At the center of pediatrics in Copenhagen the investigators experience that the children with CP with low skeletal muscle mass have more complications and admission after for example surgery is prolonged due to these complications. In order to prevent malnutrition and energy insufficiency as well as obesity and cardiometabolic disorders in adulthood, and with the aim of defining biomarkers for energy needs, the investigators wish to determine energy and protein needs in children with CP. The investigators wish to investigate the whole-body metabolism in children with cerebral palsy (CP) and compare the results with results from healthy children. The investigators aim to include 10 children (aged 2-18 years) with CP with low skeletal muscle mass and 10 healthy controls (aged 2-18 years). By using stable isotope technique the investigators will investigate systemic fat, glucose and protein metabolism together with liver protein degradation and glucose production. Furthermore, by using DXA scan the investigators will describe the quality and distribution of skeletal muscle. Lastly, the investigators will determine the skeletal muscle signal pathway and metabolism in skeletal muscle via the Bergström biopsy technique in vastus lateralis.