[The muscles work, but the brain gets tired]

Central fatigue is the term used to describe when muscle contractions become limited by the ability of the central nervous system to recruit motor neurones. Central fatigue becomes manifest when the effort is intense and is associated not only with reduced strength but also with an inability to maintain the contraction. The contractions thereby resemble those developed during partial neuromuscular blockade that mainly affect slow twitch muscle fibres. We suggest that central fatigue also manifests as a reduction in the ratio between the brain's uptake of oxygen relative to that of carbohydrate from 6 to less than 3. This imbalance between oxygen and glucose plus lactate uptake remains unsolved, but glycogen and accumulation of intermediates of metabolism are likely to play a key role.