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A sense of nervous excitement. I can see the frosty air emanating from my rivals as they execute the "starting line" jiggle on the starting line.

A quick flick of the hips.

I'm at my local 5km community parkrun and I'm serious. The countdown begins...

Hold on. I have to start my device. As does the person beside me.

The person behind us tuts. I refuse to go until… done! GPS ready, and I'm running.

During the scramble, it's difficult to see a arm without a fitness tracker on it. And it's not just us athletes who use them.

This is a explosive, multi-billion dollar market. Brands like Garmin dominate, each with an wide selection of different devices to suit a spectrum of lifestyles. According to how feature-packed the device you select, costs can vary between an affordable sum to thousands of pounds.

"I find it infuriating, I'm unable to stop checking."

"It reassures me… It acts as a friend supporting me."

"Stop telling me I've had a restless night."

A handful of the reactions I've had from like-minded device users when I ask what they think of theirs.

The Data Dilemma

My thoughts on my own device? Right now, irritating. It's telling me my speed is wrong, leading me to doubt whether the overhead branches I'm running below are interfering with my location tracking.

Millions of us attach these small gadgets onto our wrists and wear them 24/7, apparently at ease with the fact they're constantly monitoring us.

A distant memory are the days when we depended on them only to give us an rough count of our daily steps. Today, they can measure our sleep patterns, heart rate, heart rate variability (HRV), our VO2 max.

It's a challenge to find a health indicator they aren't designed to measure.

But do they do what they advertise? And are the numbers we scrutinise a accurate representation of the truth? Do they enhance our mental and physical wellbeing, or do the plethora of metrics and constant notifications just increase our everyday anxiety?

A Complicated Bond

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

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

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

"I am caring for a infant, there's no point to be notified I've not slept," she says. "I'm acutely aware all too clearly."

Why doesn't she just stop wearing it?

"I could, I suppose, but I'm in this push-pull dynamic with it," she continues. "I love it for its exercise data, but I'm unsure whether all the other things it can now do is perhaps overkill for me."

The Technology Behind the Tracking

Each model of smartwatch has its own unique way of monitoring your bodily functions and analysing your data, but the vast bulk use detectors on the reverse of the watch.

They typically shine small, green LED lights onto your skin which can detect blood flow, identifying your BPM. More advanced devices analyse changes in the skin conductance that flows through the skin to assess your recovery state.

A Fine Balance

One expert says that overall for smartwatches, it's a "difficult line" - while the ever-evolving tech could potentially prevent illness, "spotting abnormalities before we notice anything", it could also turn us the tracked into the "concerned but fine".

He points out that newer wearables can perform tests like electrocardiograms (ECGs), which regularly monitor how stable the heart is. They can flag if someone is experiencing atrial fibrillation (AF), where there is abnormal impulses in the heart causing an arrhythmia.

This does not necessarily indicate someone is about to have a heart attack, but it can give an preliminary indication they might be more susceptible of a stroke in the future.

But how to interpret these figures is complex. The expert worries that as more functions are incorporated, people may not be able to fully contextualise their data.

"I remain sceptical that the ability to monitor so many things is such a good idea," he says.

A health researcher studying patient behaviour echoes this sentiment. Research into the effect of wearable tech on a set of cardiovascular patients suggested a significant portion of those who were given devices to monitor their cardiac function experienced greater stress and were "had a higher tendency to use medical services".

She identified a cycle with her patients - they spotted a worrying figure on their watch. They became worried. Their BPM rose. They got more worried. They re-examined the data, and their anxiety rose further.

"When people are presented with data about ourselves that we don't really understand then, of course, we are going to want to know more," the psychologist says. "We check, we obsess - it becomes a vicious cycle."