The main cause of heaviness felt within the legs is often leaky, dilated leg veins, a condition called venous insufficiency. Blood circulation issues often occur in the lower part of the body, particularly the legs. This is mainly due to the veins in our legs that need to fight gravity to move blood out of the legs back up to the heart. If you have venous insufficiency the blood can end up pooling within the legs, ankles, and feet, which will result in the sensation of heaviness within the legs. Due to venous deficiency, you will feel the sensation of having heavy legs. This will occur due to the veins being heavily damaged and failing to circulate blood back to your heart. The heavy feeling may be due to the surplus of blood pooling within the legs, unable to flow sufficiently, often resulting in swelling and heaviness within the legs. If you do not get treatment for this, it can lead to spider veins or varicose veins, as well as other vein diseases. A few common causes of vein issues within the legs include sedentary lifestyle, obesity, hormones, genes, age, vitamin deficiencies, and lacking physical activity.