At the start of exercise oxygen uptake (VO₂) lags behind the energy requirement giving rise to an oxygen deficit. This is a measure of the amount of energy which has to be covered by other sources, mainly: 1) utilization of O₂ already stored in the body and 2) phosphocreatine (PCr) breakdown.

The present work was carried out to quantify the changes in the body oxygen stores from transient or steady state measurements.

Alveolar VO₂, cardiac output (Q; obtained from the signals of a photoplethysmographic system), and PCr signal, by means of ³¹P-MR spectroscopy, were measured non-invasively on 8 healthy volunteers (1.78±0.06 m; 70.1±9.7 kg) at rest and during plantar flexion exercise (8.6±2.4 W). A non-linear iterative mono-exponential fitting procedure was applied to determine the time constants of the O₂ uptake (t_VO2) and the PCr splitting (t_PCr) kinetics, respectively. Changes in the venous blood O₂ stores were calculated: 1) as the product of the alveolar steady state VO₂ above resting and the difference between the time constants t_VO2 and t_PCr () and 2) as the product of the estimated venous blood volume and the difference in the arterio-to-mixed-venous blood oxygen content between rest and steady state exercise (). t_PCr (25.0±4.9 s) was significantly shorter than t_VO2 (40.3±6.3 s; paired t-test, p<0.001, n=8). The (35.8±14.7 mL) were not statistically different from (35.3±19.5 mL; paired t-test, p=n.s., n=8) and were linearly related to each other (R²=0.595; p<0.05; n=8). The Bland–Altman plot between and shows: 1) no trend in the data as the mean venous blood oxygen store increases, 2) a 95% confidence interval from –25.3 to +24.3 mL, and 3) a bias of –0.57 mL.

Present results support the view that difference in the arterio-to-mixed-venous blood oxygen content between rest and steady state provide a good estimate of the changes in the venous blood O₂ stores at exercise onset, thus showing that it is possible to obtain information on variables characterizing the metabolic transient from data obtained at steady state, which can be considered the "memory" of the events having occurred at exercise onset.