Purpose-Females are often reported to be generally more resistant to fatigue than males for relative intensity tasks. This has been observed repeatedly for elbow flexors, whereas at the ankle sex differences appear less robust, suggesting localized rather than systemic influences. Thus, the purpose of this study was to examine sex differences in fatigue resistance at muscle groups in a single cohort and which factors, if any, predict endurance time.Methods-Thirty-two (16 female) young adults (19 to 44 yrs) performed sustained isometric contractions at 50% maximum voluntary isometric contraction to failure for elbow flexion and ankle dorsiflexion. Pain, exertion, and muscle electromyography (EMG) were assessed throughout. Selfreported baseline activity was measured using the International Physical Activity Questionnaire.Results-Females (112.3 ± 6.2 sec) were significantly more resistant to fatigue than males (80.3 ± 5.8 sec) at the elbow (p=0.001); but not at the ankle (p=0.45; 140.6 ± 10.7 vs. 129.2 ± 10.5 sec). Peak torque was greater in males than females (p < 0.0001) at the ankle (45.0 ± 1.7 vs. 30.1 ± 1.0 Nm) and the elbow (75.7 ± 3.1 vs. 34.4 ± 2.2 Nm). Peak torque was significantly related to endurance time at the elbow (R 2 = 0.30), but not at the ankle (R 2 = 0.03). Peak pain, rate of pain increase, peak exertion, EMG, and baseline physical activity did not differ between sexes.Conclusion-Sex differences in fatigue resistance are muscle group specific. Women were more fatigue-resistant at the elbow but not the ankle during a sustained isometric contraction. Further, factors which may contribute to fatigue-resistance for one muscle group (e.g. sex, peak torque) may not be critical at another.