Muscle weakness is a detrimental consequence of ageing and of prolonged disuse. One of the most notable effects of these conditions is a disproportionate loss of strength compared to that of muscle size. Although the aetiology of this phenomenon is rather complex, since it involves muscular, tendinous, neural and hormonal changes, comparison of these changes proves useful for highlighting the mechanisms underlying these adaptations and for the implementation of effective countermeasures.

We had the opportunity to study, using dynamometry, ultrasonography and MRI, the musculoskeletal adaptations of the human plantarflexors occurring with spaceflight, prolonged bed rest and ageing. After 17 days of spaceflight (Space-shuttle mission STS-78) we observed a 22% reduction in tetanic force as compared to a 7% decrease in muscle cross-sectional area (CSA), therefore providing evidence of a decrease in force per unit CSA as a result of spaceflight. This finding is further corroborated by recent observations on the musculoskeletal changes produced by simulated microgravity. Muscle strength, size and architecture and tendon mechanical properties were measured in six males aged 27-37 years before and after a 90 day-bed rest period organised by the European Space Agency (ESA) together with the French (CNES) and Japanese Space Agencies (NASDA) and performed in Toulouse, France (LTBR-2001-2). Long-term bed rest resulted in a 55% decrease in maximum isometric force compared to a 32% decrease in muscle cross-sectional area (CSA), thereby indicating a loss of force per cross-sectional area. The observed muscle atrophy was accompanied by marked alterations in muscle architecture. Muscle fibre fascicle length and pennation angle were significantly smaller after bed rest by 9% and 14%, respectively. Tendon mechanical properties were also markedly affected by the prolonged disuse period as indicated by a 32% reduction in tendon stiffness associated with a decrease in rate of force development. These alterations in muscle architecture and tendon mechanical properties are very similar with those observed in old age. Indeed, muscle fibre fascicle length and pennation angle of elderly males (n=19) aged 65-81 years were respectively found to be 8 % and 13% smaller than those of young adults aged 27-35 years (n=13). Also, similar to disuse, the stiffness of elderly tendons was found to be lower by 10% compared that of young tendons. These data indicate that in both ageing and simulated microgravity, muscle atrophy involves both a loss of sarcomeres in parallel as well as in series. These adaptations are believed to play a significant role in the loss of muscle strength and shortening velocity induced by these conditions. It was estimated that these changes in muscle architecture accounted for ~50% of the decrease in muscle strength and shortening velocity in old age. The combined effect of reduced tendon stiffness and of shorter muscle fascicle length may place muscle fibres in a non-optimal region of the length-tension relation of the muscle, thereby further contributing to the loss of muscle strength.

In conclusion, prolonged disuse, such as that induced by spaceflight, leads to changes in muscle architecture, and tendon material properties, similar to those observed in ageing. These structural alterations are believed to play a significant role in the loss of muscle strength and shortening velocity induced by these conditions.
Since muscle wasting and weakness in old age is not only due to ageing but also to reduced physical activity, the use of countermeasures will not only prove useful for preserving muscle mass and mobility but also for differentiating the effects of ageing per se from those due to disuse.

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