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A sense of building tension. Visible is the cold breath coming from my fellow runners as they perform the "pre-race" jiggle on the beginning point.

A rapid movement of the hips.

I'm lined up for my local weekly parkrun event and I'm serious. The countdown begins...

Oh, hang on. I have to sort my device. The same goes for the person next to me.

The person in the line expresses impatience. I won't start until… yes! Connection established, away I go.

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

This is a thriving, multi-billion dollar market. Names like Samsung Galaxy lead the market, all offering an broad array of diverse versions to suit a spectrum of budgets. According to how advanced the watch you select, price tags can range from £100 to a small fortune.

"It drives me insane, I can't relax."

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

"Please don't notify me about a restless night."

Just some of the comments I've had from other device users when I ask what they think of theirs.

The Monitoring Conundrum

My thoughts on my own device? Currently, annoying. It's reporting I'm not hitting my target, leading me to doubt whether the overhead branches I'm running below are disrupting my location tracking.

Countless individuals of us attach these compact monitors onto our arms and wear them all day and night, ostensibly at ease with the fact they're perpetually recording us.

Long past are the days when we depended on them only to give us an estimation of our step count. Today, they can measure our nightly rest, heart rate, oxygen saturation, our VO2 max.

It's tricky to find a bodily function they fail to monitor.

But can they do what they claim to do? And are the numbers we scrutinise a faithful picture of reality? Do they improve our mental and physical wellbeing, or do the avalanche of metrics and constant notifications just contribute to our sense of overwhelm?

A Love-Hate Relationship

"I do get a quite obsessed with mine," a wearer reveals.

While she values how her device monitors her workouts, she finds some of its other features "excessive" at times - a feeling that became apparent during a pregnancy.

Before she realised she could put it into a specific mode, her smartwatch constantly alerted her she was failing to be active enough. In a new phase, the watch insists her she's had a bad night's sleep.

"I've got a young baby, it's unnecessary for me to be notified my sleep was poor," she says. "I'm acutely aware perfectly well."

Couldn't she just stop wearing it?

"I suppose I could, perhaps, but I've got this push-pull dynamic with it," she continues. "I value it for its exercise data, I sometimes question whether the myriad other functions it can now do is maybe excessive for me."

How Do They Work?

Different brands of smartwatch has its own unique way of monitoring your bodily functions and processing your data, but the vast bulk use sensors on the reverse of the watch.

They usually shine small, green LED lights onto your wrist which can monitor blood flow, detecting your heartbeat. Sophisticated devices detect changes in the electrical current that flows through the skin to gauge your stress levels.

Expert Perspectives

One expert says that in general for smartwatches, it's a "delicate equilibrium" - while the ever-evolving tech could actually prevent illness, "identifying issues before we show symptoms", it could also turn us device users into the "anxious healthy".

He points out that the most advanced wearables can perform tests like heart rhythm analysis, which continuously monitor how healthy the heart is. They can notify if someone is experiencing an irregular rhythm, where there is abnormal signals in the heart causing an arrhythmia.

This does not mean someone is on the verge of a cardiac event, but it can give an early warning they might be more susceptible of a stroke in the long term.

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

"I'm not entirely convinced that the ability to monitor so many things is such a good idea," he says.

A health researcher focusing on cardiology agrees. Research into the impact of wearable tech on a set of cardiovascular patients suggested a noticeable percentage of those who were given devices to monitor their heart health experienced heightened worry and were "far more likely to use healthcare resources".

She identified a pattern with her patients - they saw a alarming metric on their watch. They grew concerned. Their heart rate rose. They got increasingly anxious. They looked a second time, and their heart rate rose further.

"Upon viewing stats about ourselves that we can't properly interpret then, understandably, we are going to want to know more," the psychologist says. "We look, we re-check - it becomes a self-fulfilling prophecy."