Somewhere near the top of every single one of the the roughly forty-kajillion internet listicles dedicated to the "surprising," "hidden," and "unexpected" health benefits of sex is the not-all-that-surprising-sounding factoid that bumping fuzzies basically doubles as exercise. In reality, however, there has been very little research done to support this claim.
The few studies that have investigated the physicality of sex have typically looked at things like heart rate and blood pressure – important but arguably basic physiological measurements. They've also been conducted primarily in laboratory settings – which, sure, probably falls into some specific category of kink, but for most people is probably a less-than-ideal environment for sexy time. It's not difficult to imagine, for example, how the wires from an echocardiogram, or the bulk of an oxygen-monitoring facemask, might interfere with one's (doubtless considerable) sexual talents, thereby confounding any attempt at accurate physiological measurement.