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Active Heart Rate: How to Measure It, Normal Values and Trends

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What is it?

Active heart rate is the rate at which the heart beats during activity or exercise. Heart rate is a highly responsive physiological measure, since it is in part responsible for increased cardiac output. Increasing cardiac output is vital during increased physical activity, since the metabolic demand is higher. It is important to note that heart rate is a response variable to many factors and therefore, the utility in measuring heart rate is to measure the body’s physiological reaction to the work being performed.

How is it measured?

There are many ways to measure heart rate during physical activity, but some are more reliable than others. The typical ‘gold standard’ method is through electrocardiography, whether 12-lead or single-lead. Many commercially available single-lead ecg-based chest straps are available and common among research groups due to their wireless nature, although 12-lead ECGs remain more popular among clinical settings due to the increased depth of information. 

Additionally, photoplethysmography can be used to measure pulse waves to obtain heart rate. Due to the nature of different wavelengths utilized during PPG measurements, green light LED appears to be best at detecting pulse waveforms during exercise, although still subject to some error. Red and infrared light appear to be highly prone to motion artifacts, and are not recommended for active heart rate monitoring. 

The Biostrap ecosystem contains both a chest-worn ECG-based heart rate monitor as well as a green light PPG sensor worn on the arm. The Biostrap EVO device, equipped with red and infrared PPG does not record during exercise due to motion artifacts. Therefore, the Biostrap activity heart rate monitors are recommended for use during activity to obtain proper active heart rate. 

Correlation with health

Use of heart rate during activity and exercise is not recommended for or capable of diagnosing medical conditions. However, heart rate during activity and exercise can provide a lot of information about cardiovascular health and performance. 

Increasing metabolic demand should lead to an increase in aerobic metabolism, thereby requiring increased oxygen delivery to the muscle tissues being used. This oxygen is vital for producing ATP (energy) to contracting skeletal muscles. Cardiac output (CO), as mentioned before, is the ability to supply oxygen to the contractile tissues; CO is a product of heart rate and stroke volume.

Therefore, when the heart becomes more efficient at pumping blood, increasing the stroke volume, heart rate will decrease to equal the same level of cardiac output. Typically, when examining individuals during exercise, a lower heart rate at an equivalent workload suggests increased cardiovascular health.

There are many other metrics that are designed to extract data from activity, particularly among sports and performance realms (e.g. rate of change, cardiac drift, heart rate reserve, and many more), but most will be beyond the scope of this review. 

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Normal Values

In theory, any value existing between resting heart rate and maximal heart rate are ‘typical’ values for exercising. Because exercise intensity is linearly related to heart rate, heart rate is often a reflection of relative intensity of physical activity. 

While many still suggest a percentage of maximal heart rate as a proxy of activity intensity, the heart rate reserve method (Karvonen method) is widely considered the most accurate method of monitoring intensity through active heart rate. The primary reason this is recommended is that 0% ‘work’ (sedentary) should equate to resting heart rate, but would be 0 beats per minute when using percent of maximum heart rate. 

Karvonen Formula

Heart Rate Reserve (HRR) = Maximum heart rate (MHR) – Resting heart rate (RHR)

%HRR is above resting heart rate. 

Monitoring Trends

In general, performing the same task, an individuals’ heart rate should be lower after cardiovascular fitness adaptations. 

However, many variables can influence heart rate during exercise that may alter this trend. Heat, emotional stress, caffeine consumption, movement economy, and dehydration are just some of the factors that can influence day-to-day variation in exercise heart rate at the same workload.

Cardiovascular adaptations may decrease the reactivity of heart rate to some of these influences, so a trend towards a lower heart rate should still be observed over time. 

Of note, this should not be confused with active heart rate during a single exercise bout. Heart rate should remain proportional to intensity, and thus depends on the workload applied to the activity. In theory, at a constant workload, heart rate should remain constant; however, in practice, long bouts of activity can lead to cardiac drift, which is a dissociation between heart rate and workload. This is common in lesser-trained individuals, particularly with factors that affect thermoregulation, but is also common in dehydration, hot environments, nutritional stress,  and other fatigue-related factors that influence heart rate.