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What is the effect of reduced gravity on human height?

What is the effect of reduced gravity on human height?


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There has been talk recently of building a base on either the Moon or Mars. What I'm wondering is, if you are born and grow to adulthood on the Moon, where the gravity is a tenth of the Earth's, would you be taller than if you had been born and grown up on Earth?


As far as I am aware, no one has ever been born in space, so your answer cannot be answered directly. However, it is known that the absence of gravity results in an increase in body height by a few centimeters. This is due to stretching of the vertebral column, which is no longer being pressed together by the downward pull of gravity1. For example, the astronaut Richard Hieb, who spent 2 weeks in space, found that his height had increased by 1 inch (~2.5 centimeters)2. Loss of gravity also causes the blood and other fluids to migrate from the legs to the upper part of the body, resulting in swelling of the face and protrusion of the veins of the neck1. I assume that these effects reported at zero gravity at least partially apply to 10% gravity.

Sources
- Airbus Defence & Space
- NASA


How Gravity Affects the Human Body and Ageing Process

Gravity is a force of attraction that exists involving any two masses, any two bodies, and any two particles. The bigger the object is, the more robust is its gravitational attraction. Earth’s gravity is what retains you on the ground and what triggers objects to fall. Gravity is exactly what holds the planets in their orbit all over the Sun and what retains the Moon in its orbit around Earth. The nearer you are to an item, the more powerful its gravitational pull is. Gravity is exactly what offers you excess weight. It is the pressure that pulls on all the mass in the human body. Sir Isaac Newton (1642-1727) formulated the theory of gravity when an apple fell onto his head. Also, this force has a huge effect on the human body also.

Possibly, the most apparent effect of gravity on the system is compression with the spine. Our backbone consists of vertebrae and sponge-like discs. The downward drive of gravity triggers the discs to shed moisture throughout the day, ensuing in the everyday peak lack of as many as 1/2″ – 3/4″! The humidity returns to your disc overnight, but not 100%. Over a lifetime, a person can completely eliminate amongst 1/2″ – 2″ in height!

Major Reduction doesn’t just affect the wellness of the back but acts like a “domino effect” on the rest of the human body. Your organs grow to be compressed, and also your waistline measurement raises (without obtaining real weights ). These compression wrinkles due to the fact they are, partly, a direct result of compression of your backbone! This also outcomes your capability to move and bend, which often can seriously hinder your capacity to perform very simple everyday functions.

Gravity wreaks havoc to the inside of the human body also. Over time, organs become prolapsed or slide, from their proper place within the body. Organs functionality will become fewer economical. It truly is not unusual for people today to experience bladder, kidney and digestive troubles on account of prolapsed organs. For this reason, for centuries, yoga practitioners have executed head stands to ensure the right organ placement.

Flexibility

Top loss and bigger middles inevitably result in dropped functionality. Probably the most significant aspect of retaining an active way of life while you age is sustaining the flexibility to move. Gravity can actually rob us from the ability to golfing, backyard garden, and participate in with our grandkids within our later many years.

Circulation

If gravity can avoid water from flowing uphill, it could also prevent the blood within our bodies from freely flowing upward. Over time, gravity normally takes a toll about the circulatory system, which can result in varicose veins, reduced scalp circulation and swollen limbs. Bad circulation into the eyes, ears, pores and skin, scalp and brain is a reason why our most worthy organs deteriorate about a life span.

In summary, it seems that gravity is a significant cause (but not complete) lead to getting old. Its pull places a lot of strain to the body’s organs and capabilities which results in wear and tear effortlessly and triggers ageing process more quickly.


The Effects of Zero Gravity on the Body

Before floatation tanks became more popular in spas, they were primarily used by astronauts in training . NASA still trains astronauts with floatation chambers. This is because floatation chambers simulate a zero-gravity environment. The water in a floatation chamber contains about 1,200 pounds of magnesium sulfate , causing the body to easily float on top of the water. Especially combined with the darkness and lack of sound integral to the floatation experience, it’s no wonder astronauts train in a chamber.

