Table 1 Immersive neuromuscular training intervention

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 ICAROS training device and the different VR training programs. The patient is then asked to choose the VR training setting that suits him best (e.g., flight over the mountains)

▪ To accustom the patient to the VR experience and avoid adverse events, we will slowly increase the level of immersion. We will begin with combining the ICAROS device with a tablet and move on to a VR headset once the patient feels comfortable

▪ 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 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]

▪ Within this training setting ("set") there is the possibility to repeat an individual number of training runs (“repetitions”: about 50–90 s). To determine the initial number of repetitions for a participant, the patient is asked to perform an immersive exercise as many times as is necessary until the subject reaches his or her individual determined exhaustion limit

Core sessions (Session 3–11)

▪ The immersive training programs are conducted under guidance. Participants can always ask questions and receive technical support in setting up the VR set and adjusting the ICAROS system. 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 ICAROS fights 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

▪ 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

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.