Muscle Phosphocreatine Kinetics In Children And Adults During High-intensity Exercise

Abstract

PURPOSE: To compare the age and sex differences in the kinetic response of muscle PCr during high-intensity exercise in children and adults. METHODS: Eleven adults (6 men 25 ± 5 y and 5 women 23 ± 3 y) and 11 children (6 boys 13 ± 0.2 y and 5 girls 13 ±1.3 y) exercised using their right leg on a quadriceps ergometer within a 1.5 Tesla MR scanner. Following habituation, each participant completed an incremental test to exhaustion. After a minimum 48 hours recovery, participants completed two to four constant work rate bouts on separate days. Exercise bouts consisted of 2 min rest and 7 min exercise, at an intensity equivalent to 20% of the difference between the workload at the intracellular Pi/PCr threshold and the maximal workload. 31P spectra were collected every 6 s using a 6 cm surface coil positioned beneath the right quadriceps muscle. The breakdown of muscle PCr at the onset of exercise was modelled using a single-exponential function until the onset of the PCr slow component (SC). This point was identified using an iterative fitting window - where the time constant diverged from a plateau, a PCr SC was deemed to emerge. Data are reported as mean ± standard deviation and 95% confidence intervals (CI) are presented. A two by two factorial ANOVA was utilised to determine group differences. RESULTS: No significant interaction effect was found for the fundamental time constant (boys: 31 ± 10, CI 5; girls: 31 ± 10, CI 6; men: 44 ± 20, CI 6; women: 29 ± 14 s, CI 6 s; P=0.26), or the fundamental amplitude (boys: -39 ± 10; girls: -42 ± 9; men: -36 ± 9; women: -48 ± 9%; P=0.23), or the SC PCr (as a % of the difference between the fundamental amplitude and end exercise PCr) which was similar across groups (boys: 6 ± 1; girls: 13 ± 8; men: 8 ± 5; women: 8 ± 6% P=0.15). The end-exercise PCr revealed a significant sex difference (P=0.03) but no significant age or interaction effect (boys: 55 ± 10; girls: 46 ± 14; men: 56 ± 11; women: 44 ± 8%). There was no significant difference between age (P=0.22) or sex (P=0.13) for the PCr cost of contraction at end-exercise (boys: 1.7 ± 0.5; girls: 1.3 ± 0.5; men: 1.4 ± 0.4; women: 1.2 ± 0.2 mM·W-1P=0.13). CONCLUSION: The kinetics of muscle PCr during high intensity exercise are similar in 13 y old children and adults, suggesting the phosphate-linked control of oxidative metabolism, at least during high-intensity exercise, is adult like by this age.