Fitbit updated its Charge band, adding new features to what it dubbed its best-selling line, having racked up sales of 35 million units to date.

Charge 3 gains a swimproof design, the company’s most advanced sensor and algorithm technology in a tracker form factor, and has a seven-day battery life. While it resembles earlier Charge devices, it features an “aerospace grade aluminium case and Corning Gorilla Glass 3 display” for a “slimmer, modern silhouette”.

The screen is said to be 40 per cent larger and brighter than its predecessor, with support for greyscale graphics. Another new addition is a relative SpO2 sensor (previously found in Fitbit’s smartwatch line), which the company said “allows for the potential to estimate changes in blood and oxygen levels and help track new health indicators, like sleep apnea”.

Fitbit previously identified its smartwatch business as being a key factor in offsetting declines in its core fitness tracker business, while indicating that basic wearables will continue to be an important part of the market.

To support the Charge 3 launch, Fitbit said its research showed a preference for trackers over smartwatches among consumers, due to the “slimmer design, easy-to-use features, streamlined experience and price point”.

“Charge 3 gives existing users a compelling reason to upgrade, while also allowing us to reach new users who want a sleeker, more affordable wearable in a tracker form factor. Our commitment to consumer choice and ability to meet the demand of millions of users by offering a range of products, from trackers to smartwatches, will enable us to expand our share of the overall wearables market,” said James Park, CEO of Fitbit.