Articular cartilage is essential for unconfined function of the musculoskeletal system. Because it is a bradytroph tissue, normal mechanical loading, as experienced in daily life, is essential for cartilage health. The effect of reduced mechanical loading, e.g through immobilization or microgravity, is not completely understood. Because of the slow rate of metabolism of cartilage, the time line for intervention experiments needs to be longer than for other skeletal tissues and the regenerative capacity of cartilage is very limited, once degradation occurs. Thus performing unloading experiments in healthy humans is difficult. A few studies have been performed in patient cohorts that experienced unloading due to injury and the results suggest that human cartilage health is negatively affected by unloading. Space flight research offers a unique opportunity to investigate musculoskeletal tissue adaptation to immobilization either in bed rest or space flight experiments in healthy individuals. Data on cartilage health are sparse but suggest that it is necessary to assess the risk of cartilage deconditioning during extensive human space travel. Results from this context offer the unique possibility to broaden our understanding of the role of mechanical loading for tissue health and will provide important information for patients suffering from immobilization periods due to injury or disease
