You check your smartwatch after a run. Your fitness score has dropped. You’ve burnt hardly any calories. Your recovery score is really low. It’s telling you to take the next 72 hours off exercise.

The worst bit? The whole run felt amazing.

So why is your watch telling you the opposite?

Ultimately, it’s because smartwatches and other fitness trackers aren’t always accurate.

Smartwatches can shape how you exercise

Using wearable fitness technology, such as smartwatches, has been one of the top fitness trends for close to a decade. Millions of people around the world use them daily.

These devices shape how people think about health and exercise. For example, they provide data about how many calories you’ve burnt, how fit you are, how recovered you are after exercise, and whether you’re ready to exercise again.

But your smartwatch doesn’t measure most of these metrics directly. Instead, many common metrics are estimates. In other words, they’re not as accurate as you might think.

1. Calories burned

Calorie tracking is one of the most popular features on smartwatches. However, the accuracy leaves a lot to be desired.

Wearable devices can under- or overestimate energy expenditure (often expressed as calories burned) by more than 20 per cent. These errors also vary between activities. For example, strength training, cycling and high-intensity interval training can lead to even larger errors.

This matters because people often use these numbers to guide how much they eat.

For example, if your watch overestimates calories burned, you might think you need to eat more food than you really need, which could result in weight gain. Conversely, if your watch underestimates calories burned, it could lead you to under-eat, negatively impacting your exercise performance.

2. Step counts

Step counts are a great way to measure general physical activity, but wearables don’t capture them perfectly.

Smartwatches can under-count steps by about 10 per cent under normal exercise conditions. Activities such as pushing a pram, carrying weights, or walking with limited arm swing likely make step counts less accurate, as smartwatches rely on arm movement to register steps.

For most people, this isn’t a major problem, and step counts are still useful for tracking general activity levels. But view them as a guide, rather than a precise measure.

3. Heart rate

Smartwatches estimate your heart rate using sensors that measure changes in blood flow through the veins in your wrist.

This method is accurate at rest or low intensities, but gets less accurate as you increase exercise intensity.

Arm movement, sweat, skin tone and how tightly you wear the watch can also impact the heart rate measure it spits out. This means the accuracy can vary between people.

This can be problematic for people who use heart rate zones to guide their training, as small errors can lead to training at the wrong intensity.

4. Sleep tracking

Almost every smartwatch on the market gives you a “sleep score” and breaks your night into stages of light, deep and REM sleep.

The gold standard for measuring sleep is polysomnography. This is a lab-based test that records brain activity. But smartwatches estimate sleep using movement and heart rate.

This means they can detect when you’re asleep or awake reasonably well. But they are much less accurate at identifying sleep stages.

So even if your watch says you had “poor deep sleep”, this may not be the case.

5. Recovery scores

Most smartwatches track heart rate variability and use this, with your sleep score, to create a “readiness” or “recovery” score.

Heart rate variability reflects how your body responds to stress. In the lab it is measured using an electrocardiogram. But smartwatches estimate it using wrist-based sensors, which are much more prone to measurement errors.

This means most recovery metrics are based on two inaccurate measures (heart rate variability and sleep quality). This results in a metric that may not meaningfully reflect your recovery.

As a result, if your watch says you’re not recovered, you might skip training — even if you feel good (and are actually good to go).

6. VO₂max

Most devices estimate your VO₂max — which indicates your maximal fitness. It’s the maximum amount of oxygen your body can use during exercise.

The best way to measure VO₂max involves wearing a mask to analyse the amount of oxygen you breathe in and out, to determine how much oxygen you’re using to create energy.

But your watch cannot measure oxygen use. It estimates it based on your heart rate and movement.

But smartwatches tend to overestimate VO₂max in less active people and underestimate VO₂max in fitter ones.

This means the number on your watch may not reflect your true fitness.

What should you do?

While the data from your smartwatch is prone to errors, that doesn’t mean it is completely worthless. 

These devices still offer a way to help you track general trends over time, but you should not pay attention to daily fluctuations or specific numbers.

It’s also important you pay attention to how you feel, how you perform and how you recover. This is likely to give you even more insight than what your smartwatch says.

