Skeletal muscle fatigue can limit athletic performance and the activities of daily living and is mainly determined by the balance between rates of energy use and replenishment. Differences between young and older men and women in contractile speed and aerobic capacity and variation in the duty cycle of contraction and relaxation are likely to influence development of fatigue. In the present study 37 young and older (mean age 22 and 70 yrs) men and women completed assessments of knee extensor contractile properties and two different fatigue tests. In one fatigue test a sustained isometric voluntary contraction was held at 50% of maximal force until task failure, giving no opportunity for metabolic recovery during the task. The other test consisted of a series of electrically evoked 30 Hz stimuli of 1 s contraction and 1 s rest intervals, giving brief periods between contractions for metabolic recovery. Fatigue in this latter test was presented as a fatigue index (FI), being the force after 2 minutes expressed as a percentage of the initial force. Data were examined using ANOVA and are presented as mean ± SD. Results showed that young men had the fastest contractile properties, followed by young women, and that older men and women were slowest, evidenced as a leftward shift in the torque-frequency relationship and slowing of rates of relaxation. The older subjects were able to hold the sustained contraction for around 35% longer than young (71.8±13.5 s vs 97.5±26 s; P<0.0005), irrespective of sex. In all groups task failure was almost entirely due to peripheral muscle fatigue, as assessed by superimposed electrical stimulation. However, during the intermittent contractions, which more closely resemble the recruitment patterns during activities like walking or running, there was no difference between young and old in fatigue resistance (FI: 59%±10 vs 59%±9 in young and old, respectively), but men showed 16% greater loss of force compared with women (FI: 65%±9 vs 54%±7 in women and men, respectively; P=0.001). It is concluded that women fatigue less than men during intermittent contractions but not during prolonged isometric contractions and that older muscles are more resistant to fatigue when performing sustained isometric contractions. The most likely explanation for age differences is that with ageing there is a shift towards a slower phenotype with reduced energy requirements during prolonged isometric contractions. However, during brief intermittent contractions the energetic advantage of older muscle was no longer apparent. This may be in part due to reduced aerobic capacity and ability to regenerate ATP during the recovery intervals in the older muscle and in part due to the slower, older muscles being less efficient during the initial phase of internal shortening against series compliance that occurs at the start of brief isometric contractions.