One of the major concerns of aging, but also during and after spaceflight, is loss of muscle and bone mass. In aging, this is associated with an increasing risk of fractures. Recently, the possibility of aged and aging astronauts has been arisen. Thus considering the perspectives of aging and space we want to discuss, in how far the adaptations during spaceflight and during aging interfere. In other words: does spaceflight push the astronauts along the irreversible axis of aging? And which of the spaceflight effects will be reversible? Bones adapt to their mechanical function. For convenience, a simple model has been proposed: Bone, as a 'mechanostat', keeps the strains within certain thresholds, namely one threshold for modeling, i.e. formation of new bone, and one for remodeling, i.e. repair and removal. These thresholds are usually expressed as strains. A crucial role in physiological strain detection is obviously played by the osteocytes. The largest forces in the musculo-skeletal systems arise from muscle contractions. The reason for this are the poor levers, against which the muscles pull. For example: during a one-leg vertical jump, a young subject (body weight 70 kg) exerts a vertical ground reaction force of 2500 N. Due to the lever ratio of os calcis and forefoot around the tibio-talar joint, the calf muscles must exert a force 3 times greater, so that together with the body weight the bones of the lower leg are loaded with 10000 N, i.e. 14 times the body weight. Accordingly, good correlations can be observed between muscle strength and bone strength, or muscle mass and bone mass. It is therefore reasonable to discuss the accumulated knowledge about loss of muscle and bone in a combined approach. In this respect, two points must be considered: (i) for structural adaptation of bone, the muscular variable of interest arc force and rate of force development, but not power, and (ii) women before menopause have a greater bone to muscle ratio than men.