Redditor noveltysin wore her Fitbit while she had sex, and shared the resulting graph of her heart rate with the world. Check out how well her graph matches the heart rates in Figure 1 of the classic 1956 paper Physiological Responses During Coitus.
You may have noticed some news around these weird sounding devices that measure “arousal”. They don’t. But they do measure changes in penile shape, and as such, can give users a rough estimate — in a non-invasive way — of how much blood is flowing into the penis during erection.