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A sense of feeling of anticipation. Apparent is the frosty air exhaled by my competitors as they execute the "starting line" shuffle on the starting line.

A rapid twitch of the hips.

I'm participating in my local weekly parkrun event and I mean business. Three, two, one...

Hold on. I have to adjust my watch. So does the runner next to me.

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

Amidst the crowd, it's hard to spot a arm without a fitness tracker on it. And this extends beyond us athletes who use them.

This is a booming, multi-billion pound industry. Names like Garmin lead the market, each with an broad range of various models to cater to a range of needs. Depending on how advanced the watch you choose, prices can range from a hundred pounds to thousands of pounds.

"I find it infuriating, I can't switch off."

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

"I wish it wouldn't say about a poor sleep."

Just some of the reactions I've received from other tracker enthusiasts when I ask what they think of theirs.

A Sea of Statistics

My feelings on mine? Currently, annoying. It's indicating my speed is wrong, leading me to doubt whether the trees I'm passing under are blocking my satellite signal.

Millions of us attach these small gadgets onto our wrists and use them constantly, seemingly unconcerned with the fact they're constantly tracking us.

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

It's difficult to find a physical metric they fail to monitor.

But can they do what they advertise? And are the data points we scrutinise a accurate representation of our actual health? Do they enhance our overall health, or do the plethora of readings and motivational messages just add to our everyday anxiety?

A Complicated Bond

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

While she appreciates how her device tracks her exercise, she finds some of its other features "excessive" at times - a feeling initially realised during a major life event.

Before she realised she could put it into a specific mode, her smartwatch constantly alerted her she was not being energetic enough. Later on, the watch insists her she's had a poor sleep score.

"I am caring for a young baby, it's unnecessary for me to be told I'm tired," she says. "I'm acutely aware only too well."

Couldn't she just stop wearing it?

"I suppose I could, I suppose, but I have this love-hate relationship with it," she says. "I love it for its workout tracking, 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 proprietary way of monitoring your health metrics and interpreting your data, but the vast bulk use sensors on the reverse of the watch.

They commonly emit small, green LED lights onto your wrist which can monitor blood flow, detecting your heartbeat. Higher-end devices measure changes in the electrical current that runs through the skin to assess your stress levels.

A Fine Balance

One expert says that in general for smartwatches, it's a "difficult line" - while the ever-evolving tech could actually prevent illness, "spotting abnormalities before we feel unwell", it could also turn us watch wearers into the "concerned but fine".

He points out that some of the latest wearables can conduct tests like electrocardiograms (ECGs), which regularly monitor how healthy the heart is. They can alert if someone is experiencing an irregular rhythm, where there is irregular electrical activity in the heart causing an arrhythmia.

This does not mean someone is about to have a heart attack, but it can give an advance notice they might be more at risk of a heart problem in the future.

But how to make sense of these numbers is not straightforward. The expert worries that as more functions are introduced, people may not be able to fully understand their data.

"I have some doubts that having the capacity to monitor so many metrics is such a beneficial thing," he says.

A health researcher focusing on wearable tech agrees. Research into the effect of wearable tech on a set of individuals with health issues suggested a significant portion of those who were given devices to monitor their heart health experienced greater stress and were "had a higher tendency to use doctor's time".

She noted a vicious circle with her patients - they spotted a concerning reading on their watch. They became worried. Their BPM increased. They got increasingly anxious. They looked a second time, and their heart rate increased again.

"If we see data about ourselves that we don't really understand then, of course, we are going to become concerned," the psychologist says. "We check, we obsess - it becomes a vicious cycle."