Spinal column and midbrain integration for long duration space missions

Introduction

Long-duration space missions cause detriment to bones, winnowing their structural integrity for the rapid excretion and slow replenishment of lost calcium. Human functionality in space relies on mitigating the extent of lumbar, hip and shoulder decalcification. Much of the pivotal space medicine research focuses on the problem without examination of the spine’s architectural context as a function of vertebral evolution. If this is examined, it has the potential of not only reshaping the way spinal health maintenance is viewed, but by extension, could potentially activate the concurrent midbrain structures associated with it. Anamnesis, or the ‘remembering’, of the proto-human midbrain, can potentially be a gateway for long term benefits to human health in a weightless environment.

Based on this, the exercise machinery that is proposed supports the tensioning of unloaded paraspinal muscles that, when coupled with peripheral use of VR, can redefine somato-sensory neurofeedback circuits that the brain has optimized for 1G. This has the potential to alleviate lumbar injury, reduce cognitive deficits, and improve motor performance. It also has the secondary potential of inducing global cognitive performance improvement, while potentially contributing to astronaut wellness through limbic system activation.

The weightlessness of space introduces numerous physiological problems to astronauts on long duration flights. Bone Mineral Density (BMD) reduction manifests as lost calcium in bone tissue (bone resorption) which is in disequilibrium with bone calcium absorption. The bone’s physical inner structure (it’s trabecular microarchitecture) will undergo decalcification in osteoporotic losses of about 1 to 1.5 mg/month while in space; leaving a lower mass, and dramatically lower-density bone cells (osteocytes) to bear the same weight and stress loads as before (Orwoll et al., 2013).

Vertebrae and associated bursae will also undergo significant changes from the unloading of muscle tensioning that keeps the spine and its associated disc and ligament systems in functionally straight stasis at 1G (Townsend & Klijn, 2018). This spinal change manifests as reduced axial flexibility, resulting in elongated bursae that are unable to maintain sufficient water transportation across the cell membranes as natural osmotic processes. These are contributors to a heightened inter-vertebral distance (IVD) due to increased spinal fluid This process is responsible for a 3% growth in height that the average astronaut will experience.

These deleterious symptoms of spinal elongation as related to disc height and space osteoporosis are the basis of a 4-fold increase in spinal injuries (such as disc herniation) following long duration space missions (Laws, 2019). Normal-tensioned skeletal muscle in the spine, unloaded in weightlessness, makes for this risk of injury (Townsend, 2017). This injury comes from the lateral loads imparted upon the spine during long Extra Vehicular Activities (EVA) when the body must endure high impulse force exertion of the skeleton. Collagen fibers grow, stretch and strain along the axis of spinal growth until flexion is decreased (Fortin, 2013).

Often tissue will tear from the vertebra at the bone junction when sideways loads are applied to the lumbar column (Berg-Johansen et al., 2015). The paraspinal muscle’s atrophy is the result of a marked departure from the daily loading and unloading of the spine that has evolved as a normal process in 1G. Ultrasound images of lumbar spinal vertebra record the loss of lateral flexibility, the loss of functional cross-sectional area (FCSA), lumbar flattening, and loss of range of motion (Chang et al., 2016).

Train as you fly: Lower impact workouts on ergonomic machinery

Current physical training methods by NASA involve 1.5 hours of resistance training and 1 hour of aerobic training 6 days a week, yet studies show there is still loss of muscle mass (Trappe et al., 2009; Belavý et al., 2010). The International Space Station’s (ISS) exercise device compliment (such as the Advanced Resistive Exercise Device) ARED, function to keep astronauts fit and yet are still loading spinal, wrist and pelvic areas depleted by osteoporosis. The simulation of 1G via the use of an elastic hip-and-shoulder harness connecting the astronaut to the treadmill applies substantial percussive impact to the hip joint and the femoral neck, an area specifically notable for its high decalcification during long spaceflight.

A statistic to bear in mind is that the average loss of BMD in the astronaut population will be around 10% (Grimm et al., 2016). Even if the sponsoring agency assumes that the osteoporotic losses of 1-1.5 mg/month are linear in progression, the assertion can be made that the decalcification of bone on a long-duration space mission will reach a projected geometric progression to a catastrophic failure threshold beyond its load-bearing capacity in 1G (Axpe et al., 2020). Post mission microfracture analysis supports this, and astronaut injuries to hip and spinal areas upon return to 1G confirm this is probable (Dadwal et al., 2019).

1 G Reference Frame and the Zero-G Reference Frame

The ISS exercise machine design assumes that training in space is training for reintroduction to planetary gravity. All devices are based on terrestrial movement analogues for upright ambulation. The key to understanding zero gravity and its effect on the human body is to acknowledge that it renders the human body’s resting frame of reference in a semi-fetal position called the Zero Gravity Position, or Neutral Body Position (NBP) (NASA Technology, 2013). As a human body reverts to this position for extended periods in space, placing the body back into an upright 1G position to train is logical, however, it doesn’t consider the possibilities that occur by training in the Neutral Body Position. This could potentially avoid loads and impacts to the body that would stress the human musculoskeletal system.

When considering the evolution of the vertebral spine, the fossil record shows that waterborne animals used the spine to locomote in a buoyant environment. The segmented vertebrate spine is covered in longitudinal muscle groups called the paraspinal muscles. These can contract in an asymmetric pattern that produces an undular sine wave movement in the spine which acts on the fluid medium as an inclined plane applied in series. As genetic selective pressures drove complexity in the succession of limbs from fins, the same spinal-based locomotion remained in the counter-opposing four-limbed movements of tailed quadrupeds.

