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There's a building tension. Visible is the frosty air emanating from my competitors as they execute the "pre-race" shuffle on the starting line.

A rapid movement of the hips.

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

Hold on. I need to start my device. The same goes for the runner next to me.

The person in the line expresses impatience. I'm not moving until… done! GPS ready, and I'm running.

Amidst the crowd, it's difficult to see a wrist without a smartwatch on it. And this extends beyond us runners who use them.

This is a booming, multi-billion pound market. Brands like Apple Watch lead the market, each with an broad range of various devices to suit a range of needs. Depending on how advanced the model you select, prices can range from £100 to a small fortune.

"I find it infuriating, I can't switch off."

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

"I wish it wouldn't say I've experienced a bad night's sleep."

A small sample of the comments I've received from like-minded device users when I question what they feel about theirs.

A Sea of Statistics

My thoughts on mine? At this moment, irritating. It's reporting I'm not hitting my target, leading me to doubt whether the trees I'm passing under are disrupting my GPS connection.

Millions of us attach these little monitors onto our arms and wear them constantly, apparently at ease with the fact they're constantly monitoring us.

Long past are the days when we depended on them exclusively to give us an idea of our step count. Currently, they can measure our nightly rest, heart rate, heart rate variability (HRV), our VO2 max.

It's difficult to find a health indicator they don't track.

But are they able to do what they advertise? And are the statistics we scrutinise a true reflection of the truth? Do they enhance our overall health, or do the multitude of metrics and motivational messages just add to our sense of overwhelm?

Addiction or Aid?

"I become a quite obsessed with mine," a wearer reveals.

While she appreciates how her device records her workouts, she finds some of its other features "excessive" at times - a feeling initially realised during a pregnancy.

Before she realised she could put it into a specific mode, her smartwatch repeatedly informed her she was failing to be energetic enough. In a new phase, the watch keeps telling her she's had a restless night.

"I've got a infant, it's unnecessary for me to be notified I've not slept," she says. "I'm acutely aware perfectly well."

Couldn't she just take it off?

"I suppose I could, I suppose, but I have this love-hate relationship with it," she continues. "I value it for its fitness insights, I sometimes question whether the additional features it can now do is possibly too much for me."

How Do They Work?

Every device of smartwatch has its own unique way of measuring your bodily functions and analysing your data, but the vast bulk use sensors on the back of the watch.

They typically shine minute LED lights onto your arm which can monitor blood flow, recording your BPM. Sophisticated devices analyse changes in the skin conductance that passes over the skin to gauge your stress levels.

Expert Perspectives

One expert says that for most people for smartwatches, it's a "difficult line" - while the ever-evolving tech could actually aid early diagnosis, "identifying issues before we feel unwell", it could also turn us the tracked into the "concerned but fine".

He points out that newer wearables can conduct tests like electrocardiograms (ECGs), which constantly monitor how stable the heart is. They can alert if someone is experiencing a heart arrhythmia, where there is abnormal electrical activity in the heart causing an arrhythmia.

This does not automatically signal someone is on the verge of a cardiac event, but it can give an preliminary indication they might be more at risk of a blood clot in the long term.

But how to interpret these readings is not straightforward. The expert worries that as additional features are incorporated, people may find it hard to fully understand their data.

"I remain sceptical that being able to monitor so many things is such a positive development," he says.

A professor specialising in cardiology echoes this sentiment. Her work into the effect of wearable tech on a cohort of cardiovascular patients suggested a minority of those who were given devices to monitor their condition experienced increased anxiety and were "more inclined to use medical services".

She observed a pattern with her patients - they saw a alarming metric on their watch. They felt anxious. Their pulse rose. They got even more concerned. They re-examined the data, and their heart rate rose further.

"Upon viewing data about ourselves that we can't properly interpret then, of course, we are going to become concerned," the psychologist says. "We monitor, we obsess - it becomes a self-fulfilling prophecy."