In order to overcome or mitigate the known human musculoskeletal atrophy in microgravity (Fitts et al 2001), a set of various physical exercise inflight countermeasure protocols including treadmill and bicycle ergometer (e.g., CEVIS) for cardiovascular support (aerobic interval / continuous) and a set of resistive exercise devices (IRED, ARED) are currently used by the crew members onboard the ISS. Another interesting approach uses low negative body pressure (LBMP) in combination with aerobic exercise on a treadmill to counteract cardiovascular system deconditioning and orthostatic intolerance by a thoraco-cephalic body fluid shifting which is a well-known phenomenon observed in long-duration supine body position on the ground in bed rest, in jet pilots, as well as in microgravity (Macias et al. 2005). Most of the exercise devices were tested on the ground for their feasibility and efficacy to mitigate loss in muscle mass and force in the bed rest spaceflight analogue (Alkner and Tesch 2004; Watenpaugh et al. 2007). So far, the physical training devices used onboard the ISS, for example, are equivalents of aerobic cardiovascular regimen and weightlifting that are effectively used in everyday health and fitness workout but also in athlete sports on Earth.

In microgravity, resistive-like exercise can be performed if the almost unloaded human body (loss of G-forces) is “reloaded,” for example, by elastic bungee straps and comfortable body harnessing systems that press the body and legs toward a belt conveyor or to a fixed and/or moveable platform of an inflight exercise device to produce muscle loading G-forces during, e.g., squatting bouts via simulated gravitational forces. First results from inflight resistive exercise regimen on the ISS showed some unexpected insufficient effects for musculoskeletal system support (Gopalakrishnan et al. 2009; Trappe et al. 2009). Follow-up inflight studies determined the foot forces in order to find improved ways for a better countermeasure outcome (duration/day) with increased body loading for the crew members requiring greater loadings during onboard exercise prescriptions (Genc et al. 2010), but inflight force measurements revealed that astronauts could not get even close to the pull of gravity on Earth. For example, NASA astronauts spent hours a day on the ISS for exercise on treadmills and other devices to combat muscle wasting and bone loss, however, with little avail (Witze 2014).

One possible explanation for this critical situation is given by the fact that the outcome of a strenuous resistive exercise regimen with high muscle tension, perfusion rate and neuromuscular recruitment, and maximally force output that works fine to maintain or even to build-up skeletal muscle performed under normal terrestrial G-force loading however is significantly altered following body unloading in Space possibly due to more skeletal muscle relaxation with relatively low neuromuscular activity and due to the inertial mass acceleration in microgravity. In Space, human motions may be more or less comparable to the variable biomechanical dynamics during skeletal muscle loading with very little torque observed after passive rather than active mode motions performed on Earth as, for example, confirmed by differential outcome of active and passive muscle training on muscle and bone quality in simulated microgravity with HU animals on the ground (Sun et al. 2013). Another possible explanation for an insufficient outcome of currently available inflight countermeasure protocols is given by the likely possibility of maladapted proprioceptive postural control of muscle activity in the unloaded crew member’s body in Space which may not be adequately addressed by increasing the resistive loading during exercise bouts, for example, due to altered kinematic synergies in complex movements in microgravity (Casellato et al. 2007).

The current knowledge on inflight countermeasure protocols suggests that more sophisticated countermeasure paradigms should be exploited taking into account the obvious different biomechanical and neuromuscular control mechanisms in human motion and performance on Earth and in Space. As the complex physiological mechanisms of gravitational force sensing, transmission, and support are not yet fully understood for most of the biological soft and hard tissues including their neural adaptation mechanisms in the microgravity environment, additional basic investigations are needed in this direction in order to learn more about the physiological gravisensor mechanisms of the normal and deconditioned neuromuscular system and the movement kinematics modulated by altered gravity (0–1G, ΔG). This would be important in order to find feasible ways of countermeasure protocols with new technology that is sufficiently compliant for crew members to be used in spaceflight in order to minimize at least some of their health risks onboard the ISS until 2024 (Witze 2014), and possibly thereafter, in particular using more adequate inflight countermeasure prescriptions are required for the benefit of the next generation of space travelers during their prolonged space missions to Moon and Mars.