Quantitative probing of heterogeneous regions in muscle is feasible with phosphorus-31 magnetic resonance spectroscopy because of the differentiation of metabolic patterns of glycolytic and oxidative fibers. A differential recruitment of oxidative and glycolytic fibers during exercise was demonstrated in 4 of 10 untrained young men by following changes in phosphate metabolites. Concentrations of inorganic phosphate (Pi), phosphocreatine, and ATP were estimated in the wrist flexor muscles of the forearm at rest, during two cycles of three grades ofexercise, and in recovery. At high work levels (40% of maximum strength), two distinct Pi peaks were observed and identified with Pi pools at pH 6.9 and pH 5.9-6.4, respectively. These could be accounted for as follows. At the lowest level of work (using 20% of maximum strength), early recruitment primarily of oxidative (type I) and possibly some intermediate (type HA) muscle fibers occurs with relatively little net lactate production and consequently little decrease in pH. At higher work loads, however, primarily glycolytic (type UB) muscle fibers are recruited, which have relatively high net lactate production and therefore generate a second pool of Pi at low pH. ATP depletion (35-54%) and Pi losses accompanied the reduction in ability to perform during the first exercise cycle. When the cycle of graded exercise was repeated immediately, the total Pi remained high but gave rise to only one peak at pH 6.8-7.0. These observations indicated exhaustion of glycolytic type IIB fibers, removal of lactate by high local blood flow, and sustained contractions largely by oxidative type I and HA fibers. A functional differentiation of fiber types could also be demonstrated during recovery if exercise was stopped while two pools of P1 were still apparent. In the first 3 min of recovery, the Pi peak at pH 6.86.9 disappeared almost entirely, whereas the Pi peak at pH 6.0 remained unaltered, reflecting the faster recovery of oxidative type I fibers. The potential of magnetic resonance spectroscopy to characterize oxidative and glycolytic fibers, predict capacity for aerobic performance, and signal the presence of muscle pathology is discussed. By determining the enzyme and metabolite levels in muscle homogenates and single muscle fibers from serial biopsy samples, the recruitment of oxidative and glycolytic fibers in endurance athletes and untrained subjects has been studied (9)(10)(11)(12)(13)(14). It was concluded that the oxidative muscle fibers, types I and IIA, are recruited first at low work loads (15-18) and the fast-twitch glycolytic fibers, type IIB, are predominantly recruited when the work load is raised (18,19). The oxidative fibers break down relatively little glycogen, oxidize pyruvate readily, thereby minimizing net lactate production, and maintain their content of ATP and phosphocreatine (PCr) during work relatively well. The glycolytic fibers, on the other hand, break down much more glycogen and produce more lactic acid, which they can oxidize only slowly. ...