Disparity in the effect of partial gravity simulated using a new apparatus on different rat hindlimb muscles

The days of returning to the Moon and landing on Mars are approaching. These long-duration missions present significant challenges, such as changes in gravity, which pose serious threats to human health. Maintaining muscle function and health is essential for successful spaceflight and exploration of the Moon and Mars. This study aimed to observe the adaptation of rat hindlimb muscles to partial gravity conditions by simulating the gravity of space (microgravity (µG)), Moon (1/6G), and Mars (3/8G) using our recently invented ground-based apparatus. A total of 25 rats were included in this study. The rats were divided into five groups: control (1G), sham (1G), simulated Mars (3/8G), simulated Moon (1/6G), and simulated Space (µG). Muscle mass, fiber proportion, and fiber cross-sectional area (CSA) of four types of hindlimb muscles were measured: gastrocnemius (GA), tibialis anterior (TA), extensor digitorum longus (EDL), soleus (Sol). Sol and GA exhibited the most significant alterations in response to the changes in gravity after 10 days of the experiment. A notable decline in muscle mass was observed in the simulated µG, Moon, and Mars groups, with the µG group exhibiting the most noticeable decline. In Sol, a noteworthy decline in the proportion of slow-twitch type I fibers, CSA of slow-twitch type I fibers, and average CSA of the whole muscle fibers was observed in the simulated groups. The GA red, mixed, and white portions were examined, and the GA mixed portion showed significant differences in fiber proportion and CSA. A notable increase in the proportion of slow-twitch type I fibers was observed in the simulated groups, with a significant decrease in CSA of type IIb. In EDL or TA, no discernible changes in muscle mass, fiber proportion, or fiber CSA were observed in any of the five groups. These findings indicate that weight-bearing muscles, such as Sol and GA, are more sensitive to changes in partial gravity. Furthermore, partial gravity is insufficient to preserve the normal physiological and functional properties of the hindlimb muscles. Therefore, targeted muscle interventions are required to ensure astronauts' health and mission success. Furthermore, these findings demonstrate the viability and durability of our ground-based apparatus for partial gravity simulation.