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A sense of building tension. Visible is the chilled vapour emanating from my rivals as they do the "pre-race" jiggle on the starting line.

A swift twitch of the hips.

I'm at my local weekly parkrun event and I mean business. It's almost time...

Hold on. I must adjust my watch. So does the guy next to me.

The person behind us sighs. I won't start until… there! Signal acquired, and I'm running.

During the scramble, it's difficult to spot a arm without a smartwatch on it. And this extends beyond us athletes who rely on them.

It's a explosive, multi-billion dollar industry. Brands like Apple Watch are prevalent, all offering an extensive range of diverse versions to cater to a variety of lifestyles. Based on how high tech the watch you select, prices can vary between a hundred pounds to a significant investment.

"I find it infuriating, it's impossible to switch off."

"It reassures me… It feels like a companion guiding me."

"Please don't notify me I've had a bad night's sleep."

Just some of the comments I've had from like-minded tracker enthusiasts when I ask what they think of theirs.

The Data Dilemma

My current opinion on my own device? At this moment, irritating. It's indicating I'm not hitting my target, making me wonder whether the trees I'm running below are disrupting my satellite signal.

A vast number of us attach these compact devices onto our wrists and wear them all day and night, apparently unconcerned with the fact they're constantly tracking us.

Gone are the days when we relied on them exclusively to give us an idea of our pedometer reading. Currently, they can analyse our nightly rest, blood pressure, heart rate variability (HRV), our recovery score.

It's a challenge to find a bodily function they don't track.

But are they able to do what they say they do? And are the data points we obsess over a faithful picture of our actual health? Do they boost our mental and physical wellbeing, or do the avalanche of measurements and cheerleading alerts just increase our everyday anxiety?

A Complicated Bond

"I become a bit preoccupied with mine," one user admits.

While she values how her device tracks her exercise, she finds some of its other capabilities "overwhelming" at times - a feeling that became apparent during a pregnancy.

Until she found out she could put it into a specific mode, her smartwatch kept telling her she was not being active enough. Later on, the watch insists her she's had a poor sleep score.

"I have a young baby, it's unnecessary for me to be notified I've not slept," she says. "I know that all too clearly."

Why doesn't she just stop wearing it?

"I suppose I could, perhaps, but I have this push-pull dynamic with it," she continues. "I appreciate it for its exercise data, I sometimes question whether the myriad other functions it can now do is perhaps overkill for me."

How Do They Work?

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

They commonly project small, green LED lights onto your skin which can detect capillary action, recording your BPM. More advanced devices detect changes in the bioimpedance that flows through the skin to gauge your stress levels.

The Professional Viewpoint

A professor says that in general for smartwatches, it's a "difficult line" - while the ever-evolving tech could actually save lives, "identifying issues before we notice anything", it could also turn us the tracked into the "concerned but fine".

He points out that newer wearables can conduct tests like medical-grade monitoring, which regularly monitor how normal the heart is. They can flag if someone is experiencing a heart arrhythmia, where there is irregular signals in the heart causing an uneven pulse.

This does not automatically signal someone is imminently going to suffer a cardiac event, but it can give an preliminary indication they might be more susceptible of a heart problem in the long term.

But how to interpret these numbers is complicated. The expert concerns that as further capabilities are added, people may not be able to fully comprehend their data.

"I remain sceptical that being able to monitor so many metrics is such a beneficial thing," he says.

A health researcher specialising in patient behaviour agrees. A study into the effect of wearable tech on a set of heart condition patients suggested a minority of those who were given devices to monitor their heart health experienced heightened worry and were "had a higher tendency to use medical services".

She identified a vicious circle with her patients - they spotted a concerning reading on their watch. They felt anxious. Their BPM increased. They got even more concerned. They re-examined the data, and their reading rose further.

"If we see numbers about ourselves that we lack context for then, of course, we are going to seek clarification," the psychologist says. "We monitor, we check again - it becomes a self-fulfilling prophecy."