Aim: 

Exposure to microgravity abolishes the hydrostatic gradient that keeps the blood in the lower limbs of standing humans on Earth. This induces the increase in central blood volume and initiates a neuro-humoral response leading to plasma loss. In turn, the drop of cardiac pre-load triggers morpho-functional modifications of the heart, which, accompanied by the disregulation of autonomic control system and by vascular adaptations, lead to cardiovascular deconditioning whose ominous effects become evident after landing, when the reduction of Q'_max and the resetting of the cardiovascular control determine the decrease of V'O_2max and the occurrence of orthostatic intolerance. To counteract deconditioning, artificial gravity, associated with exercise, has been proposed as an effective countermeasure. In this investigation, the physiological responses of humans performing exercise at G levels greater than 1 G were studied.

Methods: 

V'O₂, V'CO₂, V'_E, HR, SV and Q' were measured at rest and during cycling exercise at steady state (25, 50, 75 and 100 W) performed in a human centrifuge at 1, 1.5, 2 and 2.5 G on 14 male subjects.

Results: 

SV decreased with increasing G, but its decay was compensated by the increase of HR, so that Q' did not change as compared with 1 G condition. V'O₂ was proportional to the worklaod, but, as G increased, this relationship was displaced upward, so that at any given workload, V'O₂ was linearly related to G. The increase of V'O₂ at any workload was achieved at the expense of a larger peripheral extraction of O₂. Since the relationship between Q' and mechanical power was not affected by G, the relationship between V'O₂ and Q' was displaced downward by increasing G.

Conclusions: 

The increase of V'O₂ with G was likely the effect of a larger amount of internal mechanical work during pedalling. The modification of the relationship between V'O₂ and Q' with increasing G implies that the maximal aerobic power should be lower the higher the G.