In previously untrained individuals, endurance training improves peak oxygen uptake (V. O2peak), increases capillary density of working muscle, raises blood volume and decreases heart rate during exercise at the same absolute intensity. In contrast, sprint training has a greater effect on muscle glyco(geno)lytic capacity than on muscle mitochondrial content. Sprint training invariably raises the activity of one or more of the muscle glyco(geno)lytic or related enzymes and enhances sarcolemmal lactate transport capacity. Some groups have also reported that sprint training transforms muscle fibre types, but these data are conflicting and not supported by any consistent alteration in sarcoplasmic reticulum Ca 2+ ATPase activity or muscle physicochemical H + buffering capacity. While the adaptations to training have been studied extensively in previously sedentary individuals, far less is known about the responses to high-intensity interval training (HIT) in already highly trained athletes. Only one group has systematically studied the reported benefits of HIT before competition. They found that ≥6 HIT sessions, was sufficient to maximally increase peak work rate (Wpeak) values and simulated 40km time-trial (TT40) speeds of competitive cyclists by 4 to 5% and 3.0 to 3.5%, respectively. Maximum 3.0 to 3.5% improvements in TT40 cycle rides at 75 to 80% of Wpeak after HIT consisting of 4-to 5-minute rides at 80 to 85% of Wpeak supported the idea that athletes should train for competition at exercise intensities specific to their event. The optimum reduction or 'taper' in intense training to recover from exhaustive exercise before a competition is poorly understood. Most studies have shown that 20 to 80% single-step reductions in training volume over 1 to 4 weeks have little effect on exercise performance, and that it is more important to maintain training intensity than training volume. Progressive 30 to 75% reductions in pool training volume over 2 to 4 weeks have been shown to improve swimming performances by 2 to 3%. Equally rapid exponential tapers improved 5km running times by up to 6%. We found that a 50% single-step reduction in HIT at 70% of Wpeak produced peak ~6% improvements in simulated 100km time-trial performances after 2 weeks. It is possible that the optimum taper depends on the intensity of the athletes'preceding training and their need to recover from exhaustive exercise to compete. How the optimum LEADING ARTICLE