The signature symptom of long COVID and ME/CFS is post-exertional malaise—a worsening of symptoms following overexertion.
Post-exertional malaise is under-recognized and poorly understood among medical experts, but a few key clues indicate what is happening. One way researchers study post-exertional malaise is by using a two-day cardiopulmonary-exercise test, in which patients exercise to exhaustion twice in two days.
During the second day of exercise testing, by which point a patient’s post-exertional malaise has likely been triggered, the patient reaches their anaerobic threshold, which is when the body is unable to efficiently produce energy, sooner (while doing less work than on the first day).
“It’s very difficult to recover from periods of time that are spent above the anaerobic threshold,” says Mark Van Ness, a professor of health and exercise sciences at the University of the Pacific in Stockton, California, who studies post-exertional malaise but is not associated with Visible. Patients with post-exertional malaise can exceed their anaerobic threshold while doing seemingly basic tasks, such as cooking dinner or showering. “If you’re doing that throughout the day, that wears you down,” Van Ness says.
In practical terms, this means that the more patients push themselves, the worse their conditions can get. Although pacing hasn’t been widely studied, the U.S. Centers for Disease Control and Prevention and the UK’s National Institute for Health and Care Excellence recommend it as a primary tool for managing the symptoms of ME/CFS, while patient surveys report it to be one of the more effective treatment strategies. The goal is for a person to stay within their personal energy limits in order to avoid crashes.
“It’s amazing how much higher my quality of life can be when I stay on the right side of the line,” says Julia Moore Vogel, a researcher at The Scripps Research Institute in La Jolla, California, who developed long COVID after an infection in 2020.
Moore Vogel, who now studies the use of wearable tech in managing chronic illnesses, is not associated with Visible. (She manages her symptoms with the help of fitness trackers from Fitbit and Garmin.)
Visible users who have had a two-day cardiopulmonary-exercise test can customize their heart-rate limits in the app. For those who haven’t, the app recommends a starting heart-rate limit based on factors such as resting heart rate and manually recorded symptom severity.
The app is programmed to alert you when your heart rate goes above your set limit. It also tracks resting heart rate and heart-rate variability, which it measures in the morning (just after you wake up) and uses to calculate a “morning stability score.” In a 2024 study involving 4,244 Visible users, a team of researchers at Visible, New York University, Yale University, and the University of Oxford, among other institutions, found that increases in resting heart rate and decreases in heart-rate variability are associated with flare-ups in symptoms. The team published these results in a preprint and plans to submit the work for peer review.
This pacing system has some limitations. To help account for other factors that can contribute to overexertion, the app offers a manual-tracking option for cognitive and emotional exertion, as well as for other symptom tracking.
