Purpose: This study tested the hypothesis that the muscle metabolic responses of 9- to 12-yr-old children and young adults during incremental quadriceps exercise are dependent on age and sex.
Methods: Fifteen boys, 18 girls, 8 men, and 8 women completed a quadriceps step-incremental test to exhaustion inside a magnetic resonance scanner for determination of the muscle metabolic responses using 31P-magnetic resonance spectroscopy. Quadriceps muscle mass was determined using magnetic resonance imaging scans enabling comparison of metabolic data at a normalized power output.
Results: The power output and the energetic state at the Pi/PCr and pH intracellular thresholds (IT) were independent of age and sex. The rate of change in Pi/PCr against power output after the ITPi/PCr (S2) was lower in boys (0.158 ± 0.089) and girls (0.257 ± 0.110) compared with men (0.401 ± 0.114, P < 0.001) and women (0.391 ± 0.133, P = 0.014), respectively, with sex differences present for children only (P = 0.003). Above the ITpH, S2 was more rapid in the men (−0.041 ± 0.022, P = 0.003) and girls (−0.030 ± 0.013, P = 0.011) compared with boys (−0.019 ± 0.007), with no differences between the girls and the women (−0.035 ± 0.015, P = 0.479). The increase in Pi/PCr at exhaustion was lower in boys (0.85 ± 0.38) than that in men (1.86 ± 0.65, P < 0.001) and in girls (1.78 ± 1.25) than that in women (4.97 ± 3.52, P = 0.003), with sex differences in both the child (P = 0.005) and the adult groups (P = 0.019).
Conclusions: During moderate-intensity exercise, muscle metabolism appears adult-like in 9- to 12-yr-old children, although both age- and sex-related differences in the "anaerobic" energy turnover are present during high-intensity exercise.