• High-intensity interval training (HIIT): physiologically defined heart rate-controlled cycling with 80-100 rounds per minute (rpm) at 95-100% of maximum heart rate (HRmax). Participants will perform 5 × 1.5-min high-intensive exercise bouts at 95-100% of their HRmax followed by active breaks of unloaded pedalling over 2 min with the aim to achieve 60% of HRmax.
• Energy management education (IEME): face-to-face education sessions of 6.5 h in duration over a 3-week period, all conducted by a trained occupational therapist. Participants acquire knowledge and understanding about factors that influence energy and the consequences of fatigue on their habits and lifestyle. Six weeks after returning home, the participants will receive a reinforcement letter in the form of information material to remember the content of the IEME and to reinforce the implementation of the behaviour change in managing energy.

• Low-intensity training (ST): participants will exercise for 24 min continuously at 65% of participants' HRmax (60-70 rpm).
• Progressive muscle relaxation (PMR): The aim of PMR is to achieve enhanced mental relaxation by reducing muscle tension. Participants will attend six 1-h group sessions over the 3-week intervention period, instructed by a trained physical therapist. Six weeks after returning home, the participants will receive a reinforcement letter with information material for remembering the content of the PMR techniques and to reinforce the implementation of the exercises at home.

Treatment in both arms consists of specific endurance exercise modalities (HIIT or ST). Treatments differ in the applied training intensities. Participants in both arms will exercise 3 times per week over a period of 3 weeks on a bicycle ergometer. Exercise sessions will be supervised by a trained physical therapist. Exercise intensity will be heart rate monitored based on the maximum heart rate (HRmax) assessed during the initial cardiopulmonary exercise test. Exercise sessions in both arms will include a warm-up and a cool-down period at low intensity (50% HRmax) for 3 min each.

Changes of health-related quality of life will be assessed with the SF-36. The SF-36 questionnaire comprises 36 items. Scores range from 0 to 100 with higher values indicating better QoL.

Soluble factors that are known to be produced or secreted in response to exercise and are suspected to modify immune homeostasis and blood brain barrier function through their inflammatory and anti-inflammatory properties. Changes of Matrix-metalloproteinases-2 (MMP-2), Tryptophan, Kynurenine, Kynurenine acid, Interferon-gamma (IFN-Gamma) and Interleukin-6 (IL-6) will be assessed. Higher values indicate higher levels of Inflammation.