There are many reasons why the legs swell once you exercise, and common causes include excessive blood return pressure, overstrain of the knee, varicose veins of the lower extremities, and venous valve closure insufficiency. Prolonged exercise can lead to elevated venous blood return pressure and reduced return flow, causing leg bruising and edema. Excessive exercise or incorrect exercise posture leads to knee joint strain, excessive friction and pulling of the knee joint, which can also lead to significant swelling of the legs. Prolonged exercise in the case of varicose veins of the lower limbs leads to obstruction of venous blood return and swelling of the legs. The main vein branches of the lower limbs have venous valve insufficiency, and when there is a lot of exercise, the pressure of blood return is higher, and the venous valve insufficiency makes the return blood flow back to the lower limbs, resulting in swelling of the legs. There may be other reasons for the swelling of the legs when you exercise, so it is recommended to go to the hospital in time to find out the cause of the disease and target treatment.