Reliability of 31P-magnetic resonance spectroscopy during an exhaustive incremental exercise test in children

This study examined the reliability of P-31-magnetic resonance spectroscopy (MRS) to measure parameters of muscle metabolic function in children. On separate days, 14 children (7 boys and 7 girls) completed three knee-extensor incremental tests to exhaustion inside a whole-body scanner (1.5 T, Phillips). The dynamic changes in the ratio of inorganic phosphate to phosphocreatine (Pi/PCr) and intracellular muscle pH were resolved every 30 s. Using plots of Pi/PCr and pH against power output (W), intracellular thresholds (ITs) for each variable were determined using both subjective and objective procedures. The ITPi/PCr and ITpH were observed subjectively in 93 and 81% of their respective plots, whereas the objective method identified the ITPi/PCr in 88% of the plots. The ITpH was undetectable using the objective method. End exercise (END) ENDPi/PCr, ENDpH, ITPi/PCr and ITpH were examined using typical error statistics expressed as a % coefficient of variation (CV) across all three exercise tests. The CVs for the power output at the subjectively determined ITPi/PCr and ITpH were 10.6 and 10.3%, respectively. Objective identification of the ITPi/PCr had a CV of 16.3%. CVs for ENDpH and ENDPi/PCr were 0.9 and 50.0%, respectively. MRS provides a valuable window into metabolic changes during exercise in children. During knee-extensor exercise to exhaustion, ENDpH and the subjectively determined ITPi/PCr and ITpH demonstrate good reliability and thus stable measures for the future study of developmental metabolism. However, the objectively determined ITPi/PCr and ENDPi/PCr displayed poor reliability.