Treadmill tragedy as fitness fanatic, 22, stumbles backwards off running machine before plunging to her death out of gym's third-floor window during workout

Horrifying footage has emerged of the moment a gym user in Indonesia was thrown off a treadmill and plunged out of a third-floor window.

The video, from the busy facility in Pontianak, West Kalimantan, shows a row of treadmills being used on June 18, when the woman suddenly stumbles backwards.

The woman, who has only been identified with the initials FN and was reportedly 22 years old, attempts to grab on to the window frame but then disappears out of sight as she falls through.

Picture is believed to show the victim, who is 22 and has only been identified with the initials FN

The video shows a row of treadmills in the busy facility being used by gymgoers on Tuesday, June 18

Local media reports that the victim had gone to the gym with her boyfriend and younger sibling.

Her boyfriend had reportedly asked her to exercise with him on the second floor, but she wanted to work out on the treadmills on the third floor.

The woman had been exercising for just 30 minutes when she reportedly slowed to a walk, before the tragic fall took place, according to local media reports.

She was reportedly rushed to hospital with serious head injuries, but doctors were unable to save her life, according to local media reports.

An autopsy reportedly revealed that she had suffered extensive bruising and lacerations to her head.

The woman is seen stumbling backwards before plunging out of the window

Police said that the position of the treadmill in relation to the window was 'dangerous' - only 60 centimetres apart.

The low height of the wall beneath the window - just 30cm - also meant it it was easy for the woman to fall through. 

Pontianak Police Criminal Investigation Unit conducted a crime scene investigation and a number of people were suspected of opening windows at the accident scene. 

Meanwhile, K- Gym stopped operational activities for three days after the incident occurred.

The gym had warning stickers telling gymgoers not to open windows, but the sticker was reportedly damaged and writing obscured.

A personal trainer was also usually on hand to close the windows, Kompas.com reports, but they had been on break at the time of the accident.

Police continue to investigate the circumstances around the deadly fall.