YM: young men YW: young women OM: older men OW: older women MVC: maximal voluntary isometric contraction torque T: the amplitude of the superimposed doublet T: the value of the resting doublet ANOVA: Analysis of variance

Introduction: Susceptibility to muscle fatigue during aging could depend on muscle activation patterns. Methods: Young (mean age 22 yrs) and older (mean age 70 yrs) men and women completed two fatigue tests of knee extensor muscles using voluntary and electrically stimulated contractions. Results: Older subjects displayed a shift to the left of the torque-frequency relationship and held a sustained voluntary isometric contraction at 50% maximal strength for significantly longer than young (P<0.001). Young and old showed similar fatigue during electrically-induced, intermittent isometric contractions (1-s on, 1-s off for 2 min), but women fatigued less than men (P=0.001). Stronger muscles fatigued more quickly, and slower contractile properties were associated with longer sustained contractions. Discussion: The slowing and weakness of older muscle was associated with superior fatigue resistance during sustained isometric contractions. Young and old showed similar fatigue following a series of brief, intermittent contractions, but women fatigued less than men.