The weightlessness of space can be considered an analog to the buoyant hydrosphere in which vertebral species evolved. The paraspinal muscle tissue of the human back, though it may show symptoms of atrophy, fatty infiltration, and muscle fiber in aging astronauts, can replicate this spinal movement. When the shoulder and hips are engaged in a prone position exercise, (as lateral opposing movements of the hips and trunk), the spine alternately flexes and elongates along the column (such as in a low crawl). If an astronaut is starting from the Neutral Body Position and then engaging in exercise, it implies that there are no loads placed upon the cervical, thoracic, or lumbar vertebra that 1G postural muscle loading would have introduced. It also implies that the millimeter-wide tolerance gaps that hold increased hydrostatic pressure within the intra-vertebral distances (IVD) are not flexed to exacerbate pain and the inflammation of the bursae.

The prone position cycle

If the human spine is elongating for the absence of gravity, the paraspinal muscles, (for their role in postural support), could be stimulated or flexed to have a tensioning effect on the hypertonic bursae of the spinal column. Paraspinal flexion asymmetries are shown to increase in aging populations (Fortin, 2013). As decades of medical practice has shown, asymmetric paraspinal muscle groups can be made to contract through topical electric surface stimulation to correct postural deficiencies from scoliosis. 4 to 8 small topical electrodes are placed in series along the spine, with alternating current passing through them. This contracts and tones muscle and introduces a tensioning load to the spine over time that essentially corrects scoliosis (Kowalski et al., 2009). By extension, cardio/resistance exercise can also engage these muscles in series when aerobic training is conducted on the stationary prone-position cycle. Research has shown that muscle recovery is also attenuated by electro-stimulation (Sostaric et al., 2009).

A cycle for microgravity places the astronaut in a semi-prone horizontal position. It isolates the hip and shoulder group by suspending the pelvis, trunk and shoulders in a rock-climbing harness that is held inside an ergonomically flexible armature. What is noteworthy is that, for the lack of a seat, there is an assumed loss of impact to dermal soft tissue, the hip joint, the inflammation of meniscus areas, and the rider’s postural discomfort (Kwok et al., 2019).

The architecture of the Zero-G Cycle translates the Neutral Body Position into an articulated frame that swivels at the front wheelset and the hips. The circular movement in the front wheelset is made with a hand crank and the hips swivel at the end of the rear pedal drive train in the aft wheelset. There are specific angles of the machine/human interface that are to be considered in the cycle design, and all relate to the Neutral Body Position. In this position, there is a markedly lower line-of-sight made by the unloading of the cervical muscles and the elongation of the cervical column. This results in a head-down prone position and implies that there isn't a necessity to flex the neck upward to see forward (as one would do on a 1G bicycle). Given that there is nothing to see and avoid in front of a cyclist in microgravity, there can be a potentially beneficial visual-multi-sensory replacement in the form of Virtual Reality (VR) (Li et al., 2021) (see Figure 1).

Figure 1. Virtual Reality used with an exercise program enables neurofeedback to optimize cerebral and cerebellar function (Hoffman et al., 2004).

Image by Todd Richards and Aric Bills, copyright Hunter Hoffman, UW., vrpain.com

The VR connection and neuroplasticity

The exercise regimen of the Neutral Body Position cycle can reproduce the asymmetric muscle contractions that simulate a pre-hominid method of locomotion. While it is maintained in this proposal that this is central to the health maintenance of the human spinal column in zero G, it is also possible that the proprioceptive and sensorimotor systems of the astronaut, rewired by weightlessness, can be brought to an adaptive condition that closely resembles cerebellar perception and functions at 1G (Taube et al., 2004; Knierim et al., 2000). Plying deeper into the biomechanisms associated with vertebral locomotion intuits researching the role of the limbic system. Current exercise research on these areas that could be stimulated by VR situational awareness augmentation is an area in need of exploration. The human midbrain’s adaptability to weightlessness, as it is closely connected to the spinal cord, decides the efficiency of the nervous, immune, and cardiovascular systems that it regulates (Redlinger & Shao 2021). This could be programmed with neurocognitive plasticity in mind to introduce dynamic, evolving limbic system stimulation in the form of engaging interactive visual content (Esfahlani & Thompson, 2018). What is understood in VR research is that the brain’s plasticity can be a contributor to underwriting novel means of creating neuronal network group-firing (Qui et al., 2021). From the synaptic relationships that can be created by a neuronal unity of effort, cells that rewire together and fire together can overcome the plausible cognitive drawbacks of zero-G through remodeled synaptic pathway development. It is postulated that an induced cognitive, visio-spatial and memory intervention for the astronaut can be the means of optimizing their limbic systems via a VR mosaic (Sousa et al., 2021). The health of the limbic system can then be a wellness contributor to challenges such as emotional balance, hormone production and overall sympathetic nervous system homeostasis--all known regulators that are challenged in zero-G. These systems are connected to the brain as the medulla, amygdala, thalamus, and the hippocampus, and are part of the neurofeedback loop that enables the brain to function properly. In the case of a locomotion analogue, extensive study on VR’s ability to induce the neuronal network firing necessary for human ambulation has been showing significant results (Zhang et al., 2019; King et al., 2013). VR also shows a useful application for significant pain reduction during exercise (Matsangidou et al., 2019; Hoffman et al., 2004).

Theoretical framework

The functional design of existing zero-gravity exercise machinery considers the high-tension muscle loading, and long-duration workout, and this results in a durable and versatile instrument designed to be used by all types of astronauts in orbit on the ISS. Designs are adapted to be stowable, integrated into floor or wall panels, and will serve for decades as the only machinery of use for a particular type of aerobic or resistive exercise. The three exercise devices of use on the ISS are the stationary bicycle, the treadmill, and the ARED (Loehr et al., 2011). Data from the exercises are integrated into individual performance charts and this is downloaded to scientific medicine teams that monitor performance levels.

One of the means of risk mitigation to cardio and musculoskeletal systems is to supplement with vitamins and pharmaceuticals and focus training on muscle groups that bring the highest aerobic results and stress-load those skeletal areas most affected by calcium depletion in orbit (Maria et al., 2018). Though loss rates of BMD are observed to decrease slightly during spaceflight for this intervention (LeBlanc et al., 2013), the long-duration mission will still stress depleted areas of spinal muscle for the use of high-impact methods of ambulation and muscle loading such as the treadmill and Smith bench-press.

