First, you need to decide between a classic chest strap that uses an electrical pulse to read your heart rate and something that uses optical technology. Many Fitbit devices, the Apple Watch, and other wrist-based activity trackers rely on the latter, as do sports headphones that record heart rate data via in-ear measurements. Without getting too technical, chest straps read the small electrical signal your body creates to make your heart contract. Optical technology sends light into the skin and reads what reflects. Based on that information and what we know about how light scatters when it hits blood, these sensors translate the data into a pulse measurement. Electrical technology tends to be more accurate. Measurements from the arm and wrist are more difficult because those body parts can swing rapidly during activity, thus creating noisier data. In-ear optical HRMs tend to be better than wrist-based ones because the ear doesn't move nearly as much. The skin of the ear works better for optical readings than the skin on the arm, as well.