The effects of weightlessness are very different between a floatation chamber and long term space travel, however. Floating in space long-term can have some serious negative effects on the body . In a zero gravity environment, muscles shrink. This can lead to deteriorating joint function, and can also lead to pain felt throughout the body. Bones suffer as well, significantly decreasing in mass in proportion to the time spent in space. The body’s most important muscle, the heart, is not exempt from this shrinking effect.

The heart is designed to work with the gravity of Earth. The heart pumps blood strongly upward to combat the force of gravity so we can have adequate blood flow to the head. With no gravity, the heart’s upward force would be too strong. This can create swelling in the eyes and face.

The immune system also suffers. Astronauts can experience recurrence of childhood illnesses, such as chicken pox. Diseases that occur in zero gravity are also difficult to treat, as medicines don’t necessarily work the way they do on earth.

For all the detrimental effects of long term weightlessness, the short term weightlessness of the floatation chamber has been shown to have significant positive effects . The resulting muscle relaxation allows for faster healing of injuries, when applied in moderation. Patients in floatation chambers report significant improvement in muscle and joint pain whether the pain is the result of injury, genetic predisposition, or is caused by stress.

The mental relaxation achieved by a floatation chamber’s short term weightlessness is not found by any other means on Earth. When deprived of external stimuli, the brain no longer scans for threats. This is something every human brain naturally does while awake, and even while sleeping. The brain, when free from noise, sound and other distractions, enters a state similar to dreaming, but it happens while you are still awake and in control.

The positive health effects of the short term zero-gravity environment created by sensory deprivation chambers are long lasting. Depression, insomnia, pain, PMS, lack of focus, and stress are all significantly reduced. If you want to experience something as close to space travel as you can have on Earth with therapeutic effects, contact the Northwest Float Center .


The Low-down on Gravity

You can't see, touch, taste or smell it. Yet we can feel its effects every day and experience its cumulative damage on our bodies over a lifetime. No other force affects us so dramatically.

What force are we referring to? GRAVITY.

When an apple fell onto his head and he formulated the law of gravity, Sir Isaac Newton began to understand the role of gravity in controlling the moon's orbit. However, Newton probably didn't realize the profound effect of this force on the human body.

Have you ever noticed that your pants feel a little tighter around the waist at the end of the day? Have you ever adjusted your rearview mirror up in the morning and then down at night? Did you realize that after the age of 20, you've been losing an average of 1/2" in height every twenty years? Do you suffer from varicose veins, swollen feet or an aching back? If you responded yes to any of these questions, you are a victim of the inescapable, compressive force of gravity.

The results of gravity's constant downward pull on our faces, shoulders, backs, necks, chests, organs, legs and feet are painfully obvious to most of us. Gravity never gives up, nor does it discriminate. Young or old, couch potatoes or athletes - we will all experience change to our bodies as a result of life on this planet!

Exercising will help keep you fit and trim - but exercise is both beneficial and harmful to your body. How can that be? It's called compression fatigue: the more we run, the more weight we lift, the more our bodies pay the toll from gravity.

Spine
Perhaps, the most noticeable effect of gravity on the body is compression of the spine. Our spine consists of vertebrae and sponge-like discs. The downward force of gravity causes the discs to lose moisture throughout the day, resulting in a daily height loss of up to 1/2" - 3/4"! The moisture returns to the disc overnight, but not 100%. Over a lifetime, a person can permanently lose between 1/2" - 2" in height!

Waist
Height loss not only effects the health of your back, but acts like a "domino effect" on the rest of your body. Your organs become compressed and your waist measurement increases (without actual weight gain). You probably call these love handles, but we call them compression wrinkles because they are, in part, a direct result of compression of the spine! This also effects your ability to move and bend, which can seriously hinder your ability to perform simple daily activities.

Organs
Gravity wreaks havoc on the inside of your body as well. Over time, organs begin to prolapse, or fall, from their rightful place in your body. Organ function becomes less efficient. It's not uncommon for people to experience bladder, kidney and digestive problems due to prolapsed organs. In fact, for centuries, yoga practitioners have performed head stands to ensure proper organ placement.

Flexibility
Height loss and larger middles inevitably result in lost flexibility. Perhaps the most vital part of maintaining an active lifestyle as you age is maintaining the ability to move. Gravity can actually rob us of the ability to golf, garden, and play with our grandkids in our later years.

