Table 2 Non-immersive physical activity training (Control)

Opening sessions (Session 1–2)

▪ A detailed patient education will be performed to develop a better understanding of the links between fatigue and neuromuscular control

▪ The participant is introduced to the WHO physical activity training recommendations for managing fatigue

▪ The control of the individual training intensity is done via the subjective rate of perceived exertion (RPE) on a standardized 10-point Borg-scale [8]. There are different exhaustion phases according to the WHO: 1) Preparation for return to exercise: RPE 0–1, 2) Low intensity activity: RPE 2–3, 3) Moderate intensity activity: RPE 4–5, 4) Moderate intensity exercises with coordination and functioning skills: RPE 5–7 and 5) Return to baseline exercise: RPE 8–10. Patients are asked, what level of exhaustion leads to worsening of symptoms in their daily lives. During our training sessions, we will stick to the exhaustion phase below. If, for example, a subject experiences PEM when going on walks that lead to an exhaustion level of 6, we will choose phase 3) for this respective patient. During our training session we will train until he/ she feels like he/ she has reached the exhaustion level of “5” on the 10-point Borg scale. This RPE monitoring [30, 31] allows us to control the patients' exercise intensity regardless of the cardiorespiratory exercise level, since patients with Post-COVID-associated fatigue have difficulty reaching the normally recommended heart rate due to fatigue and chronotropic incompetence [32]

▪ The WHO recommendations for physical activity include a gradually increasing activity program based on the Borg scale of perceived exertion (RPE: 0–10) as explained above with respiratory exercises and stretching in the exhaustion phase 1) and specific resistance exercises (e.g., biceps curls, wall presses, arm raises, sit to stand, knee extensions, squats, and heel raises) and recommendations for independent cardiorespiratory activities (running, swimming, cycling, or dancing) in all following phases

▪ Subsequently, the patient is asked to choose the exercises that are most suitable for him/her (e.g. biceps curls, wall presses, arm raises and so on). During one training session he will complete two upper-body and two lower-body exercises. Within this training setting ("set"), there is the option to repeat an individual number of training runs (60–120 s) (repetitions). To determine the initial number of repetitions for a participant, the patient is asked to perform an exercise as many times as necessary until he/she reaches his/ her individual determined exhaustion limit

▪ In individual cases where patients arrive to the training session with an initial exhaustion level that is already above their RPE-phase, patients will be asked whether they want to do the training at all. If they want to, repetitions will be performed only on this exhaustion level and no further increase of exhaustion is allowed. In case patients do not feel fit to complete any training, the session will be rescheduled

Core sessions (Session 3–11)

▪ The WHO training programs are conducted under guidance. Participants can always ask questions and receive technical support in performing the exercises. Before and after each session, the effect of the last session and the current session, respectively, is briefly discussed

▪ Participants can increase the difficulty or the number of exercises if they stay within their RPE-phase. If training does not lead to worsening of symptoms for at least seven days and patients feel ready to, they can proceed to the next higher exhaustion phase

▪ In those cases, in which an adjustment of the training intensity should become necessary due to subjective exhaustion or Post-exertional Malaise, an adjustment of the extent and length is carried out according to the 5-phase model of the WHO recommendations [8]. That means we will return to a lower exhaustion phase

Closing session (Session 12)

▪ During the last session the subjects are offered to compare themselves with the games from the first session and to reflect on possible changes on a voluntary basis

▪ Short feedback is given and ways to improve the sustainability of the progress made are discussed