Maximal Oxygen Consumption Rates in One-Leg and Two-Leg Exercise: A Theoretical Model

Abstract

The goal of this project was to create a theoretical model to predict maximal oxygen consumption rates in one-leg and two-leg exercise. A MATLAB code was developed to simulate both capillary-level oxygen transport (in the legs) and systemic oxygen transport. Predicted values for oxygen consumption closely matched experimental data. The model was used to explain the trend of a lower maximal oxygen consumption rate in two-leg exercise compared to one-leg exercise. As activity increases from rest to one-leg exercise to two-leg exercise, the oxygen demand of the active components, the cardiac output, and the blood flow rate also increase. However, the fraction of cardiac output to the active leg(s) decreases when the second leg is activated. At the capillary level, the oxygen extraction is increased at the arteriolar end of each capillary, resulting in regions of hypoxic tissue towards the venous end. Venous oxygen saturation is decreased, leading to lower venous P02 returning to the lungs. The increased cardiac output decreases the time that the deoxygenated blood has in contact with the alveoli. As a result, arterial P 02 for blood exiting the lungs is lower. This decreases the pressure gradient between the tissue and the capillary and limits diffusive transport. In summary, the reduction of oxygen consumption rate per unit muscle mass in two-leg exercise relative to one-leg exercise is accounted for quantitatively by the model and shown to result from the combined effects of reduced flow and reduced oxygen saturation of blood to each leg in two-leg exercise.