Within working muscle, development of conditions that directly influence exercise performance is dependent on many factors, including: intensity and duration of exercise, type of skeletal muscle fibres recruited, cardiovascular support to the working fibres and the inherent metabolic characteristics of the contracting fibres. In general, it is possible to identify factors that seem to alter exercise performance only at relatively intense exercise conditions. During prolonged moderately intense exercise (e.g. 70–80% maximal oxygen consumption for at least 60–90 min) decline in performance is related to the depletion of glycogen within the working muscle. Although the cause of muscle performance decline during very intense exercise is not known, an extreme acidosis is found, especially in fast-twitch muscle, which could significantly disrupt normal metabolic and contractile processes. During fatigue caused by intense contraction conditions, ATP content decreases (by approx. 50%) and there is a stoichiometric production of IMP and ammonia in fast-twitch muscle. This loss in adenine nucleotide content is dependent on the severity of the contraction conditions relative to the functional aerobic capacity of the muscle fibre, since fast-twitch red (high mitochondria, high blood flow) and fast-twitch white (low mitochondria, low blood flow) muscles respond differently. In contrast, during similarly intense contraction conditions, rat slow-twitch muscle fibres maintain their ATP content and do not produce significant amounts of IMP. Indirect evidence suggests that a similar contrast between fibres occurs in humans during maximal exercise. Thus, there seems to be a fundamental difference between fast- and slow-twitch muscles in the management of their adenine nucleotide contents during intense contraction conditions. Whether this is related to the known differences in the fatigue process between these fibre types is not known.