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[Editor’s note: This story was contributed by Dr. Christy Lane, cofounder and COO, Vivametrica, who spoke earlier this week at VentureBeat’s HealthBeat 2014 conference. You can find more stories from HealthBeat here.]

Recently, Apple unveiled its sensor-laden smartwatch. Two of these sensors are meaningful to me as an exercise scientist: a heart rate monitor and an accelerometer. Having both of these sensors in the same gadget is an exciting opportunity to see just how reliable and helpful a health wearable can be.

Heart rate (HR) is a basic yet very powerful metric: your resting heart rate can tell a lot about your health, and your heart’s response to exercise can tell even more. Exercise physiologists have been using heart rate to prescribe and monitor athletic training programs for decades. Clinical exercise prescriptions are also based on heart rate ranges.

However, heart rate is not the be all and end all of health metrics, particularly for measuring intensity. The best physiological measure of intensity is oxygen consumption (VO2). The problem with VO2 is that it is difficult and expensive to measure. Generally, you need cumbersome equipment, a lab and an exercise physiologist to run the test. There are portable VO2 measurement units available now, but they’re pricey. Luckily, we can estimate VO2 based on heart rate. In fact, there is a linear relationship between the two. In the light-to-moderate ranges of activity, HR is a perfectly fine measure of intensity and fitness.

Now, HR is not a pure measure of exercise intensity, which is both a limitation and an advantage. Why? Because physiological processes aside from movement influence HR. Try this: put your fingers on the carotid artery in your neck and count the number of beats you feel in one minute. Now think about your most horrible boss. Think about a gut-wrenching breakup, or the traffic jam on your way to work. Now measure your heart rate again. It is probably higher now. Heart rate is influenced by emotion. It is also influenced by stress, temperature, nicotine, caffeine and health condition. This means that HR is not a perfect measure of physical activity intensity.

This is where the other Apple Watch sensor, the accelerometer, comes in. Accelerometers are fabulously accurate for measuring exercise intensity. They also work independently from the rest of your life, emotions and bodily functions. They’re a great objective and consistent tool for monitoring and prescribing physical activity.

Here’s why I’m excited about having these two sensors in one device. Think Jerry McGuire: they complete each other.  Heart rate can tell us about physiological processes that the cold and calculating accelerometers cannot. On the flip side, accelerometers provide us with accurate measurement unfettered by our morning coffee, stressful meeting or bad cough. Together, these metrics make a very powerful team.

Here are just two of the exciting applications for this power couple:

  • Stress: While an accelerometer can’t provide insight into stress, a heart rate monitor can. Long-term records of heart rate for individuals will be very powerful in stress assessment and management.
  • Cycling: People constantly complain to me that their fitness wearable device cannot account for their daily cycle. This is where heart rate comes in. Imagine a device that is smart enough to decide when HR is the better measure of intensity vs. the accelerometer.

From a scientific perspective, I am excited about how science will apply these two key health metrics, which haven’t before been readily accessible to consumers. From the perspective of a human being, I’m looking forward to foregoing the hefty heart rate monitor shoved under my sports bra. I feel better already.

Dr. Christy Lane is cofounder and COO of Vivametrica. She is an exercise scientist with expertise in physical activity and its relationship to health. Christy teaches anatomy and exercise sciences and focused her research on outcomes and treatment for common spinal disorders as well as exercise rehabilitation for special populations.

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