A hypothetical arrangement of exercise instrumentation is postulated to take advantage of space adaptation from exploitation of the ergonomic efficiencies of the Neutral Body Position (NASA Technology, 2013). Considering that the areas that are most susceptible to BMD depletion and cartilage damage are still being loaded by these exercise devices, it is possible that lumbar trauma risk mitigation is not operationally feasible. With this, then comes the risk matrix formulations where unsuitable risks are to be projected on a 600+ day Mars mission. In theory, it can be forwarded that a means of exercise that limits impact, while still maintaining high muscle loading of the musculoskeletal system is preferred over the existing methods.

Human instinct seeks to overcome the detriment brought about by long term fight-or-flight response in weightlessness. The test of this theory is to have one control group use the ISS exercise machinery and compare results to the focus group’s use of the prone-position cycle. The intent of this model is to reassess the exercise in zero gravity by removing 1G ambulation analogs and replacing them with an exercise that mimics locomotion in a buoyant environment. If the prone position cycle is of benefit, results of a six-month study should show:

Likewise, if the VR immersion therapy during exercise is to be of benefit, results should include:

This method of movement is comparable to the arrival of early multicellular organisms in the primordial ocean. Evolution has supplied the adaptation of flagellate cells that move in sinuous ways to propel a larger complex organism. This method of locomotion is the most efficient and simplest method of movement for vertebrates and invertebrates, and by extension, the existing hypothesis is that use of this spinal-based movement redevelops an instinctual mode of movement (see Figure 2). This is a key to assuaging the detriment of human spinal tissue unloading due to muscle atrophy in zero-G.

Figure 2. Richardson, T., (2023), Axial movement in immersion tank sketch, (pencil on paper/digital edit), digital collection, Las Cruces, NM, USA.

The axes of movement for spinal column exercise in a float tank are similar to thaose encountered in microgravity.

A significant study has been made of the functionality of VR or video games as a conveyance of environmental cues to a brain that is bereft of some sensory or motor activity (Ballesteros et al., 2018). Observations of the human brain while immersed in gaming show a positive correlation between participant observation and cortical mirror-neuron activation (Kern et al., 2019). It is possible that the cognitive areas of the brain in microgravity can be given corrective sensory information so that the cerebellar areas of balance and the brainstem’s areas of regulation, memory, and reaction can be given supplementary sensory information that helps to better regulate space adaptation. There is ample evidence that the brain’s ability to mitigate pain, remember lost limbs, and cue functional limb movement are possible for VR interventions (Matsangidou et al., 2019; Ramachandran, 1996).

These cited research efforts are foundational to establish the critical assertion that a better approach to risk mitigation of bone degeneration is available in the form of a novel approach to exercise. Research was sourced from Google Scholar, Pub Med, El Sevier, and the digital library of American Public University. The literature is voluminous, and successful medical interventions are also to be mentioned, as they work in concert with the exercise regimen to capitalize on the benefits of the proposed exercise. Once it is explained that such a device could reduce spinal elongation, the additive notion of cognitive and behavioral augmentation from a virtual reality (VR) system with a computer-brain interface (CBI) is introduced to expand the proposed benefits to the spinal column and midbrain.

The last part of this methodological review is to detail the exercise mechanism itself. It is proposed that a human body in the Neutral Body Position be superimposed upon the modeled cycle to emphasize the ergonomic relationship between human and machine. This also shows the range of motion of limbs, torso, and neck as they would be moving when engaged in exercise. The prone position cycle is meant to be a means of isolating human aerobic-resistive exercise movement within a weightless frame of reference (Scott et al., 2019). The qualitative appraisal of the machine hinges upon its presentation as an easily discernible, and logical system. To that end, current CAD modeling allows for 3D presentation with the user interface controlling a universal, exploded, and detailed view of the machine. This CAD drawing can be directly uploaded to a Computer Numerical Control (CNC) machine designed for parts fabrication. Basic materials for the Phase 2 prototype include 7000-series thin-walled aluminum tubing. This tubing is then extruded, bent, and brazed to existing specifications based on human anthropomorphic data.

Project objectives

Science stands to gain much from a healthy spacefaring human. That person is challenged in zero-G to remain physically fit and capable to handle the space science mission. There is ample evidence that critical mission phases are going to be tasked to astronauts whose motor coordination is not at peak performance level and whose cognitive abilities are hindered by circadian rhythm and sleep disruption (Moore et al., 2019).

Knowing that interventions such as abdominal exercise, early lumbar pain detection, and on-board ultrasound monitoring of spinal conditions can only serve to slightly modify the situation, a more comprehensive approach to spinal health and crew wellness is needed. By showing that an existing locomotion analogue in vertebrates can be integrated into an exercise intervention as part of an informed health maintenance program, a rigorous approach to reducing spinal elongation by targeting the paraspinal muscles can be made. It can be shown that interspinal electro-stimulated contraction, as used in scoliosis treatment, can significantly affect unloaded muscle tissue. This can be used in conjunction with the exercise instrument to realign a spinal column. The proposed exercise machine, through its contraction of spinal muscle, has the potential to reduce the anticipated 5 cm of average spinal growth experienced during long duration spaceflight.

In addition to the novel exercise machinery, an added benefit of midbrain stimulation through a VR mosaic of guided imagery and neurofeedback information can stimulate neural activity. This intention to exploit the neuroplasticity of the brain, in turn, could aid in central nervous system, limbic system, and immune system maintenance. Contributing to astronaut wellness, these interventions are thought to improve cognitive and motor performance.

