It is no secret that Apple has a secret testing lab for the Apple Watch where the company works on refining the various algorithms used in the smartwatch for detecting various exercises. Apple had previously given ABC an inside look at this lab, and now ahead of the September 12 event from the company, it has once again given Men’s Health magazine a look at this lab.

During the tour, Apple’s director of fitness and health technologies, Jay Blahnik, was also present and he revealed quite a fair bit about the lab and the work being done there. While Apple only launched the first Apple Watch in 2015, the secretive lab has been in operation for the last 5 years.

As for the lab itself, it is located on a side street in Cupertino, a few miles away from Apple Park. Unlike Apple Park, this secret lab is just a single-story building and looks nothing like any of Apple’s offices.

The lab consists of many expensive medical equipments and machinery including $40,000 masks, ECG machines, and more which measures the various health metrics of Apple’s employees during different stages of exercises. This includes oxygen consumption, calories burned, heart rate, glucose level, and more.

On a recent morning, about 40 employees are sweating away on different contraptions—rowers, treadmills, cable machines—as 13 exercise physiologists and 29 nurses and medics monitor data. Many of the exercisers are hooked up to a metabolic cart and ECG and are wearing a $40,000 mask apparatus that analyzes their calorie burn, oxygen consumption, and VO2 max.

Blahnik also proudly mentions that the secretive lab has collected more data on exercise and activity than any other “human performance study in history.” The company has logged over 33,000 exercise sessions comprising of over 66,000 hours of data from over 10,000 unique participants.

Apple has always emphasised on the fitness aspect of the Apple Watch. With the upcoming watchOS 4 update, Apple is going to add support for more exercises to the smartwatch including high-intensity training sessions. The update will also allow the Apple Watch to communicate with cardio machines — a first for a wearable. This will allow for precise tracking of your exercise session and provide you with accurate and relevant data.

Apple is expected to announce a new LTE variant of the Apple Watch at its upcoming September 12 event. Apart from LTE connectivity, the smartwatch is not expected to feature any other improvements.