A Malaga cardiologist has warned runners and gym-goers about the signs of rhabdomyolysis after a 24-year-old woman collapsed and died around three kilometres from the finish of the New York Marathon.

Rachel Apa was running her first marathon and had texted her father during the race to tell him how much pain she was in.

In a video posted on social media, Dr Aurelio Rojas explained how strenuous exercise can trigger rhabdomyolysis, a condition in which damaged muscle fibres break down and release their contents into the bloodstream.

One of those substances, a protein called myoglobin, can damage the kidneys. Muscle breakdown can also raise potassium levels, which in severe cases can trigger dangerous heart rhythms.

Rojas said the condition can affect otherwise healthy people who push themselves beyond what they have trained for.

It can occur during endurance events, high-intensity workouts such as CrossFit or Hyrox, or a gym session.

He advised building up training gradually and taking particular care when exercising in the heat, while dehydrated or while ill.

What are the warning signs?

Muscle pain much more severe than expected, unusual weakness and dark, cola-coloured urine are warning signs.

Symptoms can develop during or after exercise. Anyone who develops these symptoms after strenuous activity should seek urgent medical attention.

Doctors use a blood test to measure creatine kinase, also known as CK or CPK, an enzyme that rises when muscle tissue is damaged.