Which muscle has the greatest effect on the return of venous blood and lymph to the heart?

The diaphragm plays a crucial role in the return of venous blood and lymph to the heart due to its unique function as a muscle of respiration. When the diaphragm contracts during inhalation, it causes a decrease in pressure within the thoracic cavity, creating a vacuum effect. This negative pressure helps facilitate the movement of blood in the veins from the peripheral circulation back toward the heart. Additionally, as the diaphragm moves up and down, it also assists in promoting lymphatic drainage.

While the rectus abdominis, intercostals, and sternocleidomastoid muscles all play important roles in various bodily functions, they do not have the same significant direct impact on venous return as the diaphragm. The rectus abdominis is primarily involved in trunk flexion and stabilization, the intercostals aid in the mechanics of breathing but do not create the same pressure differential as the diaphragm, and the sternocleidomastoid is primarily involved in head and neck movement. Thus, the diaphragm's action is essential for efficient circulation and lymphatic flow, making it the muscle with the greatest effect on the return of venous blood and lymph to the heart.