Gravitational stress and exercise

This paper deals with the influence of G forces on certain circulatory and metabolic adjustments to dynamic exercise. Leg exercise in the sitting position increases systemic mean pressure both at normal and at increased G, thus improving G tolerance. When the force of gravity is increased to three times its normal value, leg exercise produces a much more marked increment in stroke volume than in heart rate, which contrasts with the case at normal gravity. Beta-adrenergic blockade, which strongly inhibits the heart rate response to gravitational stress, but leaves the systemic mean pressure and G tolerance unaffected, exaggerates the dominance of the stroke volume increase in exercise at increased G. With leg exercise at increasing work loads at 3G, the O2 uptake tends to level off and the arterial lactates to rise at a lower load than at normal gravity, indicating that the upper limit for the O2 uptake is lowered by the G force. A progressive increase of the venous admixture in the lungs suggests that the primary limitation of the O2 transport occurs in the pulmonary gas exchange.