Do exercise trackers work? New study finds there's a catch

If you've asked Santa for a Fitbit this year it might be a little easier walking off the Christmas kilos.

A new study has found fitness apps and trackers do make you work harder but there's a catch.

Kieran Van Broekhuizen's Apple watch has a permanent place on his wrist. A personal trainer by trade, he likes to keep a close eye on how much exercise he's doing throughout the day.

"I get daily reminders, it's kind-of like a nice kick in the butt every few hours to keep moving," he says.

And he's not the only one. A team of international researchers looked at more than 7000 case studies found smartphone fitness apps and wearable trackers do indeed boost physical activity.

"We also found that in terms of the other features that were included in those interventions, so personal text messages and personalisation... the interventions that had that were more effective," says study author Liliana Laranjo.

It varied case-to-case but the research revealed that people who track their movements had an average daily increase of almost 2000 steps a day.

While it might seem like a no-brainer, trackers are not for everyone.

"Some people are motivated to change their behaviour, others are demotivated and see it as something that tells them 'actually I've failed so I'm not doing anything now'," says Exercise New Zealand chief executive Richard Beddie.

Beddie says people should think twice before buying a device this Christmas.

"It can be a really good gift for the right person so you need to check what they'll do with that information," he says.

Van Broekhuizen is confident it'll keep his clients active over the summer break.

"When I'm not around and in their ear about it, what's better than a little vibration on their wrist telling them to get up and get moving."

And start the New Year one step ahead of those fitness resolutions.