Established models of endurance performance (Costill et al. (1973) Med Sci Sports 5(4): 248-252 and Coyle (1995) Ex. Sport Sci. Rev. 23:25-63) are based on the athlete's ability to maintain a fixed %V02peak, normally within the severe intensity domain, e.g. 88 %V02peak (exercise intensity domains being defined as; rest-moderate-heavy-severe-V02peak; Whipp (1994) Med. Sci. Sports Exerc. 26(11): 1319-13-26). The V02 slow component (V02SC) concept (Gaesser and Poole, 1996, Ex. Sport Sci. Rev. 24:35-70), which is based on observations from a healthy/sedentary populations', states that V02 continually increases within the severe intensity domain, and therefore undermines the validity of the performance models. This thesis examined the V02SC in an endurance trained cyclist population. Within the models, V02peak sets the ceiling for endurance performance. Current V02SC theory suggests that V02p"k assessment is protocol independent, as V02 continually increases within the severe intensity domain. This thesis demonstrated that V02peak was protocol dependent for 3 ramp protocols (35, 20 and 5 W.min-'), the V02SC being unable to generate a V02peak response from the 5 W.min-' protocol even though the subjects worked within the severe intensity domain. The V02SC definition states that V02 is elevated above values predicted from moderate intensity exercise at heavy, and increases continually, at severe exercise intensities. The endurance trained subjects demonstrated elevated steady state V02 responses at exercise intensities up to their endurance performance V02. This was within the severe intensity domain, thus validating the performance models for this subject population. The V02SC response in endurance trained cyclists differed from that previously observed for a sedentary/healthy population, therefore the currently accepted cause, increased fast twitch (FT) muscle fibre recruitment, may be questioned. Evidence from EMG studies suggest that muscle recruitment patterns differ between muscles, with increasing intensity (Green and Patla (1992) Med. Sci. Sports Ex. 24(1): 38-46). The recruitment patterns of three muscles were examined during incremental exercise to establish changes in both the magnitude of activation, and potential changes in fibre type recruitment (via the median frequency response). The pattern of muscle recruitment varied between both subjects and muscles. Changes in the recruitment patterns of a number of individual muscles were coincidental with the initiation of the V02SC. No coincidence between muscle fibre type recruitment (assessed via the EMG median frequency response) and the V02SC in endurance trained cyclists was observed. Therefore the V02SC may be due to changes in muscle recruitment patterns as well as FT fibre recruitment. The results of this thesis suggest that current models of endurance performance are valid for the endurance trained cyclist population studied, and that the V02SC concept should be redefined for this population. The V02SC response observed may be due to changes in muscle recruitment patterns and an increase in the number of motor units recruited, as median frequency EMG measures did not support the hypothesis that the V02SC is principally caused by an increased recruitment of FT muscle fibres.