“The research that I’ve done has shown that inability to exercise is one of the most common long-term symptoms,” Dr. Lambert said. Some people are simply too tired to exercise, she said, while others experience debilitating symptom relapses like increases in fatigue, brain fog or muscle pain. This worsening of symptoms after engaging in even just a little bit of physical activity — what is sometimes called “post-exertional malaise” — seems to be common among long Covid patients. When researchers performed an online survey of 3,762 people with long Covid, as part of a study published in August, they found that 89 percent reported post-exertional malaise.

These exercise-induced problems are not, however, merely the byproduct of becoming out of shape. The effects “are very, very different from normal and simple detraining,” said Dr. David Systrom, a pulmonary and critical care physician at Brigham and Women’s Hospital in Boston. They also don’t seem to be the result of lung or heart injury.

In one small study published in January, for example, Dr. Systrom and his colleagues compared 10 long Covid patients who had trouble exercising with 10 people who had never tested positive for Covid-19, but who had unexplained shortness of breath after exercise. The researchers found that nobody in the study had abnormal chest CT scans, anemia or problems with lung or heart function, suggesting that organ injury wasn’t to blame for their symptoms. Yet when the long Covid patients exercised on a stationary bicycle, Dr. Systrom found that some veins and arteries were not working properly, preventing oxygen from being delivered efficiently to their muscles.