Inclusion Criteria:  Covid patients -  Adults aged 18 years and over, Severe, laboratory confirmed Covid-19 infection that required hospital admission, 5-7 months post discharge from local hospital Trusts.  Controls -  No active or previous Covid infection, ie no previous positive Covid PCR test and absence of active symptoms Matched for age, gender, BMI and ethnicity.  Exclusion Criteria:  Absolute contraindications for MR scan Pre-Covid diagnosed chronic respiratory, renal, cardiovascular or cerebrovascular disease or diabetes BMI<20kg/m2 Pregnancy Those requiring oxygen therapy or non-invasive ventilation Those on anticoagulation will be included but will not have a muscle biopsy Active arthritis Overt muscle damage, e.g. plasma creatine kinase > 300 Any other conditions in addition to the above that the investigators consider may affect study measurements or safety