A woman was left with “soy sauce” coloured urine after a cycling session at her local gym.
Ms Li, from the city of Shenzhen in southern China, had gone for a 50-minute spin.
She felt tired five minutes into the session but continued until the end of the class, according to Shanghai news outlet The Paper.
Both her legs were swollen in the evening to the point where she could hardly walk the following day.
Ms Li then noticed her urine was dark like the colour of soy sauce.
She was diagnosed with rhabdomyolysis, a type of muscular injury caused by intense exercise and the breakdown of muscle tissue.
Ms Li was treated by kidney disease specialist Peng Wujian.
He said: “Rhabdomyolysis is mainly caused by exercise but can also happen after a crush injury, such as being struck by debris during an earthquake."
“Some types of medication may also lead to rhabdomyolysis as a side effect.”
Last month, scientists discovered triggered proteins triggered by working out, in a bid to develop medication that could combat frailty in the elderly.
Almost 150 genes which boost muscle were identified and isolated in the study by McMaster University in Canada.
This was done by getting participants to exercise only one leg.
The experiment hopes to develop medication which can help to combat frailty in the elderly.
Professor Stuart Phillips, who supervised the project, said: "Building and retaining muscle is critical to overall health and quality of life.
"If we can target those genes with lifestyle and drug therapies, we may be able to help seniors and others vulnerable to muscle loss."
The study was published in the life sciences journal Cell Reports.