Recently, I saw a 20-year-old patient who had come in with an episode of life-threatening arrhythmia or irregular heartbeats and fainting spells. Turns out he had stayed away from the gym for months. Then came Pulikalli — the traditional tiger dance performance of Kerala. Wanting to get into shape before the event, he suddenly returned to rigorous exercise, pushing himself through workouts that lasted nearly two hours. That evening, he noticed his urine had started looking like dark cola. He ignored it and went to sleep. Till he woke up with a pounding heartbeat and a spinning head.The underlying problem was rhabdomyolysis, a condition in which muscles break down severely and release their contents into the bloodstream. While uncommon, it can become a medical emergency — and, in severe cases, can affect the kidneys and the heart.What is rhabdomyolysis?Rhabdomyolysis occurs when muscle fibres are damaged so extensively, usually after sudden stress caused by excessive workouts, that their contents spill into the bloodstream. One of the substances released is myoglobin, a red-coloured protein that normally remains inside muscle cells. When it enters the urine in large amounts, it can turn it dark brown, red or cola coloured. That striking change in urine colour can be one of the warning signs that something more serious than ordinary post-workout soreness is happening.Also read | Gym-goer on dialysis after cola-coloured urine: How protein and creatine misuse pushed 24-year-old towards kidney failureHow can a muscle problem affect the heart?Severe muscle breakdown can release large amounts of potassium into the bloodstream. The kidneys normally help remove excess potassium, but significant muscle injury can overwhelm the body’s ability to maintain normal levels, particularly when kidney function is also affected. When potassium rises too high, it can interfere with the heart’s electrical activity.The heart may develop dangerous arrhythmias, or abnormal rhythms. In severe cases, the rhythm can deteriorate into cardiac arrest. So, the heart danger from rhabdomyolysis is primarily an electrical problem caused by severe metabolic and electrolyte disturbances, rather than a typical heart attack.Rhabdomyolysis is not the same as a heart attackThe distinction is important. A heart attack usually occurs when blood flow through a coronary artery becomes blocked, depriving part of the heart muscle of oxygen. Rhabdomyolysis, on the other hand, begins with severe muscle injury elsewhere in the body. Its cardiac complication can occur because substances released from damaged muscles—especially potassium—disturb the heart’s electrical system.A person can, therefore, have a cardiac arrest related to rhabdomyolysis without having had a conventional heart attack.Why sudden, intense exercise can be riskyStory continues below this adExercise itself is not the enemy. In fact, regular physical activity is one of the best things a person can do for cardiovascular and overall health. The problem is the sudden jump from little activity to very strenuous exercise. Someone who has been inactive for months may suddenly decide to train intensely for an upcoming event. But muscles need time to adapt. A sudden increase in exercise intensity, duration or volume can cause significant muscle injury.Extreme exertion, particularly when combined with dehydration or exercising in very hot conditions, can increase the risk of rhabdomyolysis.If severe muscle pain or swelling is accompanied by marked weakness, unusual exhaustion, dark brown or cola-coloured urine, or a noticeable reduction in urine output, it is important to seek medical attention promptly rather than waiting for the symptoms to settle on their own.Doctors can check for rhabdomyolysis with blood tests, particularly creatine kinase (CK), along with kidney function and electrolyte tests. An ECG may also be needed when there is concern about potassium abnormalities or an abnormal heart rhythm.Story continues below this adThe safer way to get back into exerciseIf you have been inactive, don’t try to make up for lost months in a few days. Start gradually and increase the duration and intensity of workouts step by step. Give the muscles adequate recovery time, and take particular care with hydration and strenuous exercise in hot weather.The heart, kidneys and muscles all have to cope with what we ask of the body. A workout should make you stronger—not leave you wondering whether a frightening warning sign can wait until morning.(Dr Shetty is lead cardiologist and medical director, Sparsh Hospital, Bengaluru)