A wrist HRM works by using light to track your blood flow. The LED lights the tiny blood vessel capillaries (about the size of a hair) that carry the blood around your body. A sensor next to the light then measure how fast the blood is being pumped past which then gives you a heart rate reading. Pretty cool huh?
Chest strap monitors measure the electrical impulses generated directly from the heart to trigger a heartbeat. Having used both wrist and chest strap and an EKG machine (which are pretty darn heavy to carry when you’re working out) the chest straps are more accurate.
Wrist monitors are great for simple measurements, like your resting heart rate, but there’s a lot of factors that can inhibit an accurate reading during a good workout. Body sweat is a known problem which can prevent the sensor from making a clear reading. A lot of people tighten the wrist strap to try to prevent this which will only restrict the blood flow more. The distance from your heart to your wrist can affect the beats per minute (BPM) as the wrist arteries are narrower and blood flow naturally slows down by the time it reaches the extremities of the body. Most modern wrist sensors have been tested under non sport related conditions without the wrist strap. For daily or light activities such as walking, slow running/cycling studies show an average 85% accuracy, but for heavier workouts over 160bpm (good aerobic zone) this percent greatly decreases which means you could be seeing misleading information or none at all.
For either wearable I would recommend getting the most out of the item by looking at what it can actually do. If you’re training to specific heart rate zones or you have a heart condition/a family history of heart conditions the chest strap are a good investment. The straps are not the easiest to wear and you do need more equipment to record the information but they will give an accurate reading. If you want to track your resting heart rate or recovery heart rate after a workout then the wrist monitors are great.
The main point of a HRM is to help you track how hard your body is working on the inside. I work to heart rate zones with a lot of clients to help them achieve their goals faster but also to keep them safe. It’s great to see people working really hard and feeling accomplished after a good workout but if you push too hard you could be putting your body under too much stress. Learning how to use heart rate information to your advantage can really help with recovery, prevent injury and build strong endurance.