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There's a building tension. Visible is the cold breath emanating from my competitors as they execute the "pre-race" wiggle on the start line.

A rapid flick of the hips.

I'm at my local weekly parkrun event and I'm serious. Three, two, one...

Oh, hang on. I need to adjust my tracker. So does the guy alongside me.

The person waiting tuts. I'm not moving until… done! GPS ready, away I go.

In the mele, it's challenging to spot a arm without a fitness tracker on it. And this isn't only us runners who rely on them.

It's a booming, multi-billion dollar market. Companies like Garmin are prevalent, each with an wide selection of diverse devices to cater to a spectrum of budgets. According to how feature-packed the device you select, price tags can start from £100 to a small fortune.

"It makes me crazy, I can't relax."

"It comforts me… It's like a supporter supporting me."

"Stop telling me I've experienced a bad night's sleep."

A small sample of the reactions I've received from fellow tracker enthusiasts when I inquire what they think of theirs.

The Data Dilemma

My thoughts on mine? Right now, frustrating. It's indicating my speed is wrong, leading me to doubt whether the trees I'm running below are blocking my GPS connection.

Millions of us fasten these small devices onto our wrists and use them all day and night, apparently unconcerned with the fact they're perpetually tracking us.

Long past are the days when we relied on them solely to give us an idea of our step count. Now, they can track our nightly rest, heart rate, glucose levels, our VO2 max.

It's tricky to find a bodily function they aren't designed to measure.

But can they do what they say they do? And are the data points we analyse a faithful picture of reality? Do they improve our daily wellness, or do the plethora of readings and constant notifications just increase our daily stresses?

A Complicated Bond

"I become a bit obsessed with mine," an individual admits.

While she values how her device monitors her exercise, she finds some of its other features "overwhelming" at times - a feeling first discovered during a pregnancy.

Prior to discovering she could adjust the settings, her smartwatch repeatedly informed her she was failing to be active enough. In a new phase, the watch keeps telling her she's had a poor sleep score.

"I have a young baby, it's unnecessary for me to be informed my sleep was poor," she says. "That fact is perfectly well."

Why doesn't she just remove it?

"I suppose I could, theoretically, but I'm in this push-pull dynamic with it," she says. "I love it for its exercise data, I sometimes question whether all the other things it can now do is perhaps excessive for me."

Inside the Device

Every device of smartwatch has its own novel way of measuring your health metrics and processing your data, but the majority use optical sensors on the back of the watch.

They typically emit tiny, coloured LED lights onto your arm which can observe capillary action, recording your heartbeat. More advanced devices analyse changes in the skin conductance that passes over the skin to get an idea of your recovery state.

A Fine Balance

One expert says that in general for smartwatches, it's a "delicate equilibrium" - while the continuously improving tech could actually prevent illness, "identifying issues before we feel unwell", it could also turn us watch wearers into the "concerned but fine".

He points out that newer wearables can conduct tests like medical-grade monitoring, which constantly monitor how healthy the heart is. They can alert if someone is experiencing an irregular rhythm, where there is abnormal electrical activity in the heart causing an arrhythmia.

This does not mean someone is imminently going to suffer a heart attack, but it can give an advance notice they might be more at risk of a stroke in the long term.

But how to make sense of these figures is complex. The expert concerns that as additional features are introduced, people may find it hard to fully understand their data.

"I have some doubts that being able to monitor so many things is such a beneficial thing," he says.

A clinical psychologist focusing on patient behaviour agrees. Research into the effect of wearable tech on a set of heart condition patients suggested a significant portion of those who were given devices to monitor their condition experienced greater stress and were "had a higher tendency to use doctor's time".

She observed a cycle with her patients - they spotted a worrying figure on their watch. They became worried. Their pulse went up. They got more worried. They looked a second time, and their heart rate went up again.

"When people are presented with stats about ourselves that we can't properly interpret then, understandably, we are going to become concerned," the psychologist says. "We monitor, we re-check - it becomes a vicious cycle."