1 Department of Exercise Science, University of Massachusetts, Amherst, Massachusetts; and 2 Department of Internal Medicine, Section of Endocrinology, Yale University School of Medicine, New Haven, Connecticut Submitted 13 May 2005 ; accepted in final form 4 July 2005 Energy for muscle contractions is supplied by ATP generated from 1 ) the net hydrolysis of phosphocreatine (PCr) through the creatine kinase reaction, 2 ) oxidative phosphorylation, and 3 ) anaerobic glycolysis. The effect of old age on these pathways is unclear. The purpose of this study was to examine whether age may affect ATP synthesis rates from these pathways during maximal voluntary isometric contractions (MVIC). Phosphorus magnetic resonance spectroscopy was used to assess high-energy phosphate metabolite concentrations in skeletal muscle of eight young (20–35 yr) and eight older (65–80 yr) men. Oxidative capacity was assessed from PCr recovery after a 16-s MVIC. We determined the contribution of each pathway to total ATP synthesis during a 60-s MVIC. Oxidative capacity was similar across age groups. Similar rates of ATP synthesis from PCr hydrolysis and oxidative phosphorylation were observed in young and older men during the 60-s MVIC. Glycolytic flux was higher in young than older men during the 60-s contraction ( P < 0.001). When expressed relative to the overall ATP synthesis rate, older men relied on oxidative phosphorylation more than young men ( P = 0.014) and derived a smaller proportion of ATP from anaerobic glycolysis ( P < 0.001). These data demonstrate that although oxidative capacity was unaltered with age, peak glycolytic flux and overall ATP production from anaerobic glycolysis were lower in older men during a high-intensity contraction. Whether this represents an age-related limitation in glycolytic metabolism or a preferential reliance on oxidative ATP production remains to be determined. acidosis; creatine kinase; glycolysis; oxidative phosphorylation; fatigue Address for reprint requests and other correspondence: J. A. Kent-Braun, Dept. of Exercise Science, Totman 108, Univ. of Massachusetts, Amherst, MA 01003 (e-mail: janekb{at}excsci.umass.edu )