The rather constant amount of glycogen found in all fibre types in human skeletal muscle provides an opportunity to study the pattern of glycogen depletion with exercise, which should give an indication of which fibres are activated to generate the force. In very light dynamic contractions repeated for hours there is a primary reliance on slow twitch (ST) fibres with no or very minor involvement of fast twitch (FT) fibres. At heavier work loads (greater than 50% Vo2max) ST fibres are depleted first but FT fibres begin to become depleted. Exhaustion at these work levels coincides with muscle fibres of all types being depleted of glycogen. The crucial role of muscle glycogen in both the metabolic response to exercise and work performance is apparent. It is more difficult to explain why extramuscular substrates (plasma free fatty acids) cannot be utilized at a high enough rate to accommodate the energy turnover needed in more intense dynamic exercise. A limitation on the uptake of free fatty acids by the muscle cell rather than its transport to the cell or oxidation within it appears to be the critical factor.