Exercise Intolerance

What exercise intolerance feels like

People describe:

• getting short of breath faster than peers or than they used to
• heavy, weak or painful muscles early in exercise
• extreme fatigue during or after activity
• palpitations or a heart rate that rises too high or too little
• dizziness or lightheadedness
• feeling unwell for hours or days after activity

The pattern and the "limiting symptom" help doctors narrow the cause.

Causes by system

Heart

• Heart failure: breathlessness with exertion, ankle swelling, needing more pillows to sleep.
• Coronary artery disease: chest pain or breathlessness with exertion.
• Cardiomyopathies and valve disease: such as aortic stenosis.
• Arrhythmias: atrial fibrillation, or a heart rate that cannot rise enough (chronotropic incompetence).
• Pulmonary hypertension: progressive breathlessness, fatigue and fainting on exertion.

Lungs

• Asthma including exercise-induced bronchoconstriction: cough, wheeze and chest tightness during or shortly after exercise.
• COPD and interstitial lung disease.
• Inducible laryngeal obstruction (vocal cord dysfunction): noisy breathing in, often in young athletes.

Blood and metabolism

• Anemia: reduced oxygen-carrying capacity; pale skin, fatigue and palpitations.
• Thyroid disease: both underactive and overactive thyroid.
• Diabetes and obesity.

Muscles and nerves

• Metabolic myopathies: inherited problems in muscle energy production, such as McArdle disease (cramps and pain early in exercise, with a "second wind") or fatty acid oxidation disorders (pain and dark urine after prolonged exercise or fasting).
• Mitochondrial diseases: fatigue, muscle weakness and exercise intolerance, often with other organ involvement.
• Myasthenia gravis, muscular dystrophies and inflammatory myopathies.

Autonomic and post-viral

• POTS: heart rate rises excessively on standing; exercise, especially upright, causes dizziness and fatigue.
• ME/CFS: the hallmark is post-exertional malaise, a worsening of symptoms 12 to 48 hours after even minor activity, lasting days.
• Long COVID: fatigue, breathlessness, POTS-like symptoms and post-exertional malaise after COVID-19.

Other causes

• Deconditioning: loss of fitness after inactivity, illness or bed rest.
• Medicines: beta-blockers, which blunt heart rate response, and statins (muscle aches in some).
• Depression and poor sleep.

How doctors investigate

A doctor asks what stops you exercising, how quickly symptoms come on, whether they are new, and about chest pain, fainting, leg swelling, cramps, dark urine, recent infections and medicines. They examine the heart, lungs and muscles and check heart rate and blood pressure lying and standing.

Tests may include:

• blood tests: blood count, iron, thyroid, kidney and liver function, blood sugar, creatine kinase (muscle enzyme), BNP for heart failure
• ECG and echocardiogram
• lung function tests and chest imaging
• exercise testing: a treadmill test or cardiopulmonary exercise test (CPET), which measures oxygen use and pinpoints whether the heart, lungs, muscles or deconditioning limit exercise
• tilt table test for POTS
• muscle biopsy and genetic tests for suspected metabolic or mitochondrial disease

Treatment

Treatment depends on the cause:

• Deconditioning: a gradual, structured exercise program.
• Heart and lung disease: medicines, inhalers, and cardiac or pulmonary rehabilitation, which improve exercise capacity and quality of life.
• Anemia and thyroid disease: iron, vitamin B12 or folate replacement, and thyroid treatment.
• POTS: increased fluids and salt, compression garments, recumbent exercise such as rowing or cycling, and medicines.
• ME/CFS and long COVID with post-exertional malaise: pacing, keeping activity within your energy limits, rather than graded exercise, which can worsen symptoms in people with post-exertional malaise.
• Metabolic myopathies: specific dietary strategies and avoiding exertion that triggers muscle breakdown.

Frequently asked questions

What causes exercise intolerance?

Common causes are loss of fitness, anemia, heart failure, asthma, COPD, thyroid disease, obesity, beta-blocker medicines and post-viral illness. Less common causes include POTS, ME/CFS, pulmonary hypertension and inherited muscle disorders.

When should I worry about getting tired easily with exercise?

See a doctor if your exercise ability is declining without explanation, or if you have chest pain, fainting, palpitations, worsening breathlessness or leg swelling. Chest pain or fainting during exercise needs emergency assessment.

What is post-exertional malaise?

Post-exertional malaise is a worsening of fatigue, pain, brain fog and other symptoms 12 to 48 hours after physical or mental effort, often lasting days. It is the hallmark of ME/CFS and is common in long COVID. It calls for pacing rather than pushing through exercise.

How is exercise intolerance tested?

Doctors start with blood tests, an ECG and an echocardiogram, plus lung function tests. A cardiopulmonary exercise test measures oxygen use, heart and breathing responses during exercise, and helps show whether the heart, lungs, muscles or fitness is the limiting factor.