Doctors have issued a warning against extreme fitness challenges after a gym-goer was hospitalised with deadly kidney failure triggered by 2,000 squats he performed as part of a ‘bet’.

The unidentified 20-year-old from Russia was confident he could complete the mammoth task in a single session.

But before he finished, his legs began to rapidly increase in size, triggering excruciating pain.

After his symptoms worsened he rushed to emergency care where medics discovered he had rhabdomyolysis — a rare muscle injury that causes tissue to rapidly break down.

If left untreated, the condition can prove fatal.

Tests also showed his kidneys were only functioning at 50 percent, according to a report released by medics at the Vladivostok Clinical Hospital, where he was treated.

Kidney damage is often seen in rhabdomyolysis as a result of a harmful protein released during the process of muscle breakdown.

This substance, called myoglobin, travels to the kidneys via the bloodstream, where it combusts into compounds that can tigger deadly kidney failure.

The unnamed man, from Russian, suffered a rare condition that causes toxic chemicals to be released into the bloodstream as a result of overworking the muscles

The Russian medics reported several telltale signs of this in their patient — including ‘dark brown’ urine and high levels of creatinine, which the kidneys usually keep in balance.

Doctors were able to treat the acute kidney failure without dialysis, but the defect remains.

The 20-year old now faces up to a year of rehabilitation, the doctors reported. 

It comes as an American woman was hospitalised last month after suffering rhabdomyolysis triggered by an intensive spin class.

‘Literally did 15 mins of one Cyclebar class and gave myself the MUSCLE DEATH DISEASE now I’m in the hospital,’ wrote TikToker Savanna Stebbins in the overlay of a video clip, as she laid in bed.

‘Beware of cycling…apparently this disease isn’t as rare as it may seem.’

Rhabdomyolysis happens when muscles are injured so badly the fibres start to breakdown, leading to muscle death. 

When this happens, toxic compounds enter the circulation system and reach the organs.

TikTok user Savanna Stebbins shared the terrifying moment she was rushed to an emergency room during a spin class earlier this month

 Symptoms include weak muscles, muscle stiffness, muscle pain and a change in urine colour.

Around 26,000 people in the United States develop this condition every year, with approximately one in 20 of those dying.

Long distance runners, people who take spin classes and others who perform high-intensity interval exercise, have a higher risk of developing the condition.

Earlier this month, doctors warned against a popular CrossFit challenge after a former World’s Strongest Man competitor claimed his arm muscles ‘blew up’ from attempting it.

Michael Congdon, 32, completed the intense Murph Challenge last May, which involves hundreds of squats, push-ups and pull-ups, as well as two one-mile runs.

But soon after the contest began, he noticed his biceps started to ‘inflate’.

Having been a top-finisher in the 2018 World’s Strongest Man competition, the 17-stone bodybuilder was confident in his ability.

However, he was soon brought to tears as his arm muscles began to break down and die.

According to the Cleveland Clinic, the most important factor for preventing rhabdomyolysis is ensuring adequate rest between exercises that are harsh on the muscles.