Nine months ago, Apple changed the way we looked at our wrists. As usual, Apple wasn’t first to the smartwatch game but they nearly nailed it on their first go. Apple Watch does so many things (and the list keeps growing) and Heath and Fitness is one of its major selling points. In fact, it’s so important that it’s prominently featured in Apple’s prototypical three-point marketing campaign.
According to Apple:
Fitness isn’t just about running, biking, or hitting the gym. It’s also about being active throughout the day. So Apple Watch measures all the ways you move…
The best watch is the one that you wear at all times. The same could be said for any fitness watch especially an activity tracker. Apple Watch may be a jack of all trades and master of none but what it isn’t is a one-trick pony (ahem, Garmin and Fitbit). Make no mistake, Apple Watch makes a fantastic fitness watch let alone smartwatch.
Since its release, the Apple Watch has been highly criticized on the accuracy of its optical heart rate monitoring.
Recently, DC Rainmaker panned Apple Watch as a fitness watch and surprisingly as a smartwatch.
Some excerpts:
“Some folks have tried to talk to things like HR accuracy, but many of their tests are flawed or silly-easy to pass.”
“Sometimes it’s mostly good, and yet other times it fails epically.”
“Cycling is the area where the Apple Watch struggled the most.”
The kickers:
“For 1/4th the cost, there are far better activity trackers from Fitbit with a crapton more features — and, they tend to do a better job too.”
“Throw out the Apple Watch if you want accuracy and reliability…”
The more I read the review, the more I realized it didn’t correlate with my Apple Watch heart rate data.
When examining my seventy-five workouts since getting the watch, only sixteen were of poor quality, five of which occurred while beta (more like alpha) testing the native Strava Apple Watch app in the summer. Seventy-nine percent of my workouts had high-quality captures, which I think is a great hit rate. On the indoor trainer, it’s been nothing but perfect.
Many sites delve into how to get the most accurate heart rate data out of Apple Watch. The combination of proper fit and body temperature is critical and will result in accurate ECG-quality data. However it’s not accuracy that we need to worry about. Sensitivity is the crux.
Understanding human physiology is important to getting optimum heart rate data. Optical heart rate sensors are based on measuring blood in the capillaries under the skin.
According to Apple:
Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate. In addition, the heart rate sensor is designed to compensate for low signal levels by increasing both LED brightness and sampling rate.
This is a key principle. When it’s cold outside, superficial capillaries constrict to keep the core warm diminishing blood flow to the skin and consequently heart rate sensitivity along with it. Additionally, exercise produces a similar effect by shunting blood from the skin to the muscles, which people don’t realize.
According to Vander, Sherman, and Luciano:
In response to sympathetic stimulation, some decrease in the blood flow through the skin can occur at the beginning of exercise. However, as the body temperature increases in response to the increased muscular activity, temperature receptors in the hypothalamus are stimulated. As a result, action potentials in sympathetic nerve fibers causing vasoconstriction decrease, resulting in vasodilation of blood vessels in the skin. As a consequence, the skin turns a red or pinkish color, and a great deal of excess heat is lost as blood flows through the diilated blood vessels. (Human Physiology 7th Edition).
For this reason, we see optical sensors struggle during the first portion of a workout when both the body is cold and muscles are hungry for blood.
It’s very evident in this high intensity workout plot done on the indoor trainer:
The heart rate in this graph clearly plummets during the first maximum effort interval (300+ watts for 3 mins). This is due to blood being shunted away from the skin to the muscles. Follwing this dip, the the sensor then needs about a minute to reacquire a signal. The second maximum effort interval of the workout at 13 min (again 300+ watts for 3 min) also indicates that I wasn’t properly warmed up.
DC Rainmaker’s results are similar:
The data from Apple Watch isn’t reliable until the user is properly warmed up.
When you gradually warm up, Apple Watch’s optical sensor is highly sensitive:
The information shown on this graph is again on the indoor trainer. I was properly warmed up (10 min instead of 5 min) before starting my intervals. There’s still a dip at 7 min indicating blood shunting to the muscles as the effort is ramped up. This shunting happens again at 14 min during the first hard effort. Furthermore, this workout also exemplifies Apple Watch’s sensor accuracy as 90% FTP (sub-threshold) interval workout perfectly correlates with Zone 3 (Tempo pace).
This is all well and good on an indoor trainer but how does Apple Watch fair under real world conditions?
According to DC Rainmaker, Apple Watch is shit. 💩
Mountain biking in the middle of January in Ohio has to be the most rigorous conditions to test of an optical heart rate sensor since it’s based so much on blood perfusion to the skin.
That day was cold. Temps just barely hit 30°F with snow covering the trail. These conditions should expose any vulnerabilities in Apple Watch’s sensor. Nonetheless, Apple Watch performed flawlessly and captured a high-quality heart rate profile.
As you can see, Apple Watch’s optical sensor struggled initially to find a signal for twenty minutes while I warmed up. Once blood was diverted back to the skin from the core and muscles, my heart rate was reliably tracked. The dip at 40 min was when I changed gloves and snapped the above photo. The key to getting a heart rate with Apple Watch is properly warming up and staying warm. A light sweat is a good indicator that you’ll get more reliable sensor readings.
Nonetheless, there is always an exception to the rule. This track was acquired during a 30–45ºF ride on a sunny day. Conditions were too warm to wear my long sleeve Winter base layer. So I went with a short sleeve Summer base layer and couldn’t warm up enough. My uninsulated arms were cold and Apple Watch struggled to find a signal. Ironically, only when I stopped to take a photo did it begin to start capturing at a high rate.
Mountain biking is a particularly hard sport for an optical heart rate sensor The constant vibrations, wind (especially when it gets cooler), and variable intensity make it far from a “silly-easy” test. Nevertheless, as long as you are properly dressed and warmed up and have the watch snug Apple Watch is solidly performs as a fitness watch.
Apple Watch heart rate sensor technology has been under intense scrutiny since its release. These concerns are baseless. The usual FUD applied to new, innovative Apple designs. Apple Watch’s sensor has been found to be ECG-accurate. A snug band and an understanding human physiology will set you up for success in getting accurate and reliable heart rate data. Dress accordingly and properly warm up and Apple Watch will solidly perform as a fitness watch.
