Abstract
The control of bipedal stance and the capacity to regain postural equilibrium after its deterioration in variable gravities are crucial prerequisites for manned space missions. With an emphasize on natural orthograde posture, computational techniques synthesize muscle activation patterns of high complexity to a simple synergy organization. We used nonnegative matrix factorization to identify muscle synergies during postural recovery responses in human and to examine the functional significance of such synergies for hyper-gravity (1.75 g) and hypo-gravity (0.25 g). Electromyographic data were recorded from leg, trunk and arm muscles of five human exposed to five modes of anterior and posterior support surface translations during parabolic flights including transitional g-levels of 0.25, 1 and 1.75 g. Results showed that in 1 g four synergies accounted for 99% of the automatic postural response across all muscles and perturbation directions. Each synergy in 1 g was correlated to the corresponding one in 0.25 and 1.75 g. This study therefore emphasizes the similarity of the synergy organization of postural recovery responses in Earth, hypo- and hyper-gravity conditions, indicating that the muscle synergies and segmental strategies acquired under terrestrial habits are robust and persistent across variable and acute changes in gravity levels.
Highlights
The gravitational force on Earth has remained constant since the formation of the planet[1]
It has been assumed that the activation patterns of postural muscles can be synthesized into synergy organizations in both animal[10] and human models[11]
These results suggest that neither a simple reflex mechanism nor a fixed synergy organization is adequate to explain the muscle activation patterns observed in postural control tasks[8,15]
Summary
The gravitational force on Earth has remained constant since the formation of the planet[1]. With an emphasize on distal muscle groups, authors demonstrated gradually increased muscle activities, facilitation of spinal reflexes and delayed onset latencies with progressively increasing gravity during upright stance[7] Some of these findings are quite intuitive, since compensating for gravitational forces and shifts of the center of mass (COM) presupposes an adequate muscle’s level of activation, which depends on the loading force and is proportional to gravitation[6]. The nonnegative matrix factorization (NNMF) approach has been validated in identifying muscle synergies during postural responses to examine the functional significance of such synergies for natural recovery responses[9,11] These results suggest that neither a simple reflex mechanism nor a fixed synergy organization is adequate to explain the muscle activation patterns observed in postural control tasks[8,15]. This synergy organization seems to be subject-specific[17], and there is increasing evidence for the robustness of those synergies across a variety of biomechanical contexts[17,18], indicating common neural mechanisms for reactive balance across different tasks[9]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