Circulation
If gravity can prevent water from flowing uphill, it can also prevent the blood in our bodies from freely flowing upward. Over time, gravity takes a toll on the circulatory system, which may cause varicose veins, decreased scalp circulation and swollen limbs. Poor circulation to the eyes, ears, skin, scalp and brain is one reason why our most valuable organs deteriorate over a lifetime.

Try this simple experiment to witness the powerful effect of gravity on the circulatory system: lift up your right arm for two minutes. Lower your arm and compare your right and left hands. Which is more pink? Now consider the effect of standing all day on your lower limbs. Our bodies subconsciously understand that we need to aid circulation from our limbs to our heart - how often do you find yourself propping up your legs on a desk or ottoman?

We may call all of these problems the unavoidable effects of aging. The truth is that they are merely a result of the constant force of gravity - and they are not unavoidable.

If you're still unconvinced about the power of gravity, consider this: astronauts grow two inches while in space! During weeks in orbit, astronauts' discs continue to absorb moisture from the blood stream. With no gravitational pull to squeeze moisture out, the discs remain plump, making their spines longer and themselves taller. In fact, space suits are designed to accommodate the extra two inches spinal stretch. Unfortunately, most of us will remain earth-bound for our entire lives. Here are some ways we compensate:

  • As fetuses, we all develop in the near-weightless environment of our mothers' wombs. During the last trimester, we actually turn upside-down to help with brain development.
  • As infants, we often slept bottoms-up! Keeping our heads lower than our hearts, we encouraged a proper supply of blood and oxygen to our brain
  • As children, we love to "escape from gravity" by riding the swings or hanging upside down on the monkey bars.
  • As adults, we prop our legs and feet on desks or stools to compensate for gravity's constant presence.

We cannot escape gravity, but we can trick it into working FOR us. How? By reversing your body's position under it's force. Use gravity to stretch and elongate your body. Remember the Nachemson study that indicated you are unable to escape compression even by lying down? This same study indicated that this compression can be overcome by applying traction of 60% of our body weight. Mechanical traction can be too awkward and complicated, and is difficult to practice at home.

The only practical way to achieve this amount of stretching force is through Inversion.

We've finally found a simple tool to help us reverse the negative, compressive force of gravity on our body! To learn more about how Inversion can help to elongate the spine, maintain proper organ position, improve circulation and increase flexibility,


Would Humans Born On Mars Grow Taller Than Earthlings?

If we ever manage to overcome the fertility and sex troubles of space , we'll probably be popping out little humanoid children on other planets. But our little tykes might not stay little for very long.

On Earth we experience the steady hand of gravity at 1 g force constantly throughout our lifetimes. On other planets in our solar system, that's just not possible. Researchers are working on ways to make artificial gravity possible in order to make long flights easier on human bodies. According to NASA, most astronauts grow about 2 inches while they're in space because the reduced gravity causes the fluid between vertebrae to expand. They lose the height within 10 days of returning to Earth's crushing gravity. Because of the growth, NASA uses space suits that have extra room to accommodate the additional height.

(You also grow taller when you sleep : As you lie in bed, gravity pushes you down and elongates your spine enough so that when you wake up you're usually about half an inch taller than the previous night.)

Grab those Martians for your basketball team Mars settlement-proponent Robert Zubrin has theorized that children born on other planets with lower gravity, like Mars, which has just one-third of Earth's gravitational pull would in fact grow taller by a few inches than they would have on Earth. While genes inherited from their parents wouldn't change, the spine could elongate more than on Earth. Fortunately, Martian kids born in a low-g environment wouldn't suffer from the muscle mass and bone problems that long-flight astronauts do.

Unfortunately, the biggest possible problem with your Galactic Globetrotters may surface if low-gravity-born humans tried to return to Earth. They'd experience three times their home gravity and could suffer serious bone problems. For example, one NASA scientist, Al Globus, gives an example of someone who weighs 160 pounds. If I went to a 3g planet, the equivalent of moving from Mars to Earth, I would weigh almost 500 pounds and would have great difficulty getting out of bed, Globus said. For children raised on the moon or Mars, attending college on Earth will be out of the question.


