It is no secret that Apple is serious about health, and now the company's director of fitness has revealed the iPhone maker has a secret gym for collecting and analysing thousands of hours of staff workouts.

To make the Apple Watch a more complete fitness tracker, the company has for several years operated a secret gym at an undisclosed location within its Cupertino, California headquarters.

The gym was revealed by fitness head Jay Blahnik this week (11 July), who explained how it acts like a laboratory to test and develop new ways of monitoring running, swimming, cycling and other forms of exercise on employee guinea pigs.

Blahnik claims the gym has collected more data than anyone else, reports MIT Technology Review, and by keeping track of oxygen consumption Apple has become the largest purchaser of metabolic carts used to track and analyse someone's breathing while they exercise.

Apple now has 50 of these machines, Blahnik said, and they are used by employees - along with a security guard - during a daily bike ride.

According to the MIT report, Blahnik said by logging the use of an indoor infinity pool, Apple has found that even regular swimmers do not swim as well as they think they do. He also revealed how Apple's gym sometimes tracks staff when they aren't actively exercising. "One day you might show up and we'll study how many calories you burn while you sit, because we need to understand that too," he said.

Apple has so far amassed 33,000 hours of data on how people run, walk, cycle swim and sit. The company's gym employs 13 physicians, exercise physiologists and specialists, plus 29 nurses and medics, Axios reported.

At the same event, held in Los Angeles and to show off Apple's work in the health and fitness sector, the company demonstrated the first gym equipment fitted with GymKit.

A way for consumers to sync data between gym equipment and their Apple Watch with a nap, GymKit will be included with the WatchOS 4 software updating coming to Apple's wearable in the autumn. Tapping the watch on compatible gym equipment like treadmills, bikes and rowing machines will send the data captured by the apparatus to the Watch and store it in the person's Health app on their iPhone for further analysis.