While you're working out, a few things are happening: Metabolic byproducts, like lactic acid, are building up in your muscles. Your heart is pumping blood into your muscles to help flush out these byproducts. This blood makes the muscles swell up. As the muscle swells up, it is harder for the blood to flow out, causing it to build up. This cycle continues as the exercising goes on. Once the exercising stops, this "pump" state goes away as the heart stops pumping as much blood into the muscles and the muscle fibers return to their normal size. This is a simplified explanation, but it will suffice for now.