&gt; <i>how do you know it would take less &#x27;effort&#x27; to get the heart rate up for out of shape people?</i><p>It&#x27;s a staple of sports medicine. Fit people start out with a lower heart rate, and unfit people&#x27;s heart rate rises much more rapidly with exercise. Get the two to climb a set of stairs - the fit person won&#x27;t notice it, the unfit person will be slightly out of breath and have a noticeably higher heart rate (in some cases, may now even be able to feel their own pulse).<p>To reach beats-per-minute X, a fit person starts lower and has a longer ramp-up.