The heart is not strong enough by itself to get the blood back up the veins in your legs and back to your heart. The human body relies on a second system to finish that task. This system involves small valves throughout the veins and muscle contractions from your skeletal muscles when you walk and move about. The valves close when blood starts to flow in one direction, so that blood in the veins can only flow in the direction back to the heart, which is up the legs. When you squeeze your leg muscles to walk, stand, kick, and move about, the muscles squeeze the veins and force the blood to get moving. Because of the valves, the blood can only move in one direction as it gets squeezed along. So it is a combination of blood pressure from the heart’s pumping action, the valves, and muscle movement that gets the blood up the legs against gravity. If the valves malfunction, then the blood falls back down to some extent after every muscle contraction and begins to pool in the veins. This causes the veins to swell with blood, which can be painful and unsightly, and is known as varicose veins.

The book Varicose Veins and Related Disorders by David J. Tibbs states:

Gravity is an ever-present force against which the limb’s venous system is designed to compete in the upright position. If the valves fail, it is gravity that causes any blood that has been shifted upwards to fall back again and this has various effects. In the superficial veins it may cause visible tortuosity (varicose veins) but, much more damaging, the reflux of blood through superficial or deep veins may cause the rapid build up of an uninterrupted column of blood to the heart with consequent high venous pressure at the ankle… Only the peripheral pumping mechanism (musculovenous pumping) can cause full venous flow against gravity. This forceful pumping action is brought about when multiple veins are compressed by contraction of surrounding skeletal muscle (Figure 1.3). The valves direct the blood heartwards and prevent it from falling back again. Thus, by this simple arrangment, the harder the muscles work, the more vigorously is the massive flow of blood generated by this activity returned towards the heart.

Put simply, if you want to literally get the blood flowing in your legs, walk around and get your leg muscles moving.

Credit:https://wtamu.edu/~cbaird/sq/2013/06/07/how-can-the-heart-be-strong-enough-to-pump-blood-up-your-legs-against-gravity/