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STEPTO, N. K., D. T. MARTIN, K. E. FALLON, and J. A. HAWLEY. Metabolic demands of intense aerobic interval training in competitive cyclists. Med. Sci. Sports Exerc., Vol. 33, No. 2, 2001, pp. 303–310. To investigate the metabolic demands of a single session of intense aerobic interval training in highly trained competitive endurance cyclists. Seven cyclists (peak O2 uptake [O2peak] 5.14 ± 0.23 L.min−1, mean ±SD) performed 8 × 5 min work bouts at 86 ± 2% of O2peak with 60-s recovery. Muscle biopsies were taken from the vastus lateralis immediately before and after the training session, whereas pulmonary gas exchange and venous blood were sampled at regular intervals throughout exercise. Muscle glycogen concentration decreased from 501 ± 91 to 243 ± 51 mmol·kg−1 dry mass (P < 0.01). High rates of total carbohydrate oxidation were maintained throughout exercise (340 μmol·kg−1·min−1), whereas fat oxidation increased from 16 ± 8 during the first to 25 ± 13 μmol·kg−1·min−1 during the seventh work bout (P < 0.05). Blood lactate concentration remained between 5 and 6 mM throughout exercise, whereas muscle lactate increased from 6 ± 1 at rest to 32 ± 12 mmol·kg−1 d.m. immediately after the training session (P < 0.01). Although muscle pH decreased from 7.09 ± 0.06 at rest to 7.01 ± 0.03 at the end of the session (P < 0.01), blood pH was similar after the first and seventh work bouts (7.34). Arterial oxygen saturation (%SPO2) fell to 95.6 ± 1% during the first work bout and remained at 94% throughout exercise: the 60-s rest intervals were adequate to restore %SPO2 to 97%. Highly trained cyclists are able to sustain high steady state aerobic power outputs that are associated with high rates of glycogenolysis and total energy expenditure similar to those experienced during a 60-min competitive ride.