Maladaptation of the human body in space can be alleviated. Pharmaceutical, dietary and exercise regimes have thus far built on past programs. These studies dealt with somatic losses that lead to lumbar trauma, cognitive deficit, and cerebellar disorientation. Lumbar injury is found to be related to the transverse torque-loading of the L1-L5 vertebrae whose increased intervertebral distance of bursae cartilage are morphologically elongated enough to rupture (Berg-Johansen et al., 2015).

Spinal muscle tissue responds to the destruction of muscle fibers through vigorous flexion by creating more muscle fibers through satellite stem cells. Those new fibers that are made to adapt to higher contractile loads also induce a higher tensile load throughout the spinal column in zero-G. In the aggregate, these are sought to be flexed in a manner that shortens intervertebral distances by a significant margin.

In space, weightlessness works against a normal spinal alignment, making the tendency for the bursae and spinal cord to become hypertonic (too high in water content). This is enabled by the atrophy of postural muscles in the spine. The widespread redistribution of fluids in zero-G could be partially due to simple hydraulics on a cellular level as osmotic dysregulation allows for dynamic fluid redistribution, thus increasing spinal column length during long-duration space flight (Iwasaki et al., 2021).

The spine connects to the brain via an archaic adaptation of vertebral evolution called the limbic system. It is responsible for fight or flight reactions and sophisticated memory-and-action responses that are tied to emotional responses. It has a direct relationship with human well-being by place and situational recognition regulating homeostasis in several systemic functions (Nielson et al., 2015). Therefore, the message that the limbic system receives through the prolonged experience of zero-G is one that significantly misdirects those systems under its control. This is due to the conflicting sensory inputs in zero-G that are irreconcilable with a 1G experiential framework. So far, no panacea has been prescribed for this limbic system challenge in zero-G.

Primary research uses a rational approach to spinal trauma risk mitigation by using the Neutral Body Position as a design key to the exercise machinery design for zero-G. Unloading the lumbar curve by suspension in 1G can significantly reduce posterior hyperflexion of the lumbar vertebra. This is meant to relieve pressure on any nerve endings that may be the result of swollen bursae or disk slippage in an inflamed intervertebral area.

Primary research has thus determined that a climbing harness, worn around the hips in a prone position, with the cyclist suspended by a single carabiner, can enable the manipulation of arm and foot pedal cranks. The center of gravity of a human body suspended in this prone position is somewhat forward of the hips with the elbows and forearms taking up a significant amount of weight on the handlebars. This position has the effect of lessening spinal tension and is effective for maximum VO₂ intake for long-duration aerobic exercise (Figure 3).

Figure 3. (a) NASA Technology (2013), Neutral body posture specifications, NASA standards inform comfortable car seats, Retrieved October 31st, 2021 from: Neutral Body Posture Specifications | NASA (b) Richardson, T., (2014), Photograph of cyclist in the aero/neutral body position, digital photograph digital collection, Las Cruces, NM, USA.

A comparison of the microgravity harness design and the Neutral Body Position as defined by NASA.

Placing the same system in zero-G makes one assumption: The lumbar column will be affected by muscle atrophy, osteoporosis, and hypertonic spinal column elongation. Therefore, exercise must consider the risk of any posterior hyperflexion of the lumbar column. Astronauts would be able to benefit from the use of an ultrasound image to determine the condition of the lumbar area prior to exercise (Marshburn et al., 2014). What is proposed is a similar suspension system as in 1G but using a harness system in a total isolation spring and collar armature. The handlebars of the 1G cycle would be replaced by a hand-driven flywheel crankset.

Determining how the human body interfaces with the exercise instrument is accomplished by using anthropomorphic data from an available database to determine average body parameters (NASA, 1995; Lewis, 2021). Once the prototype has been designed, the rationale of data accumulation is to determine human performance details based on muscle and cardiovascular performance on the 1G device. Data points can then be accumulated such as maximum pedal stroke power output. The range of articulation of the frame at the universal joints located near the shoulder and hips can be determined by human trials on the Phase 2 prototype. These are related to the established spinal column and IVD and range of motion datasets (Chang et al., 2016). Muscle power measurement determination in 1G can be made using the Cybex isokinetic dynamometer, with units of power measured in foot-pounds (torque) and multiplied by angular velocity (in radians per second) for the front and rear cranksets (Sapega & Drillings, 1983). Once in space, a torque velocity dynamometer or atrophy measurement device could be used to test individual muscle output (ESA, 2018). Comparisons can then be made to the existing ISS cycle datasets.

Cardiopulmonary data can be determined using a multifunction blood pressure cuff. Blood oxygen levels can be determined through a pulse oximeter. Maximum oxygen intake (VO₂ max) can be measured via a mask with a tubular system leading to an analytical instrument. From this, a comparison of oxygen uptake volume and exhalation volume can be made (Capritto, 2019). Research on a flywheel device in 1G indicates a VO₂ max value of about 1.2 liters/minute is to be expected for a human of average fitness (Berg & Tesch, 1998). Pre- and post-exercise muscle definition values can be accumulated over a long-term study of 16 weeks with a small representative sample of 10-20 men and women between the ages of 35 to 55. Specifically, the paraspinal tissue could be observed in pre-and post-exercise muscle biopsies. This would be an indicator of skeletal muscle status based on muscle fiber conditioning. Calf muscle tissue testing would be necessary as a determination of calf muscle response to the pedal upstroke conditioning. In addition, this type of pedaling would also load the distal femoral and humoral neck head, so applied torque to that area as well (as indicated by micro-computed tomography) would be necessary to measure as pre- and post-observation trabecular tissue volumes (Lang et al., 2006; Keyak et al., 2009).

The overall aim of VR is to enable the limbic system to sense actual locomotion for the virtual inputs that stimulate the human sensorimotor system. A separate approach to VR-assisted exercise must be made in the programming and content development. Human parameters of visual gestalt, depth perception, motion tracking, object acquisition, tactile appraisal and recognition must be re-imagined and re-drawn on the 2D planar surface of a VR computer screen.

