Simple smartwatch calculations that can save your life

A fitness tracker on your wrist may have more power than calculating steps or monitoring your sleep. A new study shows that the simple mathematical relationship between your heart rate and daily steps can make your glimpse of cardiovascular health more meaningful than traditional metrics, and they mark heart problems before they get serious.

Northwestern University researchers developed what they call “Daily Daily Heart Rate” (DHRPS), which calculates dividing a person’s average daily heart rate by its total daily steps. This novel approach was presented at the annual science conference held at the American College of Cardiology in March, and it could change how we use the data collected by millions of smartwatches around the world.

“The metrics we developed explore the heart’s response to exercise, not the exercise itself,” explains Zhanlin Chen, a medical student at Northwestern University’s Feinberg School of Medicine. “This is a more meaningful indicator because it encounters the core issue of the ability of all-day physical exercise to capture the heart’s ability to adjust under stress as physical exercise fluctuates. Our indicator is the first attempt to capture that heart with a wearable device.”

This concept addresses the basic limitations in routine fitness tracking. Although step counts can indicate activity levels, they hardly reveal the effectiveness of your cardiovascular system in handling that activity. Again, resting heart rate alone doesn’t capture your heart’s ability to react in different levels of effort.

Thousands of data reveal clear patterns

To validate their approach, the researchers analyzed impressive data sets from all NIH research programs, including FITBIT data and electronic health records of nearly 7,000 U.S. adults. The scale is surprising: 5.8 million people monitored and a staggering total of 51 billion steps.

What they found suggests that the calculation of DHRP can indeed provide valuable health insights. People with higher DHRPS scores (in the first 25 percentiles) are much more likely to have multiple cardiovascular diseases than those with lower ratings. These numbers tell a story about: about twice as likely to be type 2 diabetes, a 1.7-fold increase in the chance of heart failure, a 1.6-fold increase in the chance of hypertension, and a 1.4-fold increase in the risk of coronary atherosclerosis.

Notably, the study found that DHRP was stronger in relation to these cardiovascular diagnoses when considered daily heart rate or alone. This pattern remains true in a smaller subset of 21 participants who underwent a treadmill stress test, with DHRP more correlated with the maximum metabolic equivalent (METS) achieved during the test.

Heart disease remains the leading killer in the United States, with more lives every year than any other condition. Despite screening tests, many people will not accept recommended assessments. The potential of a passive, continuous monitoring system built into the device every day may represent a huge public health opportunity.

From research to real-world applications

The simplicity of this concept is perhaps its greatest advantage. As Chen pointed out, individuals can use data from smartwatches that have been collected to calculate this metric themselves, or they can integrate the calculations into fitness applications.

“Wearable devices are popular with consumers and wear out all day long, so they actually have tiny information about heart function,” Chen noted. “It’s a lot of information that can tell us a lot of things, and it’s necessary to further study how this details are related to patient outcomes.”

However, the researchers stressed that this study represents only the initial validation. Its cross-sectional design means they are unable to determine when to perform FITBIT measurements, which is an important limitation relative to when cardiovascular conditions are diagnosed. Interestingly, the study found no relationship between DHRP and the risk of stroke or heart attack, suggesting that this metric may better identify certain cardiovascular problems than other metrics.

Going forward, the team hopes to conduct more prospective studies with higher temporal resolutions to track DHRP at intervals per minute rather than using aggregated data over a few days. This improvement may enhance the predictive value of the metric.

The future of wearable health monitoring

This research comes at a time when wearable health technology is developing rapidly. Major smartwatch manufacturers continue to add advanced sensors to their devices, from ECG functionality to blood oxygen monitoring. The DHRP approach is unique because it does not require new sensors, which is just a smarter way to analyze data already collected.

If further validation is made, Chen recommends that DHRP or similar indicators be ultimately included in the standard heart disease risk assessment used by clinicians. This integration will represent an important step in bridging the gap between consumer fitness tracking and clinical health monitoring.

For millions of Americans who wear fitness trackers every day, the study suggests that their devices may have captured valuable health insights hidden in sight—and that could one day give early warnings about cardiovascular problems before traditional symptoms appear.

The question now is whether this mathematical relationship between heart rate and steps will be strong enough in larger populations and longer time frames to become a standard tool in preventive cardiology. If so, a fitness tracker on your wrist might be worth more credit than simply calculating your daily steps, which can quietly calculate a number that can provide meaningful insight into how your heart performs when you need it most.