Martian speciation

Solomon explained that new species evolve most commonly when a barrier prevents a population from mating, such as on an island archipelago, so species on separate Galapagos islands evolve along separate lines. With modern humanity, of course, the trend is going in the opposite direction, as people move around the planet at a rate unprecedented in human history. “So on planet Earth it would take a major change to imagine us having populations isolated long enough to have distinct species,” he said.

The gulf between Earth and Mars might present such a barrier, if the Martian colony were self-sustaining and persistent. Through natural selection, humans and any organisms they bring with them, such as a plants, may evolve and adapt to Mars' harsh environment and low gravity, which is only a third of Earth's gravity.

Further Reading

Lacking a magnetosphere, Mars is bombarded by an increased rate of radiation, which also favors speciation. Ionizing radiation causes mutation in genes, which would provide a source of new genetic variations. That could accelerate the process of adaptation. On the downside, Solomon said, the higher radiation might just kill people. Or it might cause colonists to perpetually huddle inside small habitats and space suits, leading a Morlock-like existence and facing a similar evolutionary fate.

Ultimately it still may take a long time for speciation to occur. The one solid data point we have on Earth is the colonization of the Americas, which were settled by waves of people moving across the Bering Strait around the end of the last ice age. These populations were then isolated from the rest of world for about 10,000 years. When Europeans arrived they found a distinct population of native Americans, Solomon said, but certainly not a different species. That would suggest that, on a planet with a similar atmosphere and gravity as the Earth, it would take a human population more than 10,000 years to speciate. Mars is not that planet, of course.

Another factor to consider as humans contemplate colonizing other worlds, Solomon said, is the “founder effect,” which simply means that when a small number of people establish a new population from a larger population, the genes of the founders will have a huge influence on that population moving forward. This occurred with the small bands of humans spreading out from Africa.

“I’m thinking about what the long-term fate of our species may be,” Solomon said. “When selecting colonists I don’t believe we should be trying to select what attributes we want in a new species of humans. But it’s interesting to think that if you were to take only people from certain populations, or try to include a diversity of all of humanity, how those outcomes would be very different for the potential of what might become a new species of humans.”


Human Locomotion under Reduced Gravity Conditions: Biomechanical and Neurophysiological Considerations

Reduced gravity offers unique opportunities to study motor behavior. This paper aims at providing a review on current issues of the known tools and techniques used for hypogravity simulation and their effects on human locomotion. Walking and running rely on the limb oscillatory mechanics, and one way to change its dynamic properties is to modify the level of gravity. Gravity has a strong effect on the optimal rate of limb oscillations, optimal walking speed, and muscle activity patterns, and gait transitions occur smoothly and at slower speeds at lower gravity levels. Altered center of mass movements and interplay between stance and swing leg dynamics may challenge new forms of locomotion in a heterogravity environment. Furthermore, observations in the lack of gravity effects help to reveal the intrinsic properties of locomotor pattern generators and make evident facilitation of nonvoluntary limb stepping. In view of that, space neurosciences research has participated in the development of new technologies that can be used as an effective tool for gait rehabilitation.

1. Introduction

Life evolved in the presence of gravity, which has two major impacts on motor functions: specific body orientation in space and antigravity muscle tone and specific rules of motion in the gravity field. Gravity plays an essential role in terrestrial locomotion. The dominant hypothesis regarding templates for bipedal walking in the gravity field is the pendular mechanism of walking, up to intermediate speeds, and the bouncing mechanism of running, up to the highest speeds attainable [1]. The inverted pendulum-like mechanism of energy exchange taking place during walking would be optimized at slower speeds in reduced gravity [2, 3]. Despite our intuitive appreciation for the influence of gravity, we do not fully understand how gravity interacts with other forces, such as inertia, to affect many biological and physical processes and what type of gait and/or limb synchronization (trot, gallop, lateral sequence walk, pace, skipping, etc.) would evolve at other gravity levels.

