Sarcopenia, the age-related loss of muscle mass, is becoming recognized as a major public health problem. Still, the relative contribution of its underlying mechanisms to functional impairment and disability is not clear. The following doctoral project aims at developing a better understanding regarding the effect of ageing, training and detraining on muscle quality. The findings of this research, which consists of two studies, should result in recommendations for optimized age-related preventive and/or rehabilitative exercise programs. In a first longitudinal follow-up, the effect of ageing on muscle quality in older adults (65+) will be examined. Specifically, changes in muscle mass, muscle strength, muscle density and force-velocity characteristics will be assessed with a 5 yr interval. Secondly, the adaptations in muscle quality of the knee extensors will be studied following training interventions and detraining in young (20-30 years), old (60-70 years) and very old (75-85 years) females. These interventions will differentiate between resistance training (i.e. high-intensity and explosive low-intensity) and endurance training (i.e. walking and cycling).