This is partly a matter of accuracy. Wrist heart-rate monitors rely on LEDs embedded in the device to measure the flow of blood through the capillaries. The light reflects off the arterial movement of your pulse, translates the reflection as an ebb and flow, and turns that information into the number you see on your device. But bend you wrist while pressing a barbell or during a run, and you interfere with that LED-to-capillary transmission. In our testing, we’ve found the only way to avoid this issue is to stick with very tight-fitting sport watches: if there’s a gap between the LED and your skin, you get spotty data.