Heart Rate Zones and Wearable Devices

Heart Rate Zones or HR Zones are levels of exertion during physical activities. As heart rate is readily monitored by most wearable devices, the intensity of workouts can be tracked and measured. Terra API has integrations with wearable devices that track heart rate zones directly or it can calculate it based on heart rate data.

As the intensity of a physical activity increases, the energy demands of the muscle must be supplied with more oxygen and nutrients, pushing the heart to pump blood faster to meet the demand. Eventually, at higher intensities, it will reach a point where energy demands outstrip oxygen supply and respiration in the muscle cells begins to go anaerobic (not using oxygen).

Heart rate zones are calculated based on a person's maximum heart rate, and divided into bands of intensity from low to maximal, 50% to 90%. The higher the intensity, the more muscles are utilizing anaerobic respiration versus aerobic respiration.

Unlike RPE, the Rate of Perceived Exertion to track how hard a workout is, HR zones can be measured objectively using heart rate trackers that are often installed on gym machines like treadmills, and wearable devices such as smartwatches and smart straps. Not only will HR zones tell you how hard you've pushed yourself, it also shows how long that intensity was sustained. HR zones are a crucial part of any cardiovascular and endurance training program.

What wearable devices track Heart Rate Zones

Whoop, Fitbit, Garmin, Suunto, and Apple Watch support heart rate zones (see all of them here), though Fitbit has different definitions for the zone boundaries, and in Apple watches, the user sets their own zones. But in general, any smartwatch that has a heart rate monitor can be used to monitor heart rate zones during training. Terra API makes it simple for fitness apps to extract heart rate zone data even if the user's wearable does not natively support it, through our fitness API.

How are HR zones calculated?

Heart Rate zones are fractions of the person's maximum heart rate. The maximum heart rate can either be measured (by tracking a person's heart rate over a long period, or stress testing) or estimated. The formula 220 - person's age is often a good substitute for maximum heart rate. For a 20 year old for example, their maximum heart rate is 200.

What are the heart rate zones?

Starting at 50%-60% of a person's max HR, the lowest heart rate zone is known as Zone 1. The subsequent zones at 60%, 70%, 80%, 90% etc are Zones 2, 3, 4, and 5. These will be explained below:

Zone 1 (50%-60% Max HR) - Very Light intensity

Zone 1 is the lowest intensity level for physical activity, usually on a walk or easy cycle session. Exercise in this state is entirely aerobic and can usually be sustained for long periods of time if not indefinitely.

This is the level of intensity usually recommended to build endurance as well as active recovery after higher-intensity days.

Zone 2 (60%-70% Max HR) - Light Intensity

Zone 2 is the second lowest intensity level. Exercises like a slow but steady run or swim that can be sustained for long periods. Exercise at this state is still mostly aerobic with minimal lactic acid buildup.

This is the level of intensity that an athlete should train at to improve their long-distance endurance and stamina.

Zone 3 (70%-80% Max HR) - Moderate Intensity

Zone 3 represents moderate intensity. These exercises may involve faster runs, like tempo runs or their equivalent in other cardiovascular exercises. Exercises in this state can usually be sustained for moderate periods of time but lactic acid will eventually accumulate faster than it can be removed.

Training at this intensity will improve an athlete's efficiency and improve their speed endurance.

Zone 4 (80%-90% Max HR) - High intensity

Zone 4 is a high-intensity zone, where anaerobic respiration is dominant over aerobic respiration, meaning it cannot be sustained for more than a brief period of time. This is usually the heart rate zone competitive runners in a long-distance run will compete at, during training, this zone is usually only achieved in intervals with rest periods between.

Training at this intensity will improve speed. Long periods of Zone 4 activity will require recovery.

Zone 5 (90%-100% Max HR) - Maximal Intensity

Zone 5 is the highest or maximal intensity. Anaerobic respiration causing lactic acid will build up in a matter of minutes meaning this cannot be sustained for anything longer than a sprint. This zone is usually achieved only momentarily in sports where sudden bursts of speed are required, such as football or sprint events/training.

Training at this intensity will improve maximal speed. Any sustained effort at Zone 5, even a couple of minutes may require significant recovery.

Conclusion

Depending on a person's goals, whether it is health, fitness, performance or any combination of these, any training plan will only be optimal if you can track which heart rate zones they are training at; this is where wearable technology comes in. Using Terra API, fitness apps can have integrations with fitness wearables such as smartwatches and straps that directly output heart rate zones, but even for wearables that only display heart rate, our integrations can infer the heart rate zones so you can give meaningful recommendations to your users no matter what device they have!