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A sense of building tension. Apparent is the cold breath emanating from my competitors as they do the "pre-race" jiggle on the beginning point.

A rapid flick of the hips.

I'm at my local weekly parkrun event and I mean business. The countdown begins...

Wait a second. I need to start my device. As does the runner alongside me.

The person in the line sighs. I'm not moving until… there! Signal acquired, I'm off.

During the scramble, it's challenging to spot a arm without a fitness tracker on it. And this extends beyond us runners who rely on them.

This is a thriving, multi-billion pound market. Names like Garmin dominate, each with an wide range of diverse models to fit a variety of budgets. Based on how advanced the model you choose, costs can vary between £100 to thousands of pounds.

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

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

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

A small sample of the reactions I've received from like-minded smartwatch wearers when I question what they their opinion is on theirs.

The Data Dilemma

My thoughts on my own device? At this moment, frustrating. It's reporting I'm off pace, leading me to doubt whether the overhead branches I'm running below are interfering with my GPS connection.

A vast number of us attach these little gadgets onto our arms and keep them on constantly, seemingly comfortable with the fact they're continuously recording us.

A distant memory are the days when we relied on them exclusively to give us an idea of our daily steps. Now, they can track our sleep quality, blood pressure, oxygen saturation, our body battery.

It's tricky to find a health indicator they fail to monitor.

But can they do what they say they do? And are the numbers we obsess over a true reflection of our actual health? Do they boost our overall health, or do the avalanche of metrics and motivational messages just increase our daily stresses?

Addiction or Aid?

"I find myself getting a quite preoccupied with mine," one user confesses.

While she appreciates how her device tracks her runs, she finds some of its other functions "excessive" at times - a feeling that became apparent during a pregnancy.

Until she found out she could put it into a specific mode, 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 poor sleep score.

"I have a six-month old, there's no point to be told I've not slept," she says. "I'm acutely aware perfectly well."

Is it not possible to just stop wearing it?

"It's possible, perhaps, but I have this push-pull dynamic with it," she explains. "I appreciate it for its fitness insights, but I'm unsure whether all the other things it can now do is possibly too much for me."

How Do They Work?

Each model of smartwatch has its own unique way of measuring your vitals and interpreting your data, but the majority use sensors on the underside of the watch.

They typically project small, green LED lights onto your arm which can detect capillary action, recording your heartbeat. More advanced devices detect changes in the skin conductance that passes over the skin to get an idea of your stress levels.

A Fine Balance

A professor says that overall for smartwatches, it's a "fine balance" - while the continuously improving tech could actually prevent illness, "spotting abnormalities before we notice anything", it could also turn us device users into the "anxious healthy".

He points out that newer wearables can conduct tests like heart rhythm analysis, which regularly monitor how healthy the heart is. They can notify if someone is experiencing an irregular rhythm, where there is irregular signals in the heart causing an uneven pulse.

This does not automatically signal someone is about to have a heart attack, but it can give an early warning they might be more vulnerable of a heart problem in the future.

But how to understand these figures is complicated. The expert worries that as more functions are introduced, people may find it hard to fully understand their data.

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

A professor studying wearable tech agrees. Her work into the impact 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 increased anxiety and were "had a higher tendency to use medical services".

She identified a cycle with her patients - they saw a alarming metric on their watch. They grew concerned. Their heart rate rose. They got even more concerned. They re-examined the data, and their anxiety increased again.

"If we see numbers about ourselves that we lack context for then, understandably, we are going to want to know more," the psychologist says. "We monitor, we check again - it becomes a self-fulfilling prophecy."