Sex and age are reported to influence the maximal dynamometric performance of major muscle groups, inclusive of ankle plantar-flexors. Knee flexion (KF) also impacts plantar-flexion function from where stems utilization of 0[degrees] and 45[degrees] of KF for clinical assessment of gastrocnemius and soleus, respectively. The influence of KF, sex and age on dynamometric indicators of plantar-flexion fatigue was examined in 28 males and 28 females recruited in two different age groups (above and below 40 years). Each subject performed 50 maximal concentric isokinetic plantar-flexions at 60 deg[middle dot]s-1 with 0[degrees] and 45[degrees] of KF. Maximal voluntary isometric contractions were determined before and after isokinetic trials; and maximal, minimal and normalized linear slopes of peak power during testing. Main effects of and two-way interactions between KF, sex, age and order of testing were explored using mixed-effect models and stepwise regressions. At 0[degrees] and 45[degrees], the fatigue indicators in younger and older individuals were similar and not influenced by testing order. However, peak isokinetic power and isometric torque declined to greater extents in males than females and, moreover, KF exerted greater impacts on the absolute plantar-flexion performance and maximal-to-minimal reduction in isokinetic power in males. Because KF wielded no pronounced effect on fatigue indicators, this test may perhaps be used over time with no major concern regarding the exact knee angle. Our findings indicate that sex, rather than age, should be considered when interpreting dynamometric indicators of fatigue from repeated maximal concentric isokinetic plantar-flexions, e.g., when establishing normative values or comparing outcomes.