Phosphocreatine kinetics in humans during exercise and recovery.

System linearity was assessed for exercise induced changes in energetics of forearm exercise. 31P-NMR spectroscopy techniques, with 12.5-s serial measurements of [PCr], [Pi], [ATP], and [H+] were employed during exercise and recovery transitions in four untrained men for moderate (1.7 W) and heavy (3.6 W) exercise. Signal averaging was applied and data were analyzed by regression analysis using a first-order exponential model. The time constants for both [PCr] and [Pi] responses to moderate exercise and recovery were not different both within and between nuclei ranging from 32 to 35 s (P > 0.05). The time constants derived from moderate exercise and recovery, when employed to construct predictive equations for heavy exercise and recovery, did not adequately describe [PCr] dynamics. Underestimation of the net hydrolysis of PCr during heavy exercise was associated with increases in [H+] as predicted by the creatine kinase equilibrium reaction (CKeq). Calculation of [ADP] by CKeq revealed steady state [ADP] was achieved during moderate exercise and during recovery for both intensities much earlier than during heavy exercise. We conclude that the metabolic system does not behave as a linear system. Therefore, the time constant and the net change in [PCr].W-1 must themselves be determined by work dependent combinations of other system variables.