Final answer:
The Frank-Starling mechanism is more influential during low-intensity exercises due to the increased venous return and preload leading to more forceful heart contractions without rapid energy depletion, allowing sustained activity over longer periods.
Step-by-step explanation:
The Frank-Starling mechanism has more influence during low-intensity exercise because of the physiological adaptations that occur in the cardiovascular system. During low-intensity activities, vasodilation occurs in selected vessels which reduces resistance, allowing more blood to flow into the organs they supply. This increased blood volume then returns to the venous system, enhancing venous return, and as a result, the preload, or initial stretching of cardiac muscle before contraction rises.
According to the Frank-Starling principle, a higher preload results in a more forceful contraction of the cardiac muscle, efficiently pumping blood without the need for rapid heartbeat increases. Consequently, during low-intensity exercises that utilize aerobic metabolism and fatty acids as primary energy, the heart can sustain activity for longer periods without experiencing fatigue as quickly. Otto Frank and Ernest Starling, who researched this heart relationship, concluded that the heart's ability to increase its force of contraction is related to the degree of muscle fiber stretch, which is more effectively utilized during activities of lower intensity that do not rapidly deplete energy reserves.