Effects of exercise-induced intracellular acidosis on the phosphocreatine recovery kinetics: a 31 P MRS study in three muscle groups in humans Gwenael Layec a,b,c , Emil Malucelli d , Y. Le Fur a , David Manners e , Kazuya Yashiro a , Claudia Testa e , Patrick J. Cozzone a , Stefano Iotti d,f and David Bendahan a * Little is known about the metabolic differences that exist among different muscle groups within the same subjects. Therefore, we used 31 P-magnetic resonance spectroscopy ( 31 P-MRS) to investigate muscle oxidative capacity and the potential effects of pH on PCr recovery kinetics between muscles of different phenotypes (quadriceps (Q), finger (FF) and plantar flexors (PF)) in the same cohort of 16 untrained adults. The estimated muscle oxidative capacity was lower in Q (29  12 mM min -1 , CV inter-subject = 42%) as compared with PF (46  20 mM min -1 , CV inter-subject = 44%) and tended to be higher in FF (43  35 mM min -1 , CV inter-subject = 80%). The coefficient of variation (CV) of oxidative capacity between muscles within the group was 59  24%. PCr recovery time constant was correlated with end-exercise pH in Q (p < 0.01), FF ( p < 0.05) and PF ( p <0.05) as well as proton efflux rate in FF ( p < 0.01), PF ( p < 0.01) and Q ( p = 0.12). We also observed a steeper slope of the relationship between end-exercise acidosis and PCr recovery kinetics in FF compared with either PF or Q muscles. Overall, this study supports the concept of skeletal muscle heterogeneity by revealing a comparable inter- and intra-individual variability in oxidative capacity across three skeletal muscles in untrained individuals. These findings also indicate that the sensitivity of mitochondrial respiration to the inhibition associated with cytosolic acidosis is greater in the finger flexor muscles compared with locomotor muscles, which might be related to differences in permeability in the mitochondrial membrane and, to some extent, to proton efflux rates. Copyright © 2013 John Wiley & Sons, Ltd. Keywords: mitochondrial function; muscle oxidative capacity; magnetic resonance spectroscopy; exercise; skeletal muscle; muscle acidosis INTRODUCTION It has been largely acknowledged that muscle oxidative capacity can vary according to daily-level physical activity (1). Among the corresponding changes, a variety of metabolic and cardiovascular changes including reduced utilization of muscle glycogen and exercise-induced lactate production at a given exercise intensity, increased capillary and mitochondrial densities as well as mito- chondrial enzymatic activities has been reported (1). However, while differences in muscle oxidative capacity have been related to variations in physical fitness (2–4), little is known about the po- tential differences among muscles exposed to different chronic load for a given subject (5,6). Likewise, the effects of exercise- induced acidosis on measured oxidative capacity have been addressed for a given muscle, but the corresponding effects on dif- ferent muscles of a given subject remain to be clearly documented. * Correspondence to: D. Bendahan, Centre de Resonance Magnetique Biologique et Medicale, UMR CNRS 6612, Faculté de Médecine de Marseille, Marseille, France. E-mail: david.bendahan@univmed.fr a G. Layec, Y. Le Fur, K. Yashiro, P. J. Cozzone, D. Bendahan Centre de Resonance Magnetique Biologique et Medicale, UMR CNRS 6612, Faculté de Médecine de Marseille, Marseille, France b G. Layec Department of Medicine, Division of Geriatrics, University of Utah, Salt Lake City, UT, USA c G. Layec Geriatric Research, Education, and Clinical Center, George E. Whalen VA Medical Center, Salt Lake City, UT, USA d E. Malucelli, S. Iotti Dipartimento di Farmacia e Biotecnologie dell’Invecchiamento e Malattie Nefrologiche, Università di Bologna, Italy e D. Manners, C. Testa Dipartimento di Scienze Biomediche e Neuromotorie dell’Invecchiamento e Malattie Nefrologiche, Università di Bologna, Italy f S. Iotti Istituto Nazionale Biostrutture e Biosistemi, Rome, Italy Abbreviations used: 31 P-MRS, 31 P magnetic resonance spectroscopy; PCr, phosphocreatine; Pi, inorganic phosphate; ADP, adenosine diphosphate; MVC, maximum isometric force; Τc, time constant of PCr resynthesis kinetics; Q max , oxidative capacity; Q b , basal rate of ATP synthesis; K m , affinity constant. Research article Received: 17 August 2012, Revised: 22 March 2013, Accepted: 25 March 2013, Published online in Wiley Online Library: 2013 (wileyonlinelibrary.com) DOI: 10.1002/nbm.2966 NMR Biomed. (2013) Copyright © 2013 John Wiley & Sons, Ltd.