Acta Physiol Scand 1997, 159, 209–215 Step vs. progressive exercise : the kinetics of phosphocreatine hydrolysis in human muscle T. BINZONI,,  E.HILTBRAND, T.YANO and P.CERRETELLI 1 Department of Physiology, University of Geneva, Switzerland 2 Department of Radiology, University of Geneva, Switzerland BINZONI, T., HILTBRAND, E., YANO, T. & CERRETELLI, P. 1997. Step vs. progressive exercise : the kinetics of phosphocreatine hydrolysis in human muscle. Acta Physiol Scand 159, 209–215. Received 30 November 1995, accepted 17 August 1996. ISSN 0001–6772. Department of Physiology and Department of Radiology, University of Geneva, Switzerland. It is well known that the V   2 readjustment rate of the whole body is faster when carrying out a given constant work load starting from a baseline of moderate exercise than from rest. However, it has not been established whether the above change is the result of faster kinetics of the oxidative machinery or, alternatively, the consequence of a reduced involvement of confounding factors such as anaerobic glycolysis or tissue O 2 stores. The problem, earlier approached by chemical methods, was studied in man by 31 P-NMRS assessment of the kinetics of phosphocreatine (PC) hydrolysis at the muscle level which is known to reflect the readjustment rate of the oxidative reactions. Twelve normal subjects carried out in a 90 cm bore modified Picker (15 T) magnet, a series of contractions by the plantar flexors reaching pre-set submaximal loads either in single steps (constant load, CL) or progressively (incremental exercise, I). If preceding exercise (I), compared to rest, influenced the rate of oxidations, the PC concentration at the target loads would be different for the two exercise modes, reflecting different energy deficits. This was not the case. Thus the present results show that the rate of readjustment of oxidations at the muscle level is not affected by priming exercise confirming previous findings and showing that theoretical models of V   2 control are experimentally applicable to man. Keywords muscle exercise, NMR spectroscopy, phosphocreatine (PC) hydrolysis, V   2 kinetics. So far the kinetics of readjustment of the muscle oxidative machinery in man upon step work load changes (Δw  ) has been mainly assessed from measure- ments of gas exchange kinetics at the lung level. It has been shown that, for given moderate rectangular Δw  ’s corresponding to 20–40 % of maximum O  con- sumption (V   ,max ), moderate priming exercise may elicit faster V    readjustment rates (di Prampero et al. 1989) than found in transitions from rest. However, whether the above change is the result of an acceleration of the oxidative reactions or, alternatively, the consequence of the involvement of confounding factors such as a change of tissue O  stores andor of the energy contribution by anaerobic glycolysis during the readjustment phase with a transient accumulation of lactate in the muscle [the so-called ‘ early lactate ’, EL (Cerretelli et al. 1979), influencing the shape of the V    readjustment curve] has not been established. A more direct approach was proposed (Sahlin et al. 1988) whereby the metabolic changes (high energy phos- Correspondence : Tiziano Binzoni PhD, C.M.U., Dept. of Physiology, 1, r. Michel-Servet, 1211 Geneve 4, Switzerland. phates, lactate, NADHNAD, etc.) occurring in the human quadriceps femoris muscle upon slow or fast rest to work (up to 70% V   ,max ) transients were compared. It was concluded that mitochondrial respiration in muscle is activated only by the intermediates of the creatine kinase reaction, whereas the lack of O  transport at the onset of exercise was not the determinant factor. The problem could be solved only by measuring directly, during the rest-to- work transient, the time course of oxygen con- sumption (O   ) and of lactate accumulation at the muscle level. Unfortunately, these measurements, for technical reasons, cannot be made in humans. Based on the results of the previous studies and on a recent theoretical development from this laboratory (Binzoni & Cerretelli 1994), a more sophisticated approach was adopted whereby the intracellular phosphocreatine (PC) concentration was monitored continuously by nuclear magnetic resonance spectroscopy (P-NMRS) during rest to work or lower to higher work transients.  1997 Scandinavian Physiological Society 209