1  Department of Exercise Science, University of Massachusetts, Amherst, Massachusetts 01003; 2  Exercise Science Program, Marquette University, Milwaukee, Wisconsin 53201; and 3  Magnetic Resonance Unit, Department of Radiology, University of California, San Francisco, California 94121 The purpose of this study was to compare the magnitude and mechanisms of ankle dorsiflexor muscle fatigue in 20 young (33   ± 6 yr, mean ± SD) and 21 older (75 ± 6 yr) healthy men and women of similar physical activity status. Noninvasive measures of central and peripheral (neuromuscular junction, sarcolemma) muscle activation, muscle contractile function, and intramuscular energy metabolism were made before, during, and after incremental isometric exercise. Older subjects fatigued less than young ( P  < 0.01); there was no effect of gender on fatigue ( P  = 0.24). For all subjects combined, fatigue was modestly related to preexercise strength ( r  = 0.49,   P  < 0.01). Neither central (central activation ratio) nor peripheral (compound muscle action potential) activation played a significant role in fatigue in any group. During exercise, intracellular concentrations of P i and H 2 PO increased more and pH fell more in young compared with older subjects ( P  < 0.01) and in men compared with women ( P  < 0.01). These varied metabolic responses to exercise suggest a greater reliance on nonoxidative sources of ATP in young compared with older subjects and in men compared with women. These results suggest that the mechanisms of fatigue vary with age and gender, regardless of whether differences in the magnitude of fatigue are observed. physical activity; magnetic resonance spectroscopy; central fatigue; activation; excitation-contraction coupling; metabolism