The aim of the present project is to assess the effects of the chronic diseases and their associated treatments chronic paediatric diseases (CPD), to further understand their impact on physical fitness for public health perspectives. This is an innovative approach in the treatment of chronic paediatric diseases . This project should yield results that help improving treatments for children and adolescents with chronic paediatric diseases throughout physical activity as therapy, reduced pain, fatigue and inflammation, and improvement in physical fitness and life quality. The originality and novelty of this project is to combine architectural, functional and metabolic components of skeletal muscle to further understand the impact of chronic paediatric diseases as a function of treatment, disease activity and maturation status (prepubertal, pubertal or post pubertal). This study will aim at assessing muscular function (force production capacity and fatigability) in specific or ecologic situations so as to get information about muscle functioning on isolated muscle group (here knee extensors) or during whole body exercise. Moreover, results arising from muscle architecture or quality will allow understanding the decrease in strength or endurance reported in the literature. The data collected will allow us to further understand the impact of the disease on structural, functional and metabolic parameters. Finally, the understanding of these alterations will provide information enabling to establish recommendations in physical activity (PA) to reduce or even counter the effect of the chronic inflammation and prevent at long-term overweight and cardiovascular risks. The long-term objective is to contribute establishing recommendations or guidelines for prescribing physical activity during medical therapy. Values obtained in pathological children will be compared to those of control children matched for gender and maturation.