I’m going to assume you’re talking about going from an upright to a supine position (on your back) while heart rate is significantly elevated following a vigorous aerobic workout — like running.
During a vigorous aerobic workout, a higher percentage of blood volume is directed toward legs and arms to support the working muscles and help dissipate heat. After the workout, an active, 5-10 minute cool-down — like easy walking — helps gradually return blood distribution to pre-exercise levels; less to the legs and arms, more to the central part of the body.
Certified group fitness instructors won’t have you move from peak aerobic activity to floor work without cooling down first, for good reason; exercise has a big effect on the cardiovascular system, and so do changes in posture. Skipping the cool-down and flopping down on your back rapidly increases what we call venous return – the volume of blood delivered back to the heart. A sudden increase in venous return places a higher demand on the heart — which might not be a huge deal for a healthy heart, but it can be dangerous when heart failure, arrhythmias, or other cardiac issues are present.
Here’s the tricky part — heart problems often go undiagnosed until there’s a big problem, and you don’t want to find out the hard way. Talk to your health care provider about your level of risk. And play it safe by cooling down properly after every workout, before lying down.