Humans on Earth (earthlings) are born in gravity. The Earth’s gravity (1G) is a unique environment that shapes our body and, for example, strengthens muscle and bone quality by a lifelong process of adaptation as a function of cell and tissue plasticity of many body structures including, for example, muscle and bone, cardiovascular system, and the nervous system. As our body structures and functions imperceptibly adapt to the terrestrial environment, they also help to find an adequate body equilibrium from the very first moment as a newborn or a little child, during growth and adolescence, as adults or older people, in the elderly, and finally in senescence. Movement, locomotion, and performance in 1G are, however, complex, but three main principles may be critical with respect to gravitational loading adaptation on Earth:

How

A simplistic model of nervous system motion control in higher vertebrate evolution may be easily imagined by the conserved functional development of central sensorimotor subsystem regions in the brain and spinal cord of various species with the concomitant development of the functional locomotor apparatus comprised of bones, muscles, and peripheral nerves of the trunk and limbs by comparative functional anatomy (Fig. 1.1). In evolution of the vertebrate brain functional structures that, for example, are used in body motion control, the small brain (cerebellum) of fish comprises a relatively archetypal cerebellar region (archicerebellum) with neural networks predominantly controlling body equilibrium (vestibulocerebellum) and simple eye movements (two elementary performances of fish swimming in water). As terrestrial life evolved from aqueous life, amphibian creatures living in the transition zone between water and land were more or less challenged by altered body loading forces with the inevitable need to claim against gravity using bony structures adapted for mechanical strain compensation and contracting muscle adapted to work against gravitational forces during their terrestrial stay in absence of body lifting forces in water immersion. The amphibian motions thus evolved from a second functionally more specialized cerebellar region called the paleocerebellum (spinocerebellum) with neural networks known in higher vertebrates to be responsible for postural adjustment control and fine-tuning of purposeful motor activities via brain and spinal cord to peripheral target limb muscles (muscle tone/tension, target-directed movements) necessary for elementary body motion control (in case of amphibians by creeping and hopping on terrestrial ground). However, animal life and survival on terrestrial ground was challenged by even more complex motion patterns with the need for ready sets of well-trained motion and locomotion programs, for example, to perform flight and fight reactions, to hunt food, and to reproduce in terrestrial life. Thus reptiles, birds, and mammals including man refined (updated) their motion control system by a third functionally more specialized cerebellar region (neocerebellum) with neural networks known for the typical design of even more specialized movement coordination control programs for optimal survival and fitness for life (Darwinian principle) on Earth (Fig. 1.1). In all these evolutionary steps, the movement apparatus tissue structures such as bone, muscle, and nervous system turned out to share equal plasticity mechanisms for adequate adaptation to gravitational loads in terrestrial life. Though, animal and human motions performed by the musculoskeletal system (muscle and bone) controlled by sensorimotor systems such as the neuromuscular system are far more complex than described by this simplistic view of vertebrate motion evolution. However, this example may help to imagine the impact of gravitational loading on evolution and adaptation of the body movement apparatus and motion control mechanisms on Earth. The interested reader is referred to chapters on locomotion, posture, cerebellum, and vestibular system of relevant textbooks and journal articles for more comprehensive and detailed information regarding the current understanding of motion, locomotion, and equilibrium control and the impact of gravity on Earth (Thews et al. 1989; Kandel et al. 2000; Orlovsky et al. 2001; Jamon 2014).

The term neuromuscular system used in this book is proposed in order to intuitively anticipate gravitational forces and their impact on the innervated locomotion apparatus of human being and to find a comprehensive and more integrated view on the biomechanical and functional interactions between bone and muscle (muscle-bone unit), for example, muscle force in joint dynamic stability (An 2002) with, for example, the neuromuscular system structures (nerve-muscle unit) to enable physiological human motions and performance on Earth.

The neuromuscular system on its own and its different functional tissue and cell components are likely targets to functional adaptation on Earth but also to variable changes (deconditioning) due to the microgravity environment in Space and thus should be considered as a common functional system in the future development of inflight countermeasures to minimize microgravity-related risks of impaired performance control and health of humans in Space and after their return to Earth.

Human motions and performance in Space is very different from terrestrial life on Earth (Wassersug 1999). In order to remain in space for longer periods such as during planned missions to Moon or Mars, the negative effects of microgravity on muscle and bone (DiPrampero et al. 2001; Cancedda 2001) and on sensimotor control, posture, locomotion, and spatial orientation (Reschke et al. 1998) must be overcome (Seibert et al. 2001).

Briefly, in weightlessness (or zero-G), the human body experiences only the acceleration that defines its inertial trajectory, or the trajectory of a free fall. In Space, almost any functional body system is challenged (deconditioned) by the zero-G or the microgravity (μG) environment. Without gravity, crew members perform their mission duties, for example, onboard the ISS, mostly in a “floating body position with minimal if any gravitational loading.” For example, they receive quite unaccustomed inputs from visual, tactile, and sensory information resulting in conflicting physiological stimulation with disorientation in microgravity. The crew members in Space are also challenged by hypoactivity (relatively slow body motions that require only low forces comparable to open chain muscle activation; see Sect. 2.​1.​4) as their weightless body mostly takes a squatting position that may be comparable to almost floating-induced body unloading in an ambient temperature water bath on Earth which, for example, gives a pleasant feeling of wellness and full relaxation. In Space, body unloading produces disuse-induced structural, molecular, and functional changes in skeletal muscle following either short-, medium-, or long-term exposure to a microgravity environment with consequences of impaired motion and performance control. In addition to spaceflight-induced maladaptation which is called space adaptation syndrome (SAS) or space motion sickness (Lackner and Dizio 2006), the loss in bone and muscle mass and function may be termed microgravity-induced atrophy.