Understanding locomotion characteristics is critical for those working in the area of gait biomechanics and neurophysiology of pattern generation networks and of exercise countermeasures for astronauts. Many researchers have investigated the effects of reducing and eliminating gravity on locomotive kinematics and kinetics [4–8]. Others have studied locomotion in actual weightlessness or hypogravity [9, 10]. The techniques have included supine and erect cable suspension, parabolic aircraft flights, water immersion, and centrifugal methods [6]. Increased knowledge of locomotion kinematics, kinetics, muscular activity patterns, and sensory feedback modulation may help to facilitate more effective exercise countermeasures, develop innovative technologies for gait rehabilitation, and provide new insights into our understanding of the physiological effects of gravity. In this review, we will consider the known tools and techniques used for hypogravity simulation and their effects on human locomotion.

2. Methods and Apparatuses for Reduced Gravity Simulation

Spaceflights are the more direct way to assess the effect of gravity on locomotion, but studying locomotion in actual hypogravity is demanding and expensive [6]. The drawbacks to spaceflight experiments include difficulty in using necessary data collection hardware and performing an experiment with adequate sample size. Parabolic flight offers a viable alternative, but periods of weightlessness are limited to

20 s, which only allows for acute locomotion investigations [11].

There are several apparatuses that have been used in the past to simulate reduced gravity locomotion. One of the more used systems is the vertical body weight support (BWS) (Figures 1(a) and 1(b)). These kinds of simulators are usually obtained supporting the subjects in a harness that applies a controlled upward force. For example, the WARD [12] mechanism consists of a mechanical gear driven by a pneumatic cylinder (Figure 1(b)). It is held in a cart that slides forward and backward over a track. Low-friction sliding of the mechanism ensures that only vertical forces are applied to the subject. Vertical BWS systems may also make use of a small increase in air pressure around the user’s lower body to create a lifting force approximately at the person’s center of mass [13]. Other vertical systems [8, 14] use a series of compliant rubber spring elements that are stretched to create the upward (to simulate gravity less than 1 g) or downward (to simulate gravity greater than 1 g) force (Figure 1(a)). The main limitation of these reduced gravity simulators (in addition to high local skin pressure via a harness) is that each supporting limb experiences a simulated reduction of gravity proportional to the applied force, while the swinging limb experiences 1 g.


(a) Vertical system for altered gravity simulation
(b) Vertical BWS
(c) Tilted BWS
(d) Tilted BWS
(e) Supine suspension system
(f) Passive gravity balancing system
(a) Vertical system for altered gravity simulation
(b) Vertical BWS
(c) Tilted BWS
(d) Tilted BWS
(e) Supine suspension system
(f) Passive gravity balancing system Reduced gravity simulators for locomotion. (a) Schema of the vertical system used to simulate different gravity values (redrawn from [8]). R: rubber bands, B: light metal bars, M: electric motor to stretch the elastic band system, PL: pulleys to invert the direction of the pull on the subject (dashed lines). (b) Vertical body weight support (BWS) system: subject walks on a treadmill with different levels of BWS while being supported in a harness, pulled upwards by a preset unloading force

The tilted BWS systems (Figures 1(c) and 1(d)) are constructed to simulate more realistic effects of gravity changes on both the stance and swing legs in the sagittal plane. These simulators, that have been used in the past by both Roscosmos (Russian Federal Space Agency) and NASA to train astronauts before space flights [15–17], are based on the idea of neutralizing the component of the gravity force normal to the lying surface [mg

cos(α), where α is the angle of inclination], while the component of the gravity force acting on the body and swinging limbs in the sagittal plane is reduced in relation to the tilt angle [mg sin(α)]. A similar concept has been used in the reduced gravity simulator (Figure 1(d)) designed by Ivanenko et al. (Italian patent number Rm2007A000489): the subject lies on the side on a tilted couch (up to 40° from the horizontal position) with both legs suspended in the exoskeleton and steps on the treadmill, which is tilted to the same angle [7, 18, 19]. This simulator included additional mass of the tilted chassis (

15 kg) and exoskeleton (1.5 kg for each leg). Thus the entire assembly had a mass of

18 kg that increased both gravitational and inertial forces during walking.

