Quadriceps Muscle Energetics during Incremental Exercise in Children and Adults

BARKER, A. R., J. R. WELSMAN, J. FULFORD, D. WELFORD, and N. ARMSTRONG. Quadriceps Muscle Energetics during Incremental Exercise in Children and Adults. Med. Sci. Sports Exerc., Vol. 42, No. 7, pp. 1303-1313, 2010. 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 P-31-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 P-i/PCr and pH intracellular thresholds (IT) were independent of age and sex. The rate of change in P-i/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 P-i/PCr at exhaustion was lower in boys (0.85 perpendicular to 0.38) than that in men (1.86 perpendicular to 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.