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There's a building tension. Apparent is the cold breath coming from my fellow runners as they perform the "pre-race" shuffle on the starting line.

A quick flick of the hips.

I'm participating in my local weekly parkrun event and I mean business. It's almost time...

Wait a second. I must sort my device. So does the runner beside me.

The person in the line tuts. I'm not moving until… yes! Connection established, away I go.

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

It's a booming, multi-billion dollar market. Companies like Apple Watch are prevalent, each with an broad range of diverse models to suit a variety of needs. According to how feature-packed the device you choose, costs can vary between a hundred pounds to a significant investment.

"It makes me crazy, I'm unable to switch off."

"It provides solace… It's like a companion supporting me."

"Stop telling me about a restless night."

Just some of the reactions people have shared from other tracker enthusiasts when I question what they feel about theirs.

The Monitoring Conundrum

My feelings on my own device? Currently, irritating. It's reporting I'm off pace, making me wonder whether the trees I'm running below are interfering with my location tracking.

Countless individuals of us fasten these small monitors onto our arms and keep them on all day and night, apparently unconcerned with the fact they're constantly monitoring us.

Long past are the days when we relied on them solely to give us an idea of our step count. Today, they can track our nightly rest, blood pressure, glucose levels, our VO2 max.

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

But do they do what they say they do? And are the numbers we scrutinise a accurate representation of the truth? Do they enhance our daily wellness, or do the multitude of readings and constant notifications just add to our sense of overwhelm?

A Love-Hate Relationship

"I find myself getting a a little obsessed with mine," one user reveals.

While she loves how her device records her workouts, she finds some of its other features "daunting" at times - a feeling first discovered during a pregnancy.

Prior to discovering she could put it into a specific mode, her smartwatch repeatedly informed her she was not being energetic enough. Now, the watch insists her she's had a restless night.

"I've got a infant, it's unnecessary for me to be notified I've not slept," she says. "I know that perfectly well."

Is it not possible to just take it off?

"I could, perhaps, but I'm in this love-hate relationship with it," she says. "I love it for its exercise data, but I'm unsure whether the additional features it can now do is possibly overkill for me."

How Do They Work?

Each model of smartwatch has its own novel way of monitoring your health metrics and interpreting your data, but the vast bulk use sensors on the back of the watch.

They typically emit minute LED lights onto your skin which can monitor blood flow, detecting your BPM. Sophisticated devices measure changes in the bioimpedance that passes over the skin to assess your stress levels.

A Fine Balance

A professor says that for most people for smartwatches, it's a "fine balance" - while the continuously improving tech could realistically prevent illness, "detecting disease before we show symptoms", it could also turn us device users into the "worried well".

He points out that some of the latest wearables can perform tests like heart rhythm analysis, which regularly monitor how healthy the heart is. They can alert if someone is experiencing a heart arrhythmia, where there is abnormal signals in the heart causing an irregular heartbeat.

This does not mean someone is imminently going to suffer a heart attack, but it can give an preliminary indication they might be more at risk of a blood clot in the long term.

But how to understand these numbers is complicated. The expert fears that as more functions are incorporated, people may find it hard to fully comprehend their data.

"I remain sceptical that having the capacity to monitor so much data is such a positive development," he says.

A health researcher focusing on cardiology concurs. Research into the effect of wearable tech on a cohort of cardiovascular patients suggested a noticeable percentage of those who were given devices to monitor their condition experienced increased anxiety and were "more inclined to use healthcare resources".

She observed a pattern with her patients - they spotted a worrying figure on their watch. They felt anxious. Their heart rate went up. They got more worried. They checked again, and their heart rate increased again.

"Upon viewing numbers about ourselves that we don't really understand then, naturally, we are going to want to know more," the psychologist says. "We monitor, we obsess - it becomes a feedback loop."