Severe disease from COVID-19 requires hospitalization in 20% of adults, with a third of those patients requiring intensive care unit (ICU) treatment. While a growing number of patients are surviving COVID-19 critical illness, like other ICU survivors, the majority will experience significant post-hospital disabilities in physical function and exercise capacity, new-onset disability, muscle wasting/weakness, and new cognitive deficits. These difficulties, which can persist for months to years post-ICU, are collectively known as post-intensive care syndrome (PICS) and may be more severe in COVID-19 survivors. Despite literature demonstrating the benefit of physical rehabilitation in COVID-19 and other ICU survivors, there are limited available options and data to support an optimal post-hospital physical rehabilitation strategy. Combined with the difficulties of delivering rehabilitation in- person after hospitalization during a pandemic, addressing this “rehabilitation pandemic” from COVID-19 represents an enormous public health challenge. Previous studies of home-based physical rehabilitation in ICU survivors have utilized unsupervised exercise training without a personalized exercise prescription, and none have done so while actively monitoring and targeting exercise via physiologic data such as heart rate or included strength and balance training. In debilitated older patients, high intensity interval training (HIIT) is safe and associated with significant improvements (in as little as two weeks) in cardiorespiratory fitness (CRF) that also correlates with self-reported physical function and disability. We have evaluated this approach in a successful NIH-funded phase 1 pilot study demonstrating safety and feasibility of HIIT in debilitated older adults with cancer. Adherence to exercise was 117% (meaning patients exercised more than recommended 3 sessions/week), compliance with heart rate goals was 98% (meaning patients were actually doing HIIT vs. less-intensive exercise), and on average patients improved CRF, as measured by VO2peak on cardiopulmonary exercise testing by 25%. The key to our approach is our mHealth platform, in which patients are given Apple Watches and iPhones preloaded with our software allowing us to remotely download data after each exercise session to provide personalized up-coaching via videoconferencing in home setting. As critical illness survivors, including COVID-19 ICU survivors, have impaired CRF (similar to patients in pilot study) and other manifestations of PICS, a similar monitored, personalized, structured, and progressive multi-domain physical rehabilitation intervention has significant potential for improving recovery, but has not yet been studied in COVID-19 survivors. To address this gap in knowledge, we will randomize 140 COVID-19 ICU survivors being discharged home to our REmotely Monitored, Mobile health-supported High Intensity Interval Training after COVID-19 Critical Illness (REMM-HIIT-Covid- 19) stud...