PHOSPHOCREATINE KINETICS IN HUMANS DURING EXERCISE AND RECOVERY

System linearity was assessed for exercise induced changes in energeti cs of forearm exercise, P-31-NMR spectroscopy techniques, with 12.5-s serial measurements of [PCr], [Pi], [ATP], and [H+] were employed duri ng exercise and recovery transitions in four untrained men for moderat e (1.7 W) and heavy (3.6 W) exercise. Signal averaging was applied and data were analyzed by regression analysis using a first-older exponen tial model. The time constants for both [PCr] and [Pi] responses to mo derate exercise and recovery were not different both within and betwee n nuclei ranging from 32 to 35 s (P > 0.05). The time constants derive d from moderate exercise and recovery, when employed to construct pred ictive equations for heavy exercise and recovery, did not adequately d escribe [PCr] dynamics. Underestimation of the net hydrolysis of PCr d uring heavy exercise was associated with increases in [H+] as predicte d 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 du ring heavy exercise. We conclude that the metabolic system does not be have as a linear system. Therefore, the time constant and the net chan ge in [PCr]. W-1 must themselves be determined by work dependent combi nations of other system variables.