Inclusion criteria Long stay post-ICU patients (length of stay ≥48h) between 6 weeks and 6 months after hospital discharge AND >18 years old AND able to visit the hospital 2 times a week AND a RAND-36 physical functioning subscale score < 67%  Exclusion criteria  Allergy to components of ProSource® (Allergy information: gluten-free, lactose-free, NOT cow's milk free (wey-protein), produced in a factory in which soy, wheat and nuts are processed. Inability to understand the Dutch language Actively participating in a professional physical rehabilitation program during the study period.  Note: previous participation in a concluded rehabilitation program is NOT an exclusion criterion.