Frailty is a complex phenomenon which is a result of a decline in reserve capacity. Although it is generally known that movement has a positive influence on a diversity of health outcomes in relatively healthy elderly, the effect on frail elderly is less known. Key elements in the clinical manifestation in frail elderly are muscle weakness, atrophy and fatigue. A decrease of muscle mass and muscle strength often seen in frail elderly is called sarcopenia. Scientific research indicates that an increase of physical activities appears to be the most promising treatment strategies in the rehabilitation of frail elderly. Specifically the muscles show spectacular results of training. To adequately train frail elderly knowledge of the specific biological changes in muscles due to training is essential. This masterclass will address the latest scientific knowledge on this topic.