Another class of gravity-related manipulations is “subject load device” (SLD) that applies a gravity replacement force in the direction down to the surface. This type of SLD can be used in the vertical systems to increase the gravity [8] or in the lying position (Figure 1(e)). When an astronaut walks or runs on a treadmill in weightlessness, a subject load device is used to return him or her back to the treadmill belt and to load the limbs. The gravity replacement load is transferred, via a harness, to the pelvis and/or the shoulders. Gravity simulators can simulate active treadmill running in weightlessness and provide a method of testing proposed improvements in SLD design and exercise protocols [20, 21]. In supine suspension systems (Figure 1(e)), subjects are suspended horizontally attached to latex rubber cords. A cloth sleeve and rubber cord are attached each to the upper and lower arms and legs (eight total) [20]. The limitation of this system is a local pressure on some parts of the body (e.g., shoulders) and modifications in the swing phase dynamics due to nonconstant forces of rubber cords and gravity acting in the anterioposterior direction of leg movements (Figure 1(e)).

Based on the passive gravity balancing technology, Ma et al. [22, 23] proposed a design concept of a passive reduced gravity simulator to simulate human walking or other activities in a reduced-gravity environment for potential applications of training astronauts and space travelers (Figure 1(f)). The system consists of a 3-DOF dual parallelogram mechanism, a 2-DOF torso support assembly, and a pair of 3-DOF leg exoskeletons. The weight of the body and the legs is compensated by the spring-balanced dual-parallelogram mechanism and torso-support assembly, and the weight of each leg is compensated by a leg exoskeleton. The system is capable of simulating human walking and jumping in a hypogravity environment [24]. Hardware prototyping and experimental study of the new system are currently underway.

In the following section we discuss the basic principles of adaptation of locomotion to different gravity values using the technologies described here.

3. Biomechanical Aspects of Locomotion in Reduced Gravity

Despite some differences, all reduced gravity simulation approaches show a reasonable approximation of the reduction in the gravitational force acting on the center of body mass (COM) and similar results concerning the speed of gait transitions. An important consequence of the pendulum-like behavior of the limbs in the gravity field is the principle of dynamic similarity [29], which states that geometrically similar bodies that rely on pendulum-like mechanics of movement have similar gait dynamics at the same Froude number:

where is the speed of locomotion, is the acceleration of gravity, and is a characteristic leg length. That is, all lengths, times, and forces scale by the same factors. In order to optimize the recovery of mechanical energy, the kinetic energy and the potential energy curves must be equal in amplitude and opposite in phase, as in a pendulum. Assuming that the change in kinetic energy within each step is an increasing function of the walking speed (while the change in the potential energy is proportional to gravity), the hypothesis was proposed that the inverted pendulum-like mechanism of energy exchange during walking would be optimized at slower speeds in reduced gravity [3, 10]. An optimal exchange between potential and kinetic energies of the COM occurs at Fr

0.25 [2] (Figure 2(a)). Even though specific limb segment proportions may play an essential role in the kinematics and energetics of walking [30], animal anatomy and individualized limb segment dimensions are optimized in such a way that the Froude number can explain optimal walking velocity.


(a)
(b)
(c)
(a)
(b)
(c) Biomechanical features of locomotion in reduced gravity conditions. (a) Optimal (blue) and walk-to-run transition (green) speeds as a function of gravity. Dynamically similar speeds predicted by Fr = 0.25 and Fr = 0.5 are indicated by blue and green dashed curves, respectively [25]. Green circles and stars refer to measurements of optimal walk-to-run transition speeds in simulated low-gravity conditions [5, 18]. The grey triangle indicates an earlier estimate of optimal walking speed predicted for the Moon gravitational environment by Margaria and Cavagna [3]. Blue triangles refer to the optimal speeds (at which most of the mechanical exchange between potential and kinetic energy of the body center of mass occurs) obtained in a simulation study of Griffin et al. [26]. Blue circles represent measurements of optimal speed obtained during parabolic flight [10, 27]. (b) Time course of the net vertical component of in-shoe reaction forces plotted as a function of the spatial coordinates of the foot at different reduced gravity levels. Note change in vertical scale in the 0.05 g condition. The lower right panel shows the trajectories of the center of pressure superimposed on a foot outline (adapted from [28]). (c) Maximum longitudinal foot velocity and foot excursion

On Earth, walking and running gaits are usually adopted for different speeds of locomotion, with a preferred transition occurring at

2 m/s for human adults and at slow speeds for children (Fr

0.5), in accordance with the dynamic similarity theory [29]. Different studies [4, 18] demonstrated that, at lower levels of gravity, the walk-run transition occurred at progressively slower absolute speeds but at approximately the same Froude number (Figure 2(a)).

