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There's a building tension. Apparent is the cold breath emanating from my competitors as they do the "pre-race" jiggle on the starting line.

A quick flick of the hips.

I'm at my local 5km community parkrun and I mean business. It's almost time...

Oh, hang on. I need to adjust my watch. As does the guy beside me.

The person in the line tuts. I'm not moving until… there! GPS ready, and I'm running.

During the scramble, it's hard to see a arm without a fitness tracker on it. And this extends beyond us participants who rely on them.

We're witnessing a thriving, multi-billion dollar sector. Companies like Apple Watch dominate, all offering an broad selection of diverse models to suit a spectrum of needs. According to how feature-packed the device you select, prices can start from £100 to thousands of pounds.

"I find it infuriating, I'm unable to relax."

"It comforts me… It's like a friend guiding me."

"Please don't notify me I've had a poor sleep."

A small sample of the reactions people have shared from like-minded tracker enthusiasts when I question what they their opinion is on theirs.

A Sea of Statistics

My feelings on mine? Right now, annoying. It's indicating I'm off pace, causing me to question whether the foliage I'm passing under are blocking my GPS connection.

A vast number of us strap these little devices onto our arms and keep them on all day and night, ostensibly unconcerned with the fact they're perpetually tracking us.

Gone are the days when we relied on them only to give us an idea of our pedometer reading. Today, they can analyse our sleep quality, heart rate, heart rate variability (HRV), our recovery score.

It's a challenge to find a physical metric they fail to monitor.

But do they do what they say they do? And are the data points we scrutinise a accurate representation of the truth? Do they improve our mental and physical wellbeing, or do the multitude of measurements and cheerleading alerts just increase our sense of overwhelm?

A Complicated Bond

"I find myself getting a quite obsessed with mine," one user admits.

While she appreciates how her device tracks her runs, she finds some of its other features "excessive" at times - a feeling initially realised during a major life event.

Until she found out she could put it into a specific mode, her smartwatch repeatedly informed her she was being insufficiently active enough. Later on, the watch frequently reports her she's had a bad night's sleep.

"I've got a six-month old, I don't need to be told my sleep was poor," she says. "That fact is only too well."

Couldn't she just stop wearing it?

"I suppose I could, theoretically, but I'm in this push-pull dynamic with it," she says. "I value it for its fitness insights, I sometimes question whether the additional features it can now do is perhaps too much for me."

Inside the Device

Every device of smartwatch has its own proprietary way of tracking your health metrics and processing your data, but the vast bulk use detectors on the back of the watch.

They usually shine small, green LED lights onto your skin which can monitor pulsation, identifying your pulse rate. Higher-end devices detect changes in the skin conductance that flows through the skin to gauge your nervous system activity.

Expert Perspectives

One expert says that overall for smartwatches, it's a "delicate equilibrium" - while the rapidly advancing tech could potentially save lives, "detecting disease before we notice anything", it could also turn us the tracked into the "worried well".

He points out that newer wearables can perform tests like electrocardiograms (ECGs), which constantly monitor how healthy the heart is. They can notify if someone is experiencing a heart arrhythmia, where there is erratic electrical activity in the heart causing an irregular heartbeat.

This does not mean someone is imminently going to suffer a heart attack, but it can give an preliminary indication they might be more at risk of a heart problem in the future.

But how to make sense of these numbers is not straightforward. The expert worries that as further capabilities are added, people may struggle to fully contextualise their data.

"I remain sceptical that having the capacity to monitor so many things is such a positive development," he says.

A health researcher studying cardiology agrees. A study into the effect of wearable tech on a set of cardiovascular patients suggested a significant portion of those who were given devices to monitor their cardiac function experienced heightened worry and were "more inclined to use medical services".

She identified a pattern with her patients - they noticed a concerning reading on their watch. They grew concerned. Their pulse increased. They got increasingly anxious. They re-examined the data, and their anxiety increased again.

"Upon viewing numbers about ourselves that we don't really understand then, understandably, we are going to become concerned," the psychologist says. "We look, we check again - it becomes a self-fulfilling prophecy."