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A sense of building tension. I can see the chilled vapour emanating from my rivals as they perform the "pre-race" shuffle on the starting line.

A rapid movement of the hips.

I'm participating in my local 5km community parkrun and I'm focused. Three, two, one...

Hold on. I need to adjust my device. As does the runner beside me.

The person behind us tuts. I'm not moving until… there! Connection established, and I'm running.

Amidst the crowd, it's difficult to find a person without a fitness tracker on it. And this isn't only us runners who rely on them.

It's a booming, multi-billion pound industry. Companies like Garmin dominate, all offering an broad range of different models to fit a range of lifestyles. Depending on how feature-packed the watch you go for, price tags can start from a hundred pounds to a significant investment.

"It makes me crazy, it's impossible to switch off."

"It reassures me… It's like a supporter encouraging me."

"Stop telling me I've experienced a poor sleep."

Just some of the responses people have shared from other device users when I ask what they think of theirs.

The Data Dilemma

My current opinion on my own device? At this moment, irritating. It's reporting I'm not hitting my target, making me wonder whether the foliage along my route are interfering with my GPS connection.

Millions of us attach these compact gadgets onto our wrists and wear them all day and night, seemingly comfortable with the fact they're continuously monitoring us.

Gone are the days when we relied on them only to give us an estimation of our daily steps. Currently, they can track our sleep quality, blood pressure, glucose levels, our VO2 max.

It's a challenge to find a physical metric they aren't designed to measure.

But do they do what they claim to do? And are the numbers we obsess over a true reflection of reality? Do they boost our mental and physical wellbeing, or do the multitude of metrics and cheerleading alerts just contribute to our sense of overwhelm?

A Complicated Bond

"I become a quite obsessed with mine," one user confesses.

While she values how her device records her runs, she finds some of its other functions "daunting" at times - a feeling first discovered during a significant period.

Before she realised she could adjust the settings, her smartwatch repeatedly informed her she was failing to be energetic enough. In a new phase, the watch frequently reports her she's had a poor sleep score.

"I am caring for a young baby, it's unnecessary for me to be informed I'm tired," she says. "I'm acutely aware only too well."

Why doesn't she just remove it?

"It's possible, I suppose, but I have this love-hate relationship with it," she explains. "I value it for its exercise data, I sometimes question whether all the other things it can now do is possibly overkill for me."

How Do They Work?

Every device of smartwatch has its own proprietary way of measuring your health metrics and processing your data, but the most use detectors on the reverse of the watch.

They typically emit small, green LED lights onto your wrist which can monitor capillary action, recording your heartbeat. Sophisticated devices detect changes in the electrical current that passes over the skin to gauge your stress levels.

The Professional Viewpoint

A professor says that overall for smartwatches, it's a "difficult line" - while the ever-evolving tech could actually save lives, "detecting disease before we feel unwell", it could also turn us device users into the "worried well".

He points out that the most advanced wearables can carry out tests like electrocardiograms (ECGs), which continuously monitor how stable the heart is. They can alert if someone is experiencing an irregular rhythm, where there is abnormal electrical activity in the heart causing an irregular heartbeat.

This does not automatically signal someone is about to have a cardiac event, but it can give an early warning they might be more at risk of a heart problem in the years to come.

But how to understand these readings is not straightforward. The expert fears that as more functions are introduced, people may find it hard to fully contextualise their data.

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

A professor studying cardiology echoes this sentiment. Research into the impact of wearable tech on a set of cardiovascular patients suggested a minority of those who were given devices to monitor their condition experienced heightened worry and were "had a higher tendency to use healthcare resources".

She identified a cycle with her patients - they saw a worrying figure on their watch. They felt anxious. Their heart rate increased. They got increasingly anxious. They re-examined the data, and their anxiety increased again.

"When people are presented with stats 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 feedback loop."