This is the horror moment a fitness trainer broke his arm while lifting a dumbbell in Thailand.

Wasawat Koeithong, 36, was already winded from his workout when he decided to do one last set of shoulder presses at a gym in Phuket on February 28.

He grimaced as he tried to lift a pair of 40kg (90lb) weights. However, before he could begin the set, his left arm broke with a chilling snap.

Footage shows the muscle-bound gym instructor letting go of the heavy dumbbell to nurse the injured limb. He was taken to a hospital and underwent surgery to have metal inserted into the fractured bone.

He said: ‘I am a trainer and I was lifting 40-kilogramme dumbbells which I could normally handle.

‘However, the incident occurred at the end of my workout when I had already expended most of my strength. My bone broke into two according to the x-ray.

‘I had surgery to have a metal brace inserted to support my bones. The doctors said I will need physical therapy every day for six months. I may not be able to lift heavy dumbbells as I did before.

‘I was in surgery for five hours. I was told the bone was twisted and deformed after breaking in tow. My wrist tendon was also swollen but it wasn't torn.

‘I am now undergoing physical therapy at the hospital and they expect me to recover in about six months.'