“Life is like riding a bicycle. To keep your balance, you must keep moving.” - Albert Einstein

When the FitBit activity tracker first appeared in the market in 2011, it held the promise of motivating people to become more active. Health professionals embraced the product, promoting it as a source of knowledge that could empower people to change their behavior. Many assumed that, if people could only see how much they sit around all day, they would certainly start moving more. Over a decade of research has taken the shine off of activity trackers; studies have shown that within the first two or three months people typically become more active, but these changes tend to drop off over the long term once the initial fascination with the device wears off. Overall, wearables aren’t very good at changing behavior in the long term; changes that do happen tend to be within the first few months, after which use of the device becomes inconsistent or dropped altogether.

Interestingly, the very thing that health professionals thought would motivate people—knowledge--is what most users found to be the most problematic and downright annoying feature about fitness trackers. These devices—which are now referred to as “wearables” -- collect and report a great deal of data, but it can be hard to know what to do with all that information. Wearables collect lots of data—heart rate, heart rate recovery, active calories, total calories, pace, elevation gain—the list goes on and can vary between devices. For someone who just wants to be more active or drop a few pounds, the volume of information can be overwhelming, and many devices do a poor job explaining how this data relates to health goals. The only metrics that most people understand are calories and steps, which, of all the data wearables offer, are the two that are the least accurate and are generally not very useful.

If you’ve ever used a fitness tracker you may question how accurate they are. Many have put their hand into a bag of chips only to have their wearable congratulate them on reaching a step goal. Data is only as good as the methods used to collect and analyze it; when it comes to wearables, accuracy depends on a number of factors, including a person’s age, skin color, exercise intensity, and the algorithms used to analyze the information it collects.

Although some earlier versions were positioned on the arm, chest, or hip flexor, most wearables today are worn on the wrist. An incredibly sensitive area of the body, the wrist is an important junction where nerves, arteries, and veins all converge and thread through a small tunnel that leads to the hand. The skin here is thin, making it a good place to palpate and measure the pulse. Wearables use miniature sensors that shine through the skin to track both motion and heart rate. These sensors can detect body movements that are then interpreted as steps. To measure heart rate, the sensors detect variations in reflective light from tiny pulses in the arteries. This data is then plugged into algorithms that calculate caloric expenditure, time spent standing and moving, and overall fitness levels.

The wrist is a strange place to measure one’s steps. Humans tend to use their hands quite a bit which makes accurate sensing of steps challenging—every time you swing your arm it is interpreted as a step. It is not surprising that there is a great deal of variability in the accuracy of step counting; wearables are better at counting steps during brisk walking rather than during daily activities when they are more likely to mistake arm movements for steps. They also aren’t very good at counting steps when we are strolling along at slower speeds, and frequently miscount steps when we are walking at very fast speeds. Since these devices are being used to monitor heart patients, there is a loud call from those in the medical community to improve the accuracy of measuring steps. Until they figure out how to measure steps more accurately, we are better off using common sense. With a healthy dose of awareness, most of us know if we’ve been physically active or sedentary on a given day; with its lack of accuracy, step counts can offer arbitrary and misleading information.

Several years of research that looked at a wearable’s ability to measure energy expenditure found that accuracy depended on the intensity and mode of exercise being tracked; wearables overestimated caloric expenditure for less intense aerobic exercises, such as slower paced walking, while underestimated expenditure for higher intensity activities like running or competitive cycling. These problems may be related to the algorithms used for different activities as well as the sensors themselves. Studies have shown that the sensors of some wearables are less capable of reading pulses through darker skin tones, and many have difficulty reading pulses in older exercisers. When it comes to energy expenditure, research has found that wearables don’t do a very good job at accurately measuring calories. Depending on this data too heavily can lead us to miscalculate our caloric needs if we are trying to lose weight.

