You pull on a watch, hit start, and a number appears on your wrist: 148. Is that your heart rate, or a decent guess? For a walk, it hardly matters. For training by heart rate, it matters more than most beginners expect, because a number that is 15 beats off puts you in the wrong zone.
A chest strap senses the tiny electrical signal each heartbeat sends through your chest, the same kind of signal an ECG machine reads. It sits on bare skin right next to the heart, so each beat shows up as a sharp, clean spike.
A wrist watch shines green light into your skin and measures how much bounces back. Blood absorbs green light, and each pulse changes the amount absorbed. This is called optical sensing (PPG). It works, but it is reading a blurry echo of the pulse in a limb that is also swinging, flexing and sweating.
The chart below is a simulation, not real measurements. It shows the pattern researchers and manufacturers describe: during steady effort the wrist reading follows well; when effort changes fast, or when your wrist is gripping and flexing, it lags and drifts while the strap stays close to the truth. Pick a workout and loosen the watch.
In the steady jog, a snug watch is close to the strap. Your heart rate barely moves, so a slightly delayed, smoothed reading is still a good one. That is why a wrist watch is perfectly fine for easy runs, walks and general step and sleep tracking.
In intervals, your heart rate climbs and falls within a minute. The optical reading needs several seconds of clean signal before it commits to a new number, so it trails behind on the way up and hangs on too long on the way down. Polar describes its H10 strap as excelling during hard intervals and rapid changes in intensity, while saying optical sensing is comparable for steady activity.
In lifting, your forearms tense around the bar, your wrist bends and blood flow under the sensor gets squeezed. Optical readings can wander for exactly that reason. A strap on the chest does not care what your hands are doing.
If you train by zones, for example keeping an easy run conversational, a reading that is 10 beats too low can talk you into going harder than you meant to. A reading that is too high can make you slow down for no reason. Neither is dangerous for most healthy people, but both make the training less useful. If you only want a rough sense of effort, you can also use the talk test and ignore the number entirely.
Also note what these sensors are not. Consumer straps and watches are fitness tools, not medical devices. If you feel dizzy, get chest pain, or see readings that worry you, stop and speak to a doctor. If you have a heart condition or use a pacemaker, check with your doctor before relying on any device to guide your training.
Probably not at the very start. If you walk, jog easily and lift, a watch you already own, or the effort scales you can feel (can you talk, can you do two more reps), will carry you a long way. A strap starts to earn its place when:
A strap is also not the only upgrade. Optical armbands worn on the forearm or upper arm sit away from the flexing wrist and are more comfortable for some people, though they remain optical sensors.
Most of what matters is the same at every budget. The extra money buys convenience, not a more correct heartbeat.
A wrist watch is good for steady effort and everyday tracking. A chest strap is better when effort changes quickly or when your wrist is busy gripping. Start with the effort you feel, add a strap when you find yourself training by numbers, and keep in mind that no consumer device replaces advice from a doctor.