A woman said she was “lucky to be alive” after a heavy gym workout caused her muscles to start dissolving.

Rebecca Johnson, from Brighton, spent four days in hospital after a personal trainer at LA Fitness gym in North Road pushed her to the limit during an hour-long session.

Doctors told the 30-year-old she was suffering from rhabdomyolysis, a rare condition which could have caused her kidneys to fail if left untreated.

The condition is triggered when muscles start to break down during strenuous exercise, causing toxic fluid to leak into the body.

Since her ordeal she has been unable to return to work as a goldsmith – because her arms are still too weak.

Miss Johnson, who runs regularly, paid £40 for an hour-long session at the LA Fitness gym near her home.

She told a national newspaper: “I did pull-ups, weights, bench press-ups and found it incredibly hard going. A few times I said to the personal trainer I was finding it tough and I didn’t know if I could carry on.

“But he encouraged me to dig deep to do the best possible. As he was the professional, I did as he asked. I thought no pain, no gain.”

When she woke the next day, Miss Johnson could barely move.

She said: “I was in such agony I couldn’t straighten my arms.

“It was difficult to work – I make jewellery for a living but my arms were so painful it was impossible to hold the tools.”

When she noticed that her urine was tea coloured and felt her arms beginning to swell, she called an out-of-hours doctor.

She said: “He took one look at my arms and after testing my urine said I needed to go to hospital straight away. When the doctors explained it can lead to potentially life-threatening kidney failure, I realised how lucky I was I sought help when I did. I was stunned when they said that one hour’s exercise had caused it.”

A spokesman for LA Fitness told the national newspaper: “We take the health and wellbeing of our customers very seriously and we’re really sorry that this happened to Rebecca.

“All our personal trainers are fully qualified. We understand that this particular condition is incredibly rare and cannot be pre-screened for.

“However, given the experience at Brighton we have advised our teams to ensure they are fully aware of this.”