Researchers agree that stroke volume increases above resting values during exercise such as in cross-country skiing. Most researchers agree that stroke volume increases with increasing rates of work, but only up to exercise intensities between 40% and 60% of maximal capacity. At that point, stroke volume is thought to plateau, remaining essentially unchanged up to and including the point of exhaustion.
When the body is in an upright position, stroke volume almost doubles from resting to maximal values. For example, in active but untrained individuals, it increases from about 50 at rest to 120 ml at maximal exercise. In Olympic cross-country skiers, stroke volume can increase from 80 ml at rest to 200 ml at maximal exercise.
During supine exercise, such as swimming, stroke volume also increases, but usually by only about 20% to 40%, not nearly as much as in an upright position such as in cross-country skiing. Why is there such a difference depending on body position?
When the body is in the supine position, blood does not pool in the lower extremities. Because of this, blood returns more easily to the heart, which means that resting stroke volume values are much higher in the supine position than in the upright position. Thus the increase in stroke volume with maximal exercise is not as great in the supine position as in the upright position of cross-country skiing. Interestingly, the highest stroke volume attainable in upright exercise is only slightly greater than the resting value in the reclining position. The majority of the stroke volume increase during low to moderate levels of work appears to be compensating for the force of gravity.