Unexplained muscle pain, cramping, and tiredness in active people can stem from many conditions that affect energy production — anaemia, diabetes, or hormone abnormalities, among them. One rare muscle condition that affects physical performance is McArdle’s disease. So what is McArdle’s disease, how do we diagnose it, and how is it managed?
McArdle’s disease is one of several medical causes of muscle pain in active people.
What is McArdle’s disease?
McArdle’s disease is a rare genetic muscle condition caused by a deficiency of myophosphorylase — the enzyme that breaks down glycogen (stored glucose) in muscle cells. Without this enzyme, muscles struggle to release energy, particularly during intense exercise. It’s one of a group of conditions called glycogen storage diseases.
McArdle disease symptoms
Symptoms usually appear during intense exercise or when pushing beyond your limits.
The main feature is muscle fatigue, weakness, pain, and cramping during demanding activities such as running, cycling, or weightlifting. These symptoms are typically worse at the very start of exercise, and any muscle can be affected. Over time, chronic weakness can develop even at rest, often affecting the upper arm and leg muscles.
Dark urine may follow intense exercise, caused by sudden muscle breakdown — rhabdomyolysis. Less typical symptoms include difficulty chewing or swallowing and, occasionally, acute compartment syndrome.
The “second wind” phenomenon
A hallmark of McArdle’s disease is the second wind. Unlike ordinary fatigue, people with McArdle’s often find their endurance improves after a brief rest early in exercise, as the body shifts to alternative energy sources such as fatty acids and blood glucose.
How is McArdle’s disease diagnosed?
See an experienced sports medicine doctor, who can assess you for the features of McArdle’s disease and rule out other causes of muscle pain, weakness, and fatigue.
Investigations to confirm the diagnosis and exclude other causes include:
- Blood tests — creatine kinase (CK) is often elevated during or after activity, though it can be normal in some cases. Genetic testing for PYGM mutations can confirm the diagnosis.
- Forearm exercise test — measuring lactate and ammonium before and after a brief workout. In McArdle’s, lactate fails to rise while ammonium increases, reflecting impaired glycogen breakdown.
- Muscle biopsy — can identify the enzyme abnormality in muscle tissue if needed.
- Imaging and nerve studies — MRI and nerve studies help rule out other causes of muscle pain and weakness.
Other causes of muscle pain with exercise
It’s important to exclude the other causes of exertional muscle pain: an acute muscle injury such as a calf or quadriceps muscle tear; chronic exertional compartment syndrome; nerve entrapments such as calf nerve pain or referred pain from the lumbar spine; medical problems such as anaemia, thyroid disease, diabetes, and iron deficiency; and blood-vessel entrapment such as popliteal artery entrapment syndrome.
Treatment for McArdle disease
There’s no cure for McArdle’s disease, but active people can stay fit with a targeted approach — it doesn’t have to sideline you from activity and sport.
McArdle disease diet
A few nutritional strategies can improve symptoms. A carbohydrate-rich snack about 30 minutes before exercise provides a quick energy boost and helps protect against muscle breakdown. Frequent, balanced meals with adequate protein and healthy fats support muscle repair and endurance. In some cases, creatine supplementation may improve muscle performance.
Staying active with McArdle disease
Staying fit is possible with some precautions. Favour low-intensity aerobic activities — walking, swimming, or cycling — which are safer and build endurance over time. Use gradual warm-ups, which help activate alternative energy pathways and reduce early fatigue. Avoid overexertion, since high-intensity anaerobic activity can trigger symptoms and muscle damage. And keep generally fit: the fitter you are, the less likely you are to develop symptoms during exercise.
Watch for signs of excessive muscle breakdown, such as extreme muscle pain or dark urine. Stay well hydrated, avoid prolonged endurance events such as marathons or Ironman races, and seek medical attention if you develop dark urine after exercise.
Frequently asked questions about McArdle’s disease.
What is the average age of diagnosis of McArdle’s disease?
Symptoms are often overlooked, so diagnosis is frequently delayed into adulthood. The average age of diagnosis is around 33 years.
If someone is diagnosed with McArdle disease, is genetic counselling recommended?
Yes. McArdle’s is inherited in an autosomal recessive pattern. If both parents are affected, the chance of a child having the disease is 25%, and the chance of being a carrier is 50% — so genetic counselling is worthwhile.
Can people with McArdle’s disease lift weights?
Yes. People with McArdle’s were traditionally told to avoid heavy weight training, but recent studies suggest it’s safe when supervised by a strength and conditioning coach.
Which exercises should be avoided in McArdle’s disease?
Studies suggest avoiding static muscle contractions (such as sustained handgrip), heavy loads on small muscle groups unless supervised by clinicians and fitness specialists, high-intensity dynamic activity such as competitive ball games, exercises with heavy eccentric (lengthening) loads such as jumps, and very intense aerobic exercise such as strenuous running — except in very fit, specifically trained individuals.
Is McArdle’s disease the same as rhabdomyolysis?
No, but they’re linked. McArdle’s is the underlying genetic condition; rhabdomyolysis is severe muscle breakdown that McArdle’s can trigger during intense exercise. Recurrent exertional rhabdomyolysis is one reason we test for underlying conditions such as McArdle’s.
Final word from Sport Doctor London about McArdle disease
If you have unexplained muscle pain and fatigue, see an experienced sports medicine doctor who can assess you and investigate for muscle conditions such as McArdle’s disease. Having McArdle’s doesn’t have to stop you from taking part in activity — confirming the diagnosis helps you manage symptoms and exercise safely.
To book an assessment with Dr Masci in London, contact the team here or call +44 (0) 203 488 0350.
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