Brief intense exercise followed by passive recovery modifies the pattern of fuel use in humans during subsequent sustained intermittent exercise

The role of work period duration as the principal factor influencing carbohydrate metabolism during intermittent exercise has been investigated. Fuel oxidation rates and muscle glycogen and free carnitine content were compared between two protocols of sustained intermittent intense exercise with identical treadmill speed and total work duration. In the first experiment subjects (n = 6) completed 40 min of intermittent treadmill running involving a work : recovery cycle of 6 : 9 s or 24 : 36 s on separate days. With 24 : 36 s exercise a higher rate of carbohydrate oxidation approached significance (P = 0.057), whilst fat oxidation rate was lower (P less than or equal to 0.01) and plasma lactate concentration higher (P less than or equal to 0.01). Muscle glycogen was lower post-exercise with 24 : 36 s (P less than or equal to 0.05). Muscle free carnitine decreased (P less than or equal to 0.05), but there was no difference between protocols. In the second experiment a separate group of subjects (n = 5) repeated the intermittent exercise protocols with the addition of a 10-min bout of intense exercise, followed by 43 +/- 5 min passive recovery, prior to sustained (40 min) intermittent exercise. For this experiment the difference in fuel use observed previously between 6 : 9 s and 24 : 36 s was abolished. Carbohydrate and fat oxidation, plasma lactate and muscle glycogen levels were similar in 6 : 9 s and 24 : 36 s. When compared with the first experiment, this result was because of reduced carbohydrate oxidation in 24 : 36 s (P less than or equal to 0.05). There was no difference, and no change, in muscle Free carnitine between protocols. A 10-min bout of intense exercise, followed by 43 +/- 5 min of passive recovery, substantially modifies fuel use during subsequent intermittent intense exercise.