Hunter Bennett is a lecturer in exercise science at Adelaide University. This piece first appeared on The Conversation.

If you’ve ever started a new workout routine with the best intentions only to find yourself skipping sessions by week two, you’re not alone. I’m the type to get trapped in the same cycle of burnout, where I go hard for a couple of weeks, feel exhausted, feel guilty, and repeat. For me, what finally broke that cycle wasn’t a new gym membership or a fancy fitness app, but a simple scheduling hack: the “3-3-3 rule.” I’d seen this rule applied it to general productivity, and all the same principles can apply to your fitness habits, too. Here’s how you can use the 3-3-3 rules to structure your workouts and create a habit that sticks.

What is the 3-3-3 rule?

The 3-3-3 “rule” (or “method,” or “gentle suggestion”) is essentially a weekly workout framework built around three types of movement, each done three times per week:

• Three strength training sessions. This includes lifting weights, bodyweight circuits, resistance bands, whatever builds muscle and challenges your body.

• Three cardio sessions. This includes running, cycling, swimming, jump rope, a dance class—what counts as “cardio” is up for debate, but here, I think of it as anything that gets your heart pumping.

• Three active recovery days. This includes light walking, yoga, stretching, foam rolling, and so on.

And yes, I realize this math adds up to nine intentional days of movement across a seven-day week. Here’s the thing: You do double duty some days, or skip workouts here and there, or adjust to a nine-day cycle, because the point isn’t rigid scheduling. The point is rhythm over a strict structure. For me, the 3-3-3 rule provides a sense of momentum that’s flexible enough to fit into real life, but consistent enough to actually stick to.

Why the 3-3-3 rule works for me

Before I get into how the 3-3-3 rule helped me specifically, let’s talk about why so many workout plans fall apart in the first place. I believe most of them make two classic mistakes. The first is doing too much, too soon. You go from zero to six days a week at the gym, you get burnt out, and the whole thing unravels. The second mistake is having no real structure at all—just vague intentions, like “I’ll work out when I can,” which never materializes into anything real for a lot of people.

For me, the 3-3-3 rule solves both of those problems. It gives me enough structure to build habit and momentum, but not so much intensity that my body and brain feel overwhelmed. I personally adore running, but I struggle to motivate myself to lift weights; the 3-3-3 rhythm here helped me find a middle ground between those two workouts. When I know I have three strength sessions to hit in a week (or nine-ish day cycle), I can look at my calendar and find three slots without too much drama or dread.

There’s also plenty of breathing room built into the plan, which was the biggest game changer for me. I used to have the (toxic) thought that my rest days were wasted days, which is a mentality that led to either overtraining or complete inactivity with pretty much no middle ground.

Plus, there’s something psychologically satisfying about the number three. I know and love the rule of threes in photography, comedy, survival tips, and all over the place.

How to make a 3-3-3 workout schedule work for you

The 3-3-3 rule has a ton of wiggle room for customization. Here are some ideas for how you can approach it:

For strength days, pick a format you actually enjoy. That might be a full-body circuit, a push/pull/legs split, or a class at your gym. (Boxing, anyone?) Your focus on these days should be a progressive challenge—push yourself, yes, but don’t obliterate yourself.

For cardio days, variety helps. Mix a longer, easier effort with a shorter, more intense session (like a 20-minute interval run). I know I’m biased, but cardio really shouldn’t feel like punishment.

For recovery days, resist the urge to “make them count” by sneaking in extra work. The whole point is to let your body consolidate the gains from your harder days. Walk, stretch, breathe, and trust the process.

Another practical tip: Pick a night to map out your 3-3-3 week ahead of time. You’ll probably find that the week arranges itself pretty naturally once you’re looking for those nine windows.

The bottom line

As always, consistency should always be your priority in fitness. If you’ve been struggling to find a rhythm, if your past workout plans have always fizzled out around week three, give the 3-3-3 rule an honest four-week try. Maybe start with a 1-1-1 month! After all, the 3-3-3 rule isn’t a hack to totally transform your physique, but I do think it can provide something way more valuable. Finding a routine that works for you—like the 3-3-3 rule works for me—is the first step to make exercise a reliable, sustainable part of your life.