Exposure to simulated hypogravity is the commonality between rehabilitative gait training and exercise countermeasures during human spaceflight. Patients with orthopedic or neurological disorders benefit from gait training with up to 30% body weight support (equivalent to simulated 0.7g), since less forces are acting on their lower extremities whilst gait kinematics are largely preserved. To restore walking function for those patients, in addition to preserved gait pattern, it is also important to maintain the contractile behavior of the plantar flexor muscles such as the gastrocnemius medialis (GM). However, in vivo measurements to determine whether walking with 30% body weight support (BWS) modulates GM fascicle and series elastic element (SEE) behavior have not been performed until this doctoral study.
In contrast to the intentionally applied BWS during rehabilitative gait training on Earth, astronauts exposed to micro- and hypogravity have to actively counteract the reduced loading to avoid musculoskeletal deconditioning. Therefore, International Space Station (ISS) crewmembers perform daily exercise countermeasures, including treadmill running with artificial force loading. Their maximum force loading happens to be at a similar level as the abovementioned recommendation for BWS gait training on Earth (i.e., simulated 0.7g). However, as with rehabilitative gait training, ultrasonic visualization of GM behavior during simulated running on ISS has not been performed before this doctoral study. These data now provide an insight into whether it is possible to replicate Earth-like contractile conditions in space, and thus apply similar stimuli exerted on the muscle. Moreover, for future mission scenarios it is crucial to know whether, and how, GM behavior would be modulated when reducing the hypogravity level to simulated Martian (0.38g) and Lunar gravity (0.16g).
Thus, the aim of the present doctoral study was to investigate in vivo the immediate effects of walking and running under different conditions of simulated hypogravity on GM fascicle and SEE behavior. Hypogravity was simulated on two different devices: on the Anti-Gravity Treadmill AlterG, to replicate rehabilitative gait training, and on the vertical treadmill facility (VTF) to replicate running on board ISS, Mars and Moon. Plantar forces of participants (n = 8, 32 ± 5 years, 178 ± 6 cm heights, 94 ± 6 cm leg lengths, 74 ± 7 kg body masses) were measured via force insoles to determine their stance phases and achieved loading levels. GM fascicle lengths and pennation angles were quantified using ultrasonography. Ankle and knee joint angles were recorded via electrogoniometers and analyzed to determine muscle‒tendon unit (MTU) lengths, consisting of the muscle’s contractile and series elastic elements. The lengths of the latter were calculated via an MTU model.
The results of this doctoral study are presented in three articles following the main fields of application: rehabilitation, ISS exercise countermeasures and future planetary exploration.
The main finding of the first article is that, in addition to gait kinematics, GM fascicle and SEE behavior is preserved during walking on the AlterG with 30% BWS. This is essential to recover “natural” locomotor patterns of patients and reinforces the recommendation of up to 30% BWS for rehabilitative gait training. In contrast, the results of the second and third article reveal significant differences in GM fascicle and SEE behavior, as well as gait kinematics between running on a conventional treadmill at 1g and running on the VTF at simulated 0.7g (Article 2), 0.38g and 0.16g (both Article 3). Modulation of GM behavior was found to increase with decreasing hypogravity levels. For instance, when decreasing the simulated gravity, decrements in values for SEE lengths, MTU lengths, pennation angles and shortening velocities are observed, whereas fascicle lengths increase.
These observations suggest that running on board ISS at simulated 0.7g does not provide an exact replication of Earth-like contractile behavior. Whether this functional adaptation to running under hypogravity conditions precipitates muscular deconditioning warrants further study. Nevertheless, it cannot be excluded that the observed alterations in contractile behavior, when not being compensated for elsewhere, affect the muscle’s work capacity when being re-exposed to gravitational loading. This may not only require specific attention during astronauts’ post- mission rehabilitation phase back on Earth, but also when completing mission-specific tasks after landing on planetary bodies such as Moon and Mars. Moreover, the results indicate that fascicle and SEE behavior is sensitive to small absolute changes in hypogravity levels, which questions the 1:1 transferability of Lunar to Martian surface operations. It is thus concluded that, to maintain GM muscle mass and function, exercise countermeasures such as running should be optimized, to induce an Earth-like contractile behavior, be it on ISS, Moon, or Mars.