This is the programming basis for alleviating VR motion sickness and eye fatigue (Wang et al., 2015). Once this is coded, it is a matter of immersing the human spaceflight participant into a virtual scenario that enables near, mid, and long-distance spatial orientation with additional haptic feedback for tactile sensory inputs (Schölkopf et al., 2021). This would function as an enabler of the amygdala's function for recognizing and integrating spatial frontiers stimulated by the primary visual and aural input that would have served the neuro-vestibular system in 1G.

What guides secondary research is the need to examine available medical investigations for a baseline understanding of how the human spine will change in the near and long term by studying musculoskeletal adaptations in zero-G, recovery rates for post-flight astronauts, muscular atrophy, and in-flight osteoporosis development. Secondary research also is used for the reference framework of the human neuro-cognitive system, it’s relation to the limbic system, and how zero-G causes comprehensive somatic systemic disruption.

A beneficial connection between VR and motor-cognitive rehabilitation can be asserted from existing research and this provides context for the use of VR as an exercise augmentation. Optimization of human brain activity can be the engineered result of methodical and informed VR-environmental design. If the exercise process elicits significant theta-wave activity, for instance, (i.e., higher cognitive and organized neural network activity) the EEG inputs into the AI function of the VR mosaic could be designed to ensure subsequent content generation, akin to levels in the video gaming concept.

This would translate to the appearance of theta wave activity concurrent with the learning and memory associated with VR environment interaction. It may also be an indicator of the brain’s voluntary movement choices made in that environment. Subsequently, theta wave generation would enable sensory rewards, such as color generation with higher or deeper chromatic content, easier navigability, haptic feedback that may have an expanded frequency range or lower resonance, and aural frequencies that have a particular resonance within the inner ear. These would then generate a positive neurofeedback loop for the generation of up-regulated theta wave activity (Hasselmo, 2005).

In the prone position microgravity cycle, a cyclist’s body is situated in a prone-position suspension system that isolates the hips in a harness held by a carabiner. The exercise instrument armature is articulated at the shoulder and hips with universal joints and small resistive springs allowing for lateral and ventral supination of the spinal column. Pedal cranksets are placed at the hands and feet. The foot cranksets are of special interest as they are clipped into. This allows for the pedals to be pushed and pulled during one revolution. Resistive devices such as a multi-gear system are enclosed in the cycle's two gearboxes allowing for a dial-up exertion level based on individual preferences.

The work of Berg and Tesch (1998) lays out some parameters for a cardio-resistive device on the ISS. Muscle optimization in a resistance workout can be reached with a program carried out 5 days a week for a duration of 1 hour. The eccentric and concentric muscle contractions on a cycle are less time-consuming, provide greater gains in aerobic power and endurance, and deliver greater gains in muscle strength and mass. This proposed resistive exercise device (RED) evolved to become the Interim iRED and the Advanced ARED.

With use of the iRED Loehr et al. (2011) have shown that the use of piston-resistive devices in the ARED provided improved maximal resistance energies needed to build on strength gains from the iRED’s flywheel concept. The 3000N of resistance energy provided by bilateral press actions on the iRED would be sufficient resistance for the Zero-G Cycle as proposed in a cardio-resistive capacity. The flywheel ergonometric prototype also weighed in at 27 kilograms, a markedly lower weight than the ARED. What also justifies the use of flywheel resistance is the improvement in neural drive to the muscle, defined as the sum of the spiking activities of motor neurons (Farina et al., 2014). When an increased pool of motor neurons is firing, the subsequent muscle response is directly related to the numerical quantity of the neuron population involved in the bio-electric discharge. This is the essence of enhanced muscle output.

The rationale for the microgravity cycle is to reduce muscle loss and osteoporosis through attention to areas of bone impact and calcium excretion. This would benefit crew health on long-duration spaceflight. Postural muscle groups, such as the interspinal, ankle, and knee extensors are most adversely affected by prolonged weightlessness. As Extra Vehicular Activities (EVAs) and some phases of the flight profile can deliver increased and sometimes catastrophic loading of the musculoskeletal system, concurrent stresses to the cognitive-cerebellar system are also affected. It is through the application of a novel exercise method and VR immersion that an astronaut may be enabled to better cope with those stressors.

The iRED has evolved from a flywheel instrument to a piston-resistance machine that delivers a substantially higher loading of muscle and bones. This evolution of the ARED device, in turn, led to the increased retention of muscle tissue and the decreased bone losses associated with the advanced resistive exercise device (ARED). Incorporating both exertion systems into one instrument is the intention of this research. With the flywheel resistance of eccentric and concentric muscle exertion via the pedal cranksets, sufficient loads for the hip joint and the focused loading of ham and quadricep groups can be achieved. With closer inspection, muscle optimization as the detailed exertion of calf muscles such as the soleus and gastrocnemius can also be attained.

During aerobic-resistive exercise, when the back is flexed dorsally, the latissimus dorsi are engaged. When flexed ventrally, the hip adductors and abdominal core can be exercised. When these muscles are engaged in a sequential rhythm, such as a diver swimming underwater, the spinal groups are engaged. When flexed laterally, the paraspinal, latissimus dorsi, and deltoid groups are engaged. This motion is also reproducible on a 1G cycle as a sine-wave movement of asymmetric tensioning of the muscles. Using sustained rotations of the flywheel cranksets, and for the resistive universal joints on the cycle frame, it is possible to sufficiently load the postural muscles of the back.

VR shows a general ubiquity in medicine, architecture, and entertainment, and scientific literature speaks to its effectiveness as a means of directing the human cognitive effort of repair and neural organization. It has, however, the documented effect of inducing motion sickness, eye fatigue and disorientation in a small percentage of participants. By comparison, the challenges of weightlessness offer the same in the first 48 hours of space adaptation syndrome (SANS). It is for the productive purpose of exercise augmentation that the specifics of eye fatigue must be addressed (Marshall-Goebbel et al., 2019).

