Wearable tracking device, Fitbit, has filed for an initial public offering worth up to $100m (€87.4m, £63m) and intends to lists its shares on the New York Stock Exchange.

The company, which is interviewed in the June issue of Health Club Management, has sold almost 21 million devices since 2011, with more than half of those sales made in 2014.

In 2014 the company generated US$745.4m (€652m, £471.5m) in revenue, almost triple that of the previous year and the same pace has continued into 2015.

These figures show that fitness tracking has moved out of the realms of the fitness buff and into the mainstream. However, despite Fitbit being a big player, it is facing growing competition, particularly from smart watches like the Apple Watch.