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Performance in high‐intensity exercise is dependent on the ability to activate motor units. The main aim of this study was to test the hypothesis that adult men and women (age 19–27 years) are able to maintain higher levels of voluntary activation (VA) in knee extensor muscles than boys and girls (age 12–14 years). The volunteers (n = 7 in each group) performed three 5‐s maximal voluntary contractions (MVCs) and a continuous 2‐min MVC. The VA and fatigue of the muscles was assessed by applying 250‐ms 100‐HZ test tetani (TT100HZ). During brief MVCs girls showed lower VA than women, but the difference between boys and men was not significant. During the 2‐min MVC, VA in boys and girls was more depressed than in adults. The end‐exercise values of the relative TT100HZ torque correlated with the average VA during the exercise. Thus, the results of the study support the hypothesis that children are …