Numerous studies have mapped out many parameters of optical relevance, and the work of Piumsomboon et al. (2017), demonstrates that eye strain can be dramatically reduced by using the natural tendency for the eyes to gaze upon content. This is achieved by motion tracking of the ocular movements of the participant and finding a baseline of ocular range of motion. It is then plausible that the use of dual reticles would enable ocular motion tracking. Tracking the ocular pursuit of visual content, whether it be to a singular or cluttered focal point is incorporated with nod-and-roll head tilt observation. What this can become in programming content, such as simplified focal lengths, peripheral imagery, eye-motion prediction algorithms, and adaptive depth depiction and perception, may have a substantially lower impact on the eyes. For its use in this research, VR has been shown to not only overcome the challenges of the neural-cognitive deficit but, in combination with a computer-brain interface (CBI), it has been instrumental in enabling human ambulation (Zhang et al., 2019).

Moving the spine on the prone position microgravity cycle during the prescribed exercise routine is by no means second nature to a human. These movements must be carefully learned. For this technology to be of benefit to the human user in zero-G, it must be shown that there is a connection between the sensorimotor engagement of mirror-neurons in the cerebrum (Rizzolatti, 2005; Ramachandran, 1996). and the use of composite VR imagery. This is imagery that consists of environmental, and avatar overlay in a mosaic of neural stimulation. A simulated avatar viewed during exercise can be the basis of the see-and-do action reproduction that the brain’s mirror-neuron system establishes, thus making motor coordination easier. EEG studies that are focused upon the mirror neuron system’s operation suggest that observation and execution enhance learned motor behaviors (Brunsdon, et al., 2019).

This VR augmented exercise, in turn, is undertaken to enhance the limbic system’s appraisal of acquired VR-sensory information so that it can process its information into a more comprehensive pseudo-terrestrial analog. Should the limbic system be induced to better upregulate systemic functions for this VR mosaic, then it is conjectured that health maintenance may be improved. Should the long-term exercise be undertaken with positive results, new neural pathways and network creation in the brain can be repeatedly processed and laid down as a reinforced preferential cerebral function during delta-wave and REM sleep periods. This could enhance waking cognitive performance.

Flotation of a human body in a salt-water isolation chamber will eventually disassociate cerebral and cerebellar perceptions in a passive act of deep relaxation. The observed neural-cognitive effects present in participant’s EEG data suggest multi-wavelength activity most closely associated with high mental alertness. Sensory signals from visual, auditory, olfactory, gustatory, thermal, tactile, vestibular, gravitational, and proprioceptive channels are minimized (Feinstein et al., 2018). The effect of dwelling in zero-G could be seen as a comparatively dissociative process, not unlike a water-isolation system.

The long-term effects of such a state will challenge the motor-cognitive and neuro-sensory system to adapt (Lin et al., 2020). Without the aid of artificial gravity, an adequate 1G analog must be introduced to a body coping in space without the benefit of terrestrial cues for spatial awareness. Isolation of the physical sensory inputs of the human body becomes central to the zero-G exercise so that the introduction of virtual inputs can be the means of rewriting signal paths that will enable the brain to correlate the artificial information that is meant to simulate a 1G terrestrial analog.

The benefits of vigorous exercise will result in an adaptive human response that defines the brain’s neuroplasticity. It can be combined with simultaneous cognitive training to become a multiplier of the cardiovascular program. A significant reason for this research is to stimulate limbic system function while in zero-G. Exercise studies show combined cognitive and aerobic training resulted in increased neurogenesis of hippocampal cells (Gould et al., 1999). and enhanced maintenance of the central nervous system’s neuroplasticity through increased neurotrophin creation (Kesslak et al., 1998). In addition, such training also increased spatial memory, also an important function of the hippocampus (Hötting & Röder, 2013).

Research goals

The human spine is a well-evolved structure. From an architectural point of view, it’s economy of mass and flexibility range enables human carriage and is central to human locomotion. However, upon unloading of this spinal column in zero G, numerous challenges are apparent as the human body sublimates into morphosis. This investigation compiles existing data on the spine, and postulates that the problem of lumbar trauma can be alleviated by a novel approach to exercise. This poses some questions to the investigator such as:

The human hippocampus serves to create the visual-spatial memory function. The amygdala is responsible for emotional processing and contextualization of those memories. Neurological research indicates that behavioral reactions are tied to memory. Long-term space mission studies show a marked behavioral deficit in astronauts (Koppelmans et al., 2013). However, the functions of memory and cognition are challenged in zero-G and Low-Earth Orbit, leaving the additive tasks of functional motor coordination and memory to be relearned in an environment with few spatial cues outside the encapsulated vessel. The investigation’s research of current VR medical interventions may answer the following questions about VR integration into a zero G exercise regimen. Can VR simulate a larger physical spatial range, a middle area beyond the immediate surroundings that is then processed in the hippocampus, potentially augmenting the lack of visual and physical cues that limit motor responses and coordination?

Results

Develop a means of restricting spinal elongation by a novel exercise instrument

The second phase of the Zero-G Cycle prototype is distinguished by the more complex 1G cycle concept. This is a ringed-frame and harness suspension system that relies on universal articulation points: the front and rear wheelsets, front handlebars, rear pedal cranks, and the hips (see Figures 4, 5). It will test the efficiency of the design at high rates of wheel rotation, and various angles of lateral spinal exertion, with the rider introducing twisting torque loads while pedaling. Sprint testing for physical performance values such as pedal torque and maximum oxygen uptake can be accomplished on a stationary, off-the-shelf roller-based isolation system. Electrocardiogram (ECG) testing could be used to measure heart performance and may be of interest as a neurofeedback mechanism in the zero-G final product (Paluch et al., 2017). A stationary test-stand system will also be the means to test flexion of the cycle and spine in the frontal/dorsal configuration.

