Abstract

Long-term spaceflight has a severe impact on several physiological systems. The expected main problem of planned flights to Mars (with a duration of approximately three years) is the degradation of the musculo-skeletal system. The absence of gravity and the relative inactive lifestyle cause atrophy of both muscle and bone. The latter is also known as osteoporosis. The decrements in muscle mass attribute to the severe losses in muscle strength, thereby endangering the health and productivity of spacetravellers in-flight and after return to earth. The exact relationship between muscle atrophy and decreased muscle strength is still unclear, as is the degree of the influence of neuromotor changes on this relationship. The losses in bone mineral density have not yet reached clinical significance during spaceflights, but could become the mission limiting factor of future flights as fracture-risk increases with the continuous in-flight breakdown of bone tissue.

The most important and most effective countermeasure against the musculo-skeletal problems encountered during spaceflight is exercise. In-flight exercise has been performed for decades, but may not yet have been used to it’s full potential. The transference of recent findings and beliefs of earth-based exercise physiology to practical use in a weightless environment may help to optimize in-flight exercise protocols.

Recent findings indicate that the muscle strength increasing potential of resistance training is enhanced when maximal eccentric contractions are included. Current in-flight exercise protocols hardly include this type of contractions. Osteoporosis can be prevented, or maybe even reversed, by the application of exercises that provide transient high impact forces. This is believed to be the optimal stimulus for bone enhancement. Exercise devices that could provide these stimuli are currently under design. It is important to determine the exact loads that these devices impose on the musculo-skeletal system, so their practical use can be based on their scientifical relevance rather than mere expected benefits.On earth, an exercise protocol that aims at enhancing the musculo-skeletal system relies also on the constant presence of gravitational forces when exercises are not being performed. It is difficult to transfer any results from earth-based research to the practice of spaceflight, when such a "back ground strain" is absent. The Russian designed TNK V-1, or "Penguin suit" may supply this strain, and may increase the effects of exercises performed while wearing this suit. However, the exact loads this suit provides have never been quantified.

Both muscle and bone tissue can benefit from the same exercises, as the loads placed on muscles also produce loads on the skeleton. An important feature of exercises that aim to enhance both muscle and bone tissue is that they are mainly site-specific. Since the whole body is affected by the absence of gravity, all major muscle groups must be included in exercise protocols. However, the legs and trunk are continuously acting against gravitational forces on earth, and consequently suffer the most when these forces disappear. Therefore exercises of the legs and trunk should be stressed most during long-term missions.