentage and glycogen depletion within type II fibers during CON (r = -0.85, P<0.05).
Conclusion: The results demonstrate that the decrease in running economy, subsequent to 60 min 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 fibers was associated with the glycogen depletion pattern during constant-load, but not variable-intensity exercise.