Figure 4. Photograph of phase 1 cycle, digital photograph, digital storage, Las Cruces, NM, USA.

The Phase 1G prototype was a proof-of-concept that tested the prone position as a frame and saddle that the rider rested upon.

Figure 5. (a) NASA Technology (2013), Neutral body posture specifications, NASA standards inform comfortable car seats Retrieved October 31st, 2021 from: Neutral Body Posture Specifications | NASA Photo-edited with cycle overlay (b) Anthropomorphic study of cyclist and microgravity cycle.

Neutral Body Posture Specifications | NASA. The cycle flexes with the cyclist’s exertion and contractions. The Neutral Body Position is used as a reference framework.

This stationary cycle system will also serve to set universal-joint values for lateral and ventral articulation. A 1G prototype will also need a complex locking mechanism or neutral-positioning spring mechanism for the universal-joint to keep from sagging under the cyclist’s weight. However, in zero-G, this would be of less concern. Angles of U-joint movement should not exceed 20 degrees; front-to-back and left-to-right of the frame centerline. Combined with the 270-degree rotation of the handlebars, the 40 degrees of movement in the frameset would enable precise, short radius turning under real-world conditions. It is also important to keep the cyclist in a position to see forward without hyperextension of the cervical vertebrae. In zero-G, the spinal cord swells for the redistribution of fluids and thus hyperextension could increase the risk of cervical lesions. A pair of refractive prism eyeglasses are proposed to keep the cyclist in 'head down' position.This would be of benefit for the 1G prototype.

By extending this idea to the microgravity cycle it is possible to maintain the ergonomic positioning of the exercising astronaut. Introducing a head-mounted VR display (VR HMD) would replace the refractive prism eyeglasses. Though data indicates that the neck strain associated with a crash helmet is significant in studies of the head movements of high-performance jet pilots, the VR HMD is substantially lighter, causing less of a problem with associated head movements (O’Connor et al., 2020). Another approach to the introduction of VR stimulus would be to use a hemispheric VR projector. This allows the exercise participant’s head to move freely and makes the immersive display more analogous to 1G. While this may alleviate connectivity and wiring issues, it may be less effective in rendering comprehensively visceral stimulus inputs to the central nervous system.

Develop an exercise regimen of exertion levels without stressing postural bones

The muscle tissue of the paraspinal group allows for flexion along two planes of movement. Considering both as comparative to x and y axes, we can develop the Phase 3 Microgravity prototype into a flexible instrument that accommodates two distinct workouts (see Figure 6). The cumulative effect of these programs is to thoroughly train and contract postural muscles to hold the spine as close to its 1G baseline condition as possible. As this instrument also emphasizes aerobic-resistive training, repetition and loading variance would be needed to produce muscle growth, endurance, and tone in the abdominal core, hip adductors, and shoulders. Biceps and triceps could also benefit from a similar loading to the distal area of the distal humeral neck head by an eccentric/concentric workout to the pectoral, deltoid, and latissimus dorsi muscle groups.

Figure 6. Richardson, T., (2023), Anthropomorphic and axial study of cyclist in prone position cycle, (pencil on paper/digital edit), digital collection, Las Cruces, NM, USA.

The axes of movement for a microgravity cycle.

It is also possible to focus exercise on the dexterity of muscles needed for intricate off-axis maneuvering on EVAs such as handhold gripping, area transitioning, and tool implementation. The 3-axes of movement and physical load vectors of a 1G terrestrial workplace are replaced in zero-G by the combined load vectors applied from literally every conceivable angle. Training the muscles to adapt to unusual impulse forces applied to and by the upper body calls for an exercise instrument adaptation that enables the shoulder group (deltoid, supraspinatus, infraspinatus, teres minor, and teres major muscles) to exert and repulse force throughout an increased range of motion that is necessary to protect the shoulder and rotator cuff.

The crankset arm of the prototype would be designed with the NASA Man-Systems Integration requirements for effective muscle loads. What a fully rotatable, locking ball-shaped handgrip on the front crankset provides is the multi-positioned range of motion of the upper arms when they are engaged in the workout. This ranging of exertion angles would engage a higher effective area of the deltoids, latissimus dorsi, biceps, and triceps, and this would create muscle fiber creation more adept at handling and exerting force at oblique angles. The handgrip can also be tensioned to provide resistance training to the flexor digitorae, flexor carpii, and brachioradialis muscles.

Discussion

On Earth, the human hippocampus functions to contextualize a much larger physical domain with recognizable distances that are perceived and processed in the brain as a working and remembered cognitive product of mapped-out actions that are connected to motor responses. Astronauts, while on Earth are vehicle operators, and they engage significant portions of the midbrain in the navigational movement, memory, and recall of processed encoded information created from interacting with their physical environment.

Though not a survival behavior that the limbic system has evolutionarily processed, like food-seeking, threat avoidance, or intimate bonding (Mogenson et al., 1980). the navigable socio-physical movement could be interpreted as a vital survival-mode analog in modern humans. These activities engage the midbrain’s ability to transit successfully and so mediate a healthy response that regulates emotion, avoids depression, and builds a complex understanding of environments.

Proposed VR content, through its depiction of navigable areas, structures, and time, helps to build a larger spatial dimension beyond the spacecraft hull, by the creation of an artificial environment that elicits similar sensory-inputs signals that would allow the brain to build a 4D terrestrial analog model in the zero-G environment of a spacecraft. The amygdala acts in concert with the hippocampus and is inferred to be central in socially interactive behaviors, stress responses, emotional damping, and the emotional processing of fear, anxiety, and aggression (Pompiano et al., 2004). The VR content could serve to have memorable content that would engage the limbic system’s ability to develop those situational occurrences from VR into functional learned memory.

