The best fitness tracker? Your brain

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The most sophisticated fitness tracker available, according to a report last month in the journal Neuron, is the one hardwired into your brain.

In a study of treadmill-running rats with brain implants, researchers at Boston University demonstrated the existence of "odometer neurons" that fired at regular intervals corresponding to how far the rats had run and how long they had been running. Some of the neurons fired every eight seconds, for example, while others would fire every 400 centimetres.

What's remarkable is that the rats were on a treadmill, which means they had no external cues (such as landmarks or passing scenery) to help them. The brain itself, it turns out, is sensitively attuned to the passage of time and distance – a finding that gives added resonance to an ongoing debate about whether high-tech fitness monitoring gadgets are really superior to old-fashioned gut feelings.

Over the past few years, wearable fitness technology has jumped into the mainstream and become more sophisticated. Instead of just tracking your speed, you can monitor your running stride and lactate levels in real time; instead of heart rate, you can track heart-rate variability, which is the subtle changes in the time between successive heart beats.

To be useful, of course, the information provided by these devices has to be accurate. Researchers at the University of Toronto recently published an analysis in the journal BMC Research Notes of the three most popular free pedometer apps for smartphones – Accupedo, Moves and Runtastic Pedometer – and found that all three had "an unacceptable error percentage," sometimes higher than 50 per cent, in their estimates of the number of steps taken by subjects.

Still, there's little doubt that more specialized fitness devices such as GPS watches can offer greater precision than, say, your odometer neurons. The more important question is whether this additional precision is useful – and whether learning to rely on it has any downsides. Does your brain need practice to get better at self-monitoring?

"I'm sure it does," says Howard Eichenbaum, the senior author of the Boston University study. "I suspect that folks who run on a treadmill every day can make a pretty good guess about how long and for what distance they have been on a treadmill."

This ability isn't just a neat parlour trick. The problem with external measurements of your effort is that they can't take into account bad weather, a long day at work or other factors that might make the same pace feel easy one day and hard the next. That means you'll sometimes push too hard on days when you needed a break and not hard enough on those days when everything feels easy but your watch tells you you're at the "right" pace.

Some of these problems can be avoided by gauging your body's internal response through heart rate or more exotic measurements such as lactate or glucose levels. But even heart rate isn't a perfect gauge of effort, since it's affected by factors such as how hydrated you are. A lower-than-usual heart rate during exercise can mean you're getting fitter – or that you're dangerously overtrained.

The same issues arise when you try to monitor overall levels of fatigue and recovery before or after exercise, an area in which heart-rate variability is currently garnering a lot of hype.

Researchers at Deakin University in Australia recently combined the results of 56 studies that compared the effectiveness of "objective" and "subjective" measures of training fatigue. The objective category included simple measures such as heart-rate variability and blood pressure, as well as blood and saliva tests of hormone levels, immune function, inflammation and so on. The subjective category involved questionnaires that, in essence, asked the athletes how they were feeling.

The results, published in the British Journal of Sports Medicine, found that, in about half the studies, the subjective and objective measures were equally good at detecting when the athletes were training too hard. In the remainder of the studies, the subjective measures were more sensitive 85 per cent of the time.

So why is it that fitness technology is so alluring? Part of the problem is that relying on feelings seems "soft" and open to manipulation, says Anna Saw, the lead author of the Deakin review. "Athletes may deliberately be dishonest if they want to manipulate how they are perceived by their coach," she points out. And we're not above lying to ourselves, either.

For that reason, fitness technology can be a valuable reality check to make sure we're reading our gut feelings correctly. The data collected by fitness devices can also be a great motivational tool; graphing your progress can be, dare I say it, fun.

But it's also worth making sure that you don't neglect your odometer neurons, and all the other subtle and complex self-monitoring tools that come preloaded in the human brain. My litmus test is this: Before you start your workout, imagine that you've forgotten all your fitness technology at home. Can you still do the same workout? If not, there's a problem.