Cardiac output during physical exercises following real and simulated space flight.

During moderate physical work (after 5 min) the cardiac stroke volume in the cosmonauts who made 2--8-day flights in the Soyuz spacecraft was lower and the pulse rate was higher than the pre-flight level. By the end of the 49-day bed-rest experiment the stroke volume during physical exercises (after 5 min) was lower than at the 5th min of the pre-test period in both the supine and sitting positions. The cosmonauts performed physical exercises in the sitting position. Therefore, it can be assumed that the major factor changing cardiac output may be the effect of the earth's gravity against the background of decreased orthostatic tolerance. Results of simulated experiments give evidence that in both cases one of the major factors responsible for changes in the cardiac output was a decline in the contractile capacity of the myocardium.