The triceps surae muscle-tendon unit is one of the most commonly injured in elite and recreational athletes, with high prevalence in middle-age adults. Performing maximal numbers of unilateral heel-raises is used to assess, train, and rehabilitate triceps surae endurance; and conventionally prescribed in 0° knee flexion for gastrocnemius and 45° for soleus. However, the extent of muscle-selectivity conferred through change in knee angle is lacking for heel-raises performed to volitional fatigue. This study investigated the influence of knee angle on triceps surae muscle fatigue during heel-raises and determined whether fatigue differed between middle-age and younger-age adults. Forty-eight healthy individuals of 18-25 and 35-45 years performed maximal numbers of unilateral heel-raises in 0° and 45° knee flexion. Median frequencies and linear regression slopes were calculated from soleus, gastrocnemius medialis, and gastrocnemius lateralis surface electromyographic signals. Stepwise mixed-effect regressions were used for analysis. Subjects completed an average of 45 and 48 heel-raises in 0° and 45° knee flexion; respectively. Results indicated that the three muscles fatigued during testing as all median frequencies decreased and regression slopes were negative. Consistent with muscle physiology and fiber typing, fatigue was greater in gastrocnemius medialis and lateralis than soleus (p<.001). However, knee angle did not influence triceps surae muscle fatigue parameters (p=.814), with similar soleus and gastrocnemius fatigue in 0° and 45° knee flexion. These findings contrast with the traditionally described clinical use of heel-raises in select knee angles for gastrocnemius and/or soleus. Furthermore, no difference in triceps surae fatigue between the two age groups was able to be determined, despite the reported higher prevalence of injury in middle-age individuals.