According to a new study, researchers estimated that the inactivity of astronauts during spaceflight presents a significant risk to their muscles. Scientists have simulated the impact of  21 day spaceflights on the body, and the impact of low gravity environments such as the Moon or Mars. The study was published in The Journal of Physiology.

During spaceflights, and in future habitats on the Moon or Mars, humans are or will be exposed to a condition of "microgravity," in other words to a gravitational field much smaller compared to that present on Earth. This environmental stress has negative consequences on many organs, systems, and functions of the body, including bone, muscle, plus the cardiovascular, respiratory and nervous systems.

Spaceflights and reduced gravity habitats will expose astronauts also too low oxygen environments. The hypoxic stress will, therefore, be superimposed upon that related to reduced gravity. Mitochondria are found in cells, and they use oxygen to produce energy in a process called oxidative metabolism. People use their skeletal muscles for movement, and therefore to be active requires oxidative metabolism by these muscles.

The muscles need for oxygen led researchers to think that the low oxygen ("hypoxic") environments of spaceflight or future planetary worlds would impair function. However, they found that inactivity itself may have a more pronounced effect on skeletal muscle than a lack of oxygen. The results demonstrated that hypoxia did not aggravate the impairments following microgravity.

Given that inactivity and hypoxia are frequently associated with several important cardiovascular and respiratory diseases there is a suggestion that for skeletal muscle, inactivity is worse compared to hypoxia. If confirmed, this finding would have obvious relevant consequences on therapeutic and rehabilitative interventions: correction of hypoxia could be less critical than correction of inactivity.

As well as for astronauts, this research could have a significant impact on patients with chronic diseases characterized by immobility and hypoxia. The researchers evaluated the mechanisms by which the tissue obtains energy by burning sugars and fats by the intervention of oxygen during exercise, on the knee extensors of one leg in 11 active men. Mitochondrial respiration was also evaluated on skeletal muscle fibers by biopsy.

Future studies will have to investigate in more detail the mechanisms responsible for the observed findings. The results obtained on skeletal muscles, moreover, will have to be interpreted in conjunction with those deriving from other studies of the PlanHab project, dealing with the cardiovascular, respiratory, immune and central nervous systems and metabolism.

It is important to note that another study from the research group (the PlanHab Project), looking at whole-body exercise, found that hypoxia did, in fact, aggravate the impairment caused by microgravity alone. This suggests that the effects of hypoxia could be more significant during whole-body exercise, in which the stress on the cardiovascular system is more pronounced.