Heart rate variability (HRV), one of the important measures utilised in wearable technology, offers information on the physiological state of the body. A new study using Garmin watches set out to find out how accurate these types readings are in assessing our mental and physical readiness. It turns out its a mixed bag.
With the rise of smartwatches and other wearable technologies, automated measurement of a range of psychophysiological markers, such as heart rate and accelerometer data, to provide personalized feedback has found widespread use. Many of us have, in fact, become vitals metrics junkies!
HRV, in particular, has become popular. Nowadays, a lot of smartwatches come equipped with sensors that spit out such readings. They can monitor variations in HRV and give users an assesment of their physiological state. The data can be utilised for a number of things, such as tracking sleep patterns, keeping tabs on stress levels, and figuring out how much physical exercise has an effect on the body. The measure is heavily used by devices such as WHOOP, and more recently Garmin and Polar.
This increased use of HRV in smartwatches has opened up new avenues for health and wellness research and development. The purpose of this new study was to investigate the relationship between resting HRV during sleep and perceived mental and physical fitness.
Difference between physical and mental fitness
Physical and mental fitness are both essential for our daily functioning. Both are similar in some ways, but they are very different in others.
Physical fitness is the body’s ability to perform physical activities effectively and efficiently. It includes aspects such as strength, endurance, flexibility, and cardiovascular health.
Mental fitness, on the other hand, refers to the mind’s ability to deal with daily stressors and challenges, maintain positive thinking and emotional well-being, and effectively perform cognitive tasks. Emotional regulation, memory, attention, and problem-solving abilities are all components of mental fitness.
HRV is a measure of both of these. This is because both physical and mental fatigue can contribute to HRV values. For example, physical fatigue occurs when the body’s energy resources are depleted. In this case the autonomic nervous system (ANS) shifts to sympathetic dominance, lowering HRV. Mental fatigue or stress from prolonged and intense cognitive activity can have the same effect.
The study – its a mixed picture
Over the course of a month and a half, 63 military personnel wore a Garmin wrist-worn wearable and completed a short morning questionnaire on their perceived mental and physical fitness. The heart rate and accelerometer data were collected using a custom-built smartphone app. The data was then subjected to sleep detection and artefact filtering algorithms.
Resting HRV during sleep was found to be a small predictor of perceived physical fitness, with a marginal R2 of.031, but not of mental fitness. An R2 value of 1 indicates that the model is perfectly fitting, while a value of 0 indicates that the model does not explain any of the variance in the dependent variable. The items on perceived mental and physical fitness were strongly correlated (r =.77). This indicates that resting HRV is more related to perceived fitness’s physical component than its mental component.
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The findings of this study emphasise the significance of resting HRV as a predictor of perceived physical fitness. More research, however, is needed to better understand the relationship between mental fitness and resting HRV.
The results are encouraging for smartwatches and fitness trackers that use HRV to determine your fatigue and physical readiness to train. But it also leads to the conclusion that you should not put as much stock into mental stress figures provided by these same wearables.
Source: de Vries, H., Oldenhuis, H., van der Schans, C. et al. Does Wearable-Measured Heart Rate Variability During Sleep Predict Perceived Morning Mental and Physical Fitness?. Appl Psychophysiol Biofeedback (2023). https://doi.org/10.1007/s10484-022-09578-8
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