Although the new generation wearables can store and track a great deal of health information—some can even tell you if your earbuds are too loud--these devices were originally designed to simply measure our fitness levels. Our growing understanding of the intricacies of metabolism is shifting how health professionals view the role exercise plays in weight loss. We now know that exercise is incredibly important for immunity, stress resilience, heart health, diabetes prevention and management, as well as sleep hygiene and mental health. For weight loss, exercise plays a role, but it is a minor one and is not effective, by itself, to move the needle on the scale. Diet—what and how much we eat--has been shown to be the primary means by which a person loses weight. Investing in a wearable and then using it only to track calories is like buying the newest iPhone and using it only to send emails.

As our understanding of chronic disease deepens, fitness and body size are two different variables, with fitness being more important for cardiovascular and metabolic health, as well as our overall risk of morbidity and mortality. Although not as accurate as exercise testing done with professional equipment in a medical setting, wearables do a decent job tracking metrics that measure our general cardiovascular fitness. The data they offer can be much more meaningful to our fitness goals and go much further to help us to stay healthy rather than simply focusing on weight.

Fitness means different things to all of us, from being able to carry golf clubs, to hiking a mountain on the trip of a lifetime. From a physiological standpoint, fitness reflects how well our cardiovascular system delivers and utilizes oxygen. We can assess our fitness quite easily by measuring our heart rate, which most wearables do reasonably well (the exception being, as noted earlier, darker skin tones and some older exercisers). Many wearables offer information on heart rate trends for exercise sessions, allowing users to track changes when performing the same workout over time. As fitness improves, our heart rate will decrease as it performs at the same intensity of exercise, showing that the heart is getting stronger and does not need to work as hard to pump blood throughout the body.

We think of a healthy heart as having a steady, rhythmic beat, but the opposite is true—the more fit we are, the more variable our heart rate will be. At rest, a healthy heart shows small variations in the length of a beat and the timing between them. These changes are more dramatic during an exercise session; a healthy heart can shift from a high heart rate to a much lower one quite quickly. Variability indicates that the heart is adapting to its conditions and making adjustments to changing demands. A heart with low variability is doing all it can just to keep up. It may take longer for a less conditioned heart to reach a higher heart rate and much longer to return to a resting rate. Many wearables can measure heart rate variability by tracking fluctuations from one beat to the next. After an exercise session, wearables track variability by measuring how quickly your heart returns to a resting state; the faster your heart rate changes, the healthier your heart.

VO2max is a precise assessment of our overall cardio-respiratory health and is typically measured in a lab with special equipment. If you’ve ever had this test done, you most likely will not have fond memories. The workload is extreme and very uncomfortable—either running very fast on a treadmill or cycling hard on a stationary bike--as you breathe into a tube taped over your mouth that is collecting samples to be analyzed by sophisticated equipment. VO2max is the “gold standard” method of assessing one’s aerobic fitness, measuring the capacity of the heart and lungs to take in oxygen, distribute it throughout the body, and convert it into energy. Luckily, we can skip the torture of a lab test and get estimates of our VO2max with our wearables. Using algorithms that use heart rate and motion data, wearables can provide a rough estimate of how well we respond to the intensity and frequency of exercise. VO2max is a highly individualized number, but one that we can target for improvement. In many ways, it is the most reliable way of assessing how fit we are. Since fitness is more important for our morbidity and mortality, VO2max is a metric that has considerably more meaning than measuring calories.

Health data can provide us with coordinates about where we are and the direction in which we are moving, but it only stimulates change in our behavior when it has meaning, and when we know how to use it. While tracking health data, it is helpful to know what you intend to do with the information. How is it helpful? What will you change once you have that data? Importantly, we need to keep data in perspective, as it only tells us about one aspect of fitness. Our subjective experience of movement is just as important to our health; it is the real spark that motivates us to put on our shoes and get moving. A wearable tells us nothing about the feeling of accomplishment experienced when you run to the top of a hill that you could barely walk up six months earlier. We need to keep sight of why we want to be fit, focusing on the values that fuel our need to be healthy and live a long life. Without that context, all that data just becomes noise, and a device that could support us in living a healthy life gets lost in the dresser drawer.

For this week, I ask you to contemplate this question: How fit do you want to be and for what reason? So, for those who use activity trackers, what information can the device provide that will help you reach those goals? And if you are counting your steps, as so many people do, where are those steps taking you? Importantly, what do you hope you will find once you get there?