The scientific revelations of 1967 regarding the relationship between glycogen and exercise endurance set in motion two decades of research mostly oriented toward the practical application of those early findings. It is surprising that, in spite of the considerable time that has elapsed between those initial studies and the wealth of knowledge accumulated since, we still do not know the biochemical mechanism that explains why carbohydrate is essential to the ability of the muscle fiber to maintain high force outputs. It may be that the discoveries of 1967 were so widely accepted and so essentially unchallenged that we narrowed our vision as to the potential mechanisms responsible for the observations reported. The time has come for us to lay this issue to rest and answer the question: Why is glycogen availability necessary to perform prolonged, heavy-intensity exercise?