The extent of the growth changes in maximal work output during 10 s (MWO10), 30 s (MWO30), and 90 s (MWO90) of maximal repetitive knee flexions and extensions assessed on a modified Hydra-Gym machine was investigated in 84 boys and 83 girls, 9–19 yr of age. Body weight, fat mass and fat free mass by underwater weighing, and thigh volume and cross-sectional area were also determined. No difference was observed in the absolute MWO10, MWO30, and MWO90 between girls and boys at 9 and 11 yr of age. However, significant differences appeared between genders from 13 yr of age onward, anaerobic performances of the knee extensor muscles of girls representing about 75% or even less of those of boys. The analysis of variance revealed that maximal work output during the three knee extension tests was significantly greater in males as well as in females from 9 to 18 yr, regardless how performance was related to morphological characteristics. Performance in absolute values or expressed per unit of body weight, fat free mass, and thigh cross-sectional area for the MWO10, MWO30, and MWO90 tests were almost always significantly lower in both genders when performances of the 9-yr-old group were compared with those of the 13-yr-old group or older groups. Improvement in maximal work output during the 10-s, 30-s, or 90-s knee extension tests with age occurred mainly between 9 and 15 yr in both genders. The results of the present study show that there are gender differences in predominantly anaerobic performances during growth and reveal that increase in muscle mass does not appear to be the only factor responsible for the age- related increment in the anaerobic working capacity of the knee extensor muscles.
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