Aim: 

At the start of exercise oxygen uptake (VO₂) lags behind the energy requirement giving rise to an oxygen deficit. Previous works show that changes of the body O₂ stores greatly affect the time course of the VO₂ kinetics, as measured at the mouth, while they do not influence the time course of the phosphocreatine (PCr) breakdown, mirror image of the muscle VO₂. The present work was carried out to quantify the changes in the body oxygen stores and to assess their role in determining the metabolic adjustment at exercise onset.

Methods and Results: 

Alveolar VO₂ and PCr signal, by means of ³¹P-MR spectroscopy, were measured on 8 healthy volunteers (1.78±0.06 m; 70.1±9.7 kg) during plantar flexion exercise. The two time constants t_VO2 and t_PCr were calculated from the best fit of a mono-exponential function: 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). Changes in the venous blood O₂ stores (DO₂^s) were calculated as the product of the alveolar steady state VO₂ above resting and the difference between the two time constants. DO₂^s (mL) were not related to the t_PCr, while they were linearly related to t_VO2 (s), the regression being described by: t_VO2 = 0.36·DO₂^s + 27.47 (R² = 0.712, p<0.001).

Conclusion: 

Present results support the view that the PCr splitting kinetics is not affected by the changes in the body O₂ stores while the O₂ uptake kinetics at the mouth is really affected by the O₂ stores involvement.