Measures maximum oxygen consumption, resting heart rate, etc.
Sensor accuracy has already reached a level suitable for medical use.

Optical heart rate sensors are increasingly being incorporated into wearable devices.
The applications for this sensor are diverse, providing wearers with information ranging from sports and fitness activities to health status. This information allows the wearer to accurately understand their current condition and take quick action.

So, what can product developers measure with this sensor? The table below lists some of the most common measurements used in fitness applications today.

Measurement items	definition	Athletic ability	fig​
Maximum oxygen Consumption (VO₂ max)	Aerobicability. You may notice gradual changes in your cardiovascular health.	The higher it is, the better the athletic ability.	The higher it is, the lower the risk of death from heart disease and the faster the recovery.
Stable time heart rate	Heart rate while awake and not engaged in strenuous activity	The lower the better.	The higher it gets, the more advanced the cardiovascular disease becomes.
heart rate resilience	Heart rate changes over 1 minute after intense exercise	The higher it is, the better the exercise endurance.	font-size:10px;">The higher it is, the healthier your cardiovascular system is.
heart rate reaction	Heart rate changes during the first minute of exercise	The higher it is, the less prepared your heart is for exercise.	A high heart rate response and a decreased ability to increase heart rate may indicate atherosclerosis.
Cardiac efficiency	Average cadence divided by average resting heart rate	The higher it is, the less heartbeats are needed for physical activity.	If your heart efficiency is low, you may have high blood pressure.
Heart rate variability (HRV)	Statistical variability of heart rate intervals	H It is caused by psychosocial stress or excessive exercise.	It may indicate atrial fibrillation and arrhythmia.

While most wearable devices on the market today are designed for sports and fitness, their applications could expand to include medical applications. This is because the accuracy of the photoplethysmogram (PPG) sensors used in wearable devices has already improved to a level suitable for medical applications.

So, let's take a look at what optical heart rate sensors can measure and how they could be used in future health and medical wearables.

Maximal oxygen consumption (VO₂ max)
To understand your health, you need to measure physical activity and your body's responses to it. Some wearable devices can measure maximal oxygen consumption (VO₂ max), which can help measure chronic changes in cardiovascular health. A higher VO₂ max indicates improved performance during aerobic exercise. Wearable devices that measure VO₂ max can also be used for medical purposes. A higher VO₂ max is associated with a lower risk of death from heart disease and improved recovery capacity.

Resting heart rate
Resting heart rate is an indicator of fitness level. A relatively lower resting heart rate, especially when not engaged in strenuous activity, indicates better health. Lowering your resting heart rate can reduce your risk of developing cardiovascular disease.

Heart rate recovery
Heart rate recovery refers to the ability to return to normal levels after physical activity, and can be used to predict health and cardiac function. A healthier heart recovers faster. A higher heart rate recovery indicates greater endurance and cardiovascular health. Heart rate recovery is measured by measuring the difference between the heart rate during exercise and the heart rate 1-2 minutes after stopping exercise.

Heart rate response
Another useful measurement using a heart rate sensor is the heart rate response to exercise, measured approximately one minute after starting exercise. Increased muscle activity activates the sympathetic nervous system. The poorer your fitness, the stronger your heart rate response to exercise. Regular exercise allows your body to adapt. While your heart rate may seem to increase with exercise, it actually takes more intense exercise to achieve the same heart rate. A high heart rate response and increased cardiac activity, coupled with a decreased ability to elevate your heart rate, may indicate arterial disease.

Cardiac efficiency
Regular exercise improves cardiac efficiency, which is the ratio of cardiac activity to the energy expended to achieve it. The more efficient your heart is, the more you can perform physical activities with fewer heartbeats. Cardiac efficiency is a key indicator of heart health. Low cardiac efficiency is associated with many cardiovascular diseases, including high blood pressure.

HRV (Heart rate variability)
HRV refers to the degree of heart rate variability, or the subtle changes between one cardiac cycle and the next. HRV is linked to mental and physical exertion, allowing it to monitor the effects of stress on the body. HRV can also predict arrhythmia and atrial fibrillation. While previously only used in elite sports, HRV is now being utilized by the general public.

For more information on how HRV is measured and what the results indicate, see this article on heart rate variability by Dr. Chris Eschbach, director of the Biometrics Laboratory at Valencell.

organize
Recently, new wearable devices have been announced that can measure atrial fibrillation, arrhythmia, and core body temperature. All of these features utilize optical heart rate sensors using PPG. The features discussed above represent only a small fraction of the possibilities. Today, countless wearable devices already have these features ready to be used.

For more information on the above, see How Sensor Systems Work and What They Measure and 10 Lessons Valence Learned from Developing Over 50 Biometric Wearable Devices .

This article is based on a contribution from Texas Instruments.