The research of Toril and company (2016), shows that visuospatial working memory and episodic memory are enhanced by video gaming. Through these memorable experiences, exercising astronauts could catalog a series of learning events. It is also of interest to note that the crew’s common access to the VR program library would create a shared, and thus socially interactive, temporal component of the virtual space. By sharing a common VR experience through interactions, social networking, like increased neural networking during workouts, could ensue.

These shared virtual spaces, through socially interactive personal feedback, could access the amygdala’s rewards-based behavioral function (Janak & Tye, 2015). What has been learned of interactivity between core crew members who are fully engaged in the mission is that the higher statistical rates of interpersonal relation were attributed to better qualitative crewmember adaptability during the long 520-day conjunction-class Mars mission simulation (Basner et al., 2014). When shared learning from the VR exercise landscapes is tied to the Freudian pleasure principle, the effort-driven tasks in VR could bring an adaptive behavior to an inferred reward (Cherry, 2020). In the case of the Zero-G Cycle, that could be the content generation for the sake of the EEG’s beta and theta-wave upregulation, endorphin and dopamine release, and cognitive boost that vigorous exercise creates. The focus of a functional EEG neurofeedback program associated with VR is clarified by selecting a narrow wavelength range and covering the surface area of the head with as many electrodes as possible (Rogala et al., 2016). In coding VR for an environment of recreational engagement, the additive use of timed dietary intake with sports supplements and prescribed pharmaceuticals could set the blood-brain interface for a physiologically and psychologically beneficial experience (Zeredo et al., 2019; Wang et al., 2015).

A final example of VR content and its use in limbic system stimulation arises out of the need for situational awareness. On long-distance transoceanic airline flights, a live video feed from an external camera that provides additional information of global position, speed, direction, and altitude is included in the passenger’s video screen. As user preference would decide the VR program used on the prone position cycle, a similar offering would enable a further investigation of external environmental cues that provide the limbic system with an accurate appraisal of position and orientation in space. The VR display could include an external camera input from the transiting spacecraft. A synthetic panorama of cameras would enable 3D/360-degree viewing of the external spacecraft environment. If planetary orbit or the en-route portion of the flight is to be of benefit to the exercising spacefarer, perhaps a telescopic view of the origin or destination planet would serve to anchor perceptions of movement, distance, and velocity. Over the course of a lunar transit or a longer mission, this may act as a baseline boundary for spatial perceptions processed by the hippocampus.

Conclusions

Given that current zero-G exercise machinery intends to reproduce the acts of walking and cycling in zero-G, it has yet to alleviate the significant impulse-force loading risk to areas of bone mineral density depletion in the skeleton. Instead of a 1G postural baseline for zero-G, exercise, the novel system proposes that the Neutral Body Position is the more adaptive exercise baseline and derives its ergonomic sensibilities from this assumption. Less impact is applied to the pelvic seat, coccyx, hip joint, knees, and shoulder joints for the prone-position cycle’s ability to transfer muscle energy through the suspension frame to the dual cranksets.

The predicted geometric losses of bone and cartilage function, due to space osteoporosis, will be significantly greater the longer each mission gets (Axpe et al., 2020). Though current measures have demonstrably enhanced musculoskeletal strength due to pharmaceutical interventions, dedicated physical conditioning is only one part of the applied scientific research into crew health for the long space voyage. There are risks to the lumbar column and load-bearing bones in space and post-flight observations of the multi-year replenishment of the body’s normal calcium levels reinforce this.

In an aerobic-resistive exercise program, this novel exercise instrument is primarily aimed at contracting the intraspinal muscle group, while also engaging the back and core muscles--re-conditioning the spine to a 1G baseline of flexibility that would be preferred to the almost total loss of lumbar range of motion endured in a long-duration flight (Chang et al., 2016). Conditioning of the spine by a whole-body workout creates the asymmetric spinal flexion from neck to hip, making the action the central method of movement. Since a reduction in spinal elongation holds the key to reducing possible lumbar trauma and cartilage depletion, and possible fluid redistribution in the spinal column, the prone-position cycle is offered as a remedy for the problem.

Humankind has adapted to an existence far from the hunter-gatherer nomadic lifestyle, and in doing so, has lost the daily engagement of muscles that only a concerted effort of physical conditioning can replace in the modern context of existence. Likewise, the central nervous system, encapsulated in a spacecraft, endures challenges to the élan vital that is derived from the navigable existence of effort-based outcomes. It is the inferred loss of neurocognitive acuity in space that the brain must make up for in a psycho-motor conditioning system meant to awaken the midbrain’s so-called reptile complex to replicate an ancient sensory-driven adaptation to life on Earth. In this way, neural pathways and the bioelectric signals processed between regions of the human brain can be remade to better adapt to weightlessness during long-duration spaceflight (Katafuchi et al., 1995).

The current physiological and neurological research into space adaptation intuits that the brain is challenged in neurocognitive areas, and that it could benefit from area-specific applied stimulus to areas of the midbrain to recreate a level of functionality that is like its normal capacity in 1G (Klein et al., 2019). While there is little gravitational, or sensori-motor input and limited environmental to inform the limbic system in zero-G, a recreation of the basic elements of cerebellar and cerebral actions that anchor the 1G terrestrial experience could be reproduced by a virtual means. Ample research exists as to the productive capacity of virtual reality usage, but until research uses VR in zero-G, the question as to whether it can benefit crew well-being as an augmentation to the lack of sensory input has yet to be answered.

To develop such a physical conditioning system enables a stake-holding space agency with an opportunity to also include cerebellar and cognitive conditioning as well. While the human sensory systems are open and actively re-wiring in zero-G, the virtual information that is included during exercise can be a means of reorienting the midbrain, essentially re-tasking neural pathways that have been void of specific sensorimotor inputs since the arrival in zero-G. This method of enhancing global cognition with the intent of engineering a specific neurocognitive altered state, can also have the added measure of helping to balance dysregulation of sympathetic and parasympathetic systems dependent upon a healthy midbrain.