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A sense of building tension. Apparent is the cold breath emanating from my competitors as they do the "starting line" shuffle on the start line.

A swift twitch of the hips.

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

Oh, hang on. I need to start my watch. So does the guy next to me.

The person waiting tuts. I won't start until… yes! Connection established, and I'm running.

During the scramble, it's difficult to spot a wrist without a fitness tracker on it. And it's not just us runners who wear them.

This is a thriving, massive industry. Names like Samsung Galaxy are prevalent, each with an broad selection of various models to cater to a range of lifestyles. Depending on how advanced the device you choose, price tags can vary between a hundred pounds to a significant investment.

"It makes me crazy, I can't relax."

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

"I wish it wouldn't say I've experienced a restless night."

A small sample of the responses I've received from other tracker enthusiasts when I inquire what they think of theirs.

The Data Dilemma

My feelings on my own device? Currently, irritating. It's telling me I'm not hitting my target, leading me to doubt whether the overhead branches I'm running below are interfering with my satellite signal.

Millions of us attach these little gadgets onto our arms and use them constantly, seemingly at ease with the fact they're constantly recording us.

A distant memory are the days when we used them solely to give us an idea of our step count. Today, they can measure our sleep patterns, blood pressure, oxygen saturation, our recovery score.

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

But are they able to do what they say they do? And are the data points we analyse a accurate representation of the truth? Do they boost our overall health, or do the multitude of readings and motivational messages just increase our daily stresses?

A Complicated Bond

"I become a bit fixated with mine," one user reveals.

While she loves how her device monitors her runs, she finds some of its other features "daunting" at times - a feeling initially realised during a pregnancy.

Prior to discovering she could put it into a specific mode, her smartwatch kept telling her she was being insufficiently active enough. In a new phase, the watch keeps telling her she's had a restless night.

"I've got a six-month old, there's no point to be told my sleep was poor," she says. "I'm acutely aware only too well."

Why doesn't she just take it off?

"It's possible, I suppose, but I have this complicated dynamic with it," she continues. "I appreciate it for its workout tracking, I just wonder whether the additional features it can now do is maybe overkill for me."

Inside the Device

Each model of smartwatch has its own proprietary way of measuring your vitals and processing your data, but the vast bulk use sensors on the back of the watch.

They commonly project small, green LED lights onto your wrist which can detect blood flow, detecting your BPM. Higher-end devices detect changes in the electrical current that runs through the skin to get an idea of your recovery state.

A Fine Balance

One expert says that in general for smartwatches, it's a "delicate equilibrium" - while the rapidly advancing tech could actually aid early diagnosis, "spotting abnormalities 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 constantly monitor how stable the heart is. They can notify if someone is experiencing a heart arrhythmia, where there is irregular impulses in the heart causing an arrhythmia.

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

But how to understand these numbers is not straightforward. The expert concerns that as additional features are added, people may not be able to fully understand their data.

"I'm not entirely convinced that having the capacity to monitor so many metrics is such a beneficial thing," he says.

A health researcher specialising in wearable tech agrees. A study into the influence of wearable tech on a cohort of individuals with health issues suggested a significant portion of those who were given devices to monitor their cardiac function experienced heightened worry and were "far more likely to use medical services".

She identified a cycle with her patients - they spotted a worrying figure on their watch. They grew concerned. Their BPM increased. They got increasingly anxious. They re-examined the data, and their reading rose further.

"If we see numbers about ourselves that we can't properly interpret then, of course, we are going to seek clarification," the psychologist says. "We monitor, we obsess - it becomes a feedback loop."