A mathematician on Twitter has a favourite term she uses to describe reducing anything and everything one encounters in life to numbers: “mathfuckery”. To quote from what I consider a seminal Twitter thread, people who indulge in mathfuckery “like the flashy terms and ideas, and the fancy graphs and apps, but they have no desire to grapple with reality, and how little we actually know.”

This always reminds me of wearables and, specifically, the ones that people use for fitness tracking.

From the most extravagant Apple Watch that costs over Rs 50,000 down to the obscure Chinese smart bands (with sketchy names like Digibuff and Welrock) that cost in the region of Rs 500, they all broadly perform some of the same functions: tracking your sleep, your steps, your fitness activities, measuring some health parameters like heart rate, blood oxygen, etc, and offering some measure of interaction with your smartphone.

The history of fitness wearables as we know them today can be roughly traced back to the 2008 launch of Fitbit’s first device (called simply the Fitbit) which you clipped onto your clothing. From there, it has grown to be a $32 billion industry worldwide, propelled in no small measure due to the outsize success of the Apple Watch.

This phenomenon has embedded the idea in the public consciousness that you can and should “math” your way to fitness. As with all mathfuckery, this is a complex beast and it’s useful to have an understanding of what fitness trackers can and cannot do before you plonk down a certain amount of money on your next new gadget.

To get my personal biases out of the way, I don’t believe it’s worth spending tens of thousands of rupees on a fitness tracker or wearable. But since my brief is not to scold rich people for spending their money, I’ll get down to the actual functional aspects of these devices and where they do and don’t live up to their promise.

Steps

This was the main selling point of the original Fitbit, and the thing most trackers do quite well. They count steps and mostly do a solidly reliable job of it. This also gave rise to the 10,000-steps dogma, which is very popular around the world, and I have mixed feelings about. On the one hand, this target is extremely useful to motivate people, and strapping on a tracker and cranking out these steps is one of the simplest ways to introduce movement into an otherwise sedentary lifestyle. But the simple fact is that 10,000 is a very arbitrary number, and does not and should not be treated as some sort of holy grail.

Placing something like the number of steps you do within a broader fitness context is much more likely to give you a good understanding of its significance and utility. For instance, I try to hit a certain number of steps in a day to supplement my strength training workouts.

Distance

Many fitness trackers nowadays come with the ability to use your phone’s GPS to accurately track the distance that you have run, walked or jogged. There are even some (pricier) models that have an in-built GPS module so that you can go for a run without being weighed down by your phone. These (mostly quite accurate) measurements, combined with time, etc, give you a very good and reliable indicator of the work you’ve put in and the progress you’re making, so it’s a useful feature if running or walking is a part of your fitness regimen.

Sleep

Most fitness trackers track your sleep with varying degrees of sophistication. Some will stop at telling you what time you fell asleep and woke up, others will tell you the duration of your sleep cycles (deep, light, REM, etc) and yet others will even give you suggestions on what you can do to sleep better. There is some utility in this, but here it’s worth remembering that tracking your sleep is less precise than tracking your steps or distance, and in some devices it can be wildly inaccurate.

This functionality is less usable in devices like the Apple Watch that have less than a single day’s battery life, and more so in devices like the Xiaomi Mi Band which last for weeks, so you don’t have to decide between charging your device overnight and learning about your sleep.

Heart rate

Most trackers these days also include an optical heart rate sensor. By and large, these do a good job of giving you an idea of your heart rate during various times of the day including when you are active, but there are some limitations to the accuracy of this data. It’s still a useful feature and there are many known cases of the feature providing an actionable early warning sign for people regarding underlying health issues. The Apple Watch also has an EKG function but unlike the heart rate function, it’s not very accurate, and having the feature on your device can be a double-edged sword with false negatives providing a potentially false sense of comfort.

Blood oxygen

This is the latest hot feature among wearables, especially in light of the Covid pandemic when everyone has been rushing (sensibly) to buy pulse oximeters to keep tabs on the oxygen saturation in their blood. In short, do not rely on your wearable for SpO2 measurements. They need to be perfectly positioned to even take a shot at doing the measurement and can do a far worse job than the fingertip pulse oximeters. And for something like SpO2, where the margins are so fine, it’s best to pay no heed to the readings from an unreliable device.

Calories burnt

Wearables use a combination of data and algorithms to try and work out the energy that you expend. But according to a 2017 Stanford study “no wrist-worn monitoring devices report energy expenditure within an acceptable error range”. So, it’s best to not get hung up on the “calories” number that your tracker throws up beyond it being a rough guideline number.

Our instinct for mathfuckery drives us to place too much of a reliance on these numbers. If your game of football felt really strenuous but you “only burnt 250 calories”, don’t worry about it. Your tracker probably also told you the steps and the distance you ran and even your heart rate, which are all more useful numbers for you to draw conclusions from.

There are several more functions that these devices perform, but these are some of the headline features for which people buy fitness tracking wearables. And as you can see, they don’t quite deliver on all fronts equally.

So, while evaluating the data that your fitness tracker throws up (and, of course, while evaluating whether you should spend on a cheap tracker or a fancy smartwatch), it’s very important to keep these limitations in mind. It’s also important to not let numbers dictate your fitness regimen. For instance, if you lift weights, a fitness tracker adds very little value, whereas if you’re a runner, they can be very useful. Remember to keep these aspects in mind as well before you make a purchasing decision.

Contact the author on Twitter @vinayaravind.