The investigators focus on the rare Glycogen Storage Disease type 2 (Pompe Disease), a genetic disease characterized by gradual replacement of muscular tissue with glycogen and fat, resulting in loss of muscle mass and muscle strength. Pompe disease is of particular interest since a new drug has recently been developed and approved for this, hitherto untreatable, condition. This treatment is expensive, and it is therefore of great importance to have objective methods of examination to monitor the disease and the effects of the treatment.