The skeletal muscles, the muscles that power voluntary movements of the body, aid the circulatory system as the primary method by which deoxygenated blood is pushed back to the heart through the veins. Veins have thin walls compared to arteries and are easily compressed as skeletal muscles contract, forcing blood along the vein. Backflow is prevented by the presence of valves within the veins, a feature arteries largely lack.
Skeletal muscles are the main driver of venous blood; simply breathing also contributes to this process. When a person inhales, the diaphragm moves downward, decreasing the pressure in the thorax and increasing pressure in the abdomen. This decrease in pressure draws in air from the trachea, but it also draws in blood from the body, which is compounded by the increased pressure in the abdomen forcing blood out.
Veins can constrict, but they do not do this in a rhythmic fashion that sends blood back to the heart over time. Rather, veins constrict mostly to increase blood pressure. This becomes necessary in several situations, most notably when blood loss occurs. Gravity helps blood return to the heart from everywhere above the heart, most notably the brain. However, it makes return more difficult from the legs, so in general gravity is not an overall aid.