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The effects of a priming exercise bout on both muscle energy production and the pattern of muscle fibre recruitment during a subsequent exercise bout are poorly understood. The purpose of the present study was to determine whether a prior exercise bout which is known to increase O₂ supply and to induce a residual acidosis could alter energy cost and muscle fibre recruitment during a subsequent heavy‐intensity knee‐extension exercise. Fifteen healthy subjects performed two 6 min bouts of heavy exercise separated by a 6 min resting period. Rates of oxidative and anaerobic ATP production, determined with ³¹P‐magnetic resonance spectroscopy, and breath‐by‐breath measurements of pulmonary oxygen uptake were obtained simultaneously. Changes in muscle oxygenation and muscle fibre recruitment occurring within the quadriceps were measured using near‐infrared spectroscopy and surface …