Despite similarities in approximating reduced gravity, there are nevertheless essential differences between different simulation approaches. The variables that showed the greatest differences between vertical and tilted reduced gravity systems (Figure 1) were maximal longitudinal foot velocity and longitudinal foot excursion (Figure 2(c)), in agreement with significant influences of gravity on swing leg dynamics [7]. Even though the maximal longitudinal foot velocity for the tilted BWS condition decreased only slightly relative to the vertical BWS, however, the actual decrement was much more obvious if one takes into account that it was significantly compensated for or masked by increments in the stride length [7]. A previous modeling study also predicted differential effects of gravity during stance and swing phases [31]. In fact, the changes in the longitudinal foot excursion were basically opposite for the vertical and tilted BWS systems (Figure 2(c)). For the former system the amplitude of longitudinal foot motion decreased, while for the latter system it increased relative to the 1 g condition. Considering a monotonic (presumably proportional [32]) relationship between the stride length and the maximal foot velocity at a given gravity level (1 g), the peak foot velocity would be expected to be

1.5 times higher for the vertical than for tilted BWS condition if the stride lengths were similar (Figure 2(c)). The previous studies on parabolic flights investigating the effect of gravity on walking mechanics demonstrated increments in the swing phase duration (by 29% at 0.25 g [33] see also [11]), in line with the substantial contribution of gravity to the swing leg. Overall, the findings demonstrate that gravity acting on both stance and swing legs plays an important role in shaping locomotor patterns.

4. Nonlinear Reorganization of EMG Patterns

It is known that load plays a crucial role in shaping patterned motor output during stepping [34–36], and humans produce a specific heel-to-toe rolling pattern during stance in normal gravity conditions. Ground contact forces reflect the net vertical and shear forces acting on the contact surface and result from the sum of the mass-acceleration products of all body segments while the foot is in contact with ground [37]. Simulating reduced gravity between 0.05 and 1 g reveals drastic changes of kinetic parameters but limited changes of the kinematic coordination [28]. The reported accurate control of limb/foot kinematics [28] may depend on load- and displacement-compensation mechanisms working effectively throughout a wide range of ground contact forces, from full body weight up to <5% of its value. The peak vertical contact forces decrease proportionally to gravity, but at 0.05 g they are applied at the forefoot only (Figure 2(b)). During lower limb loading, a variety of receptors can be activated, such as Golgi tendon organs, cutaneous receptors of the foot, and spindles from stretched muscles [36]. These sensory signals interact with central rhythm-generating centers and help in shaping the motor patterns, controlling phase-transitions, and reinforcing ongoing activity [38, 39]. For instance, loading of the limb enhances the activity in antigravity muscles during stance and delays the onset of the next flexion [40]. It is important to understand the mechanisms of sensorimotor adaptation to the biomechanics of locomotion and foot placement/loading in heterogravity, especially to longer-term changes of load.

A key feature of adaptation to hypogravity is a remarkable nonlinear scaling of muscle activity patterns contrary to monotonic changes in foot loading. The simplest kind of change with simulated reduced gravity [28] was seen in ankle extensors: the mean amplitude of activity decreased systematically with decreasing simulated gravity, consistent with their antigravity function [35, 41]. By contrast, the behavior of other muscles could not be predicted simply on the basis of the static load during stance. The amplitude and pattern of muscle activity generally depended on speed and could vary nonmonotonically with body unloading. There was also a complex reorganization of the pattern of activity of thigh muscles with decreasing simulated gravity, as well as noteworthy individual differences [28]. Figure 3(a) illustrates an example of nonlinear reorganization of EMG patterns in one subject walking at 3 km/h. With body weight unloading, gluteus maximus and distal leg extensors decreased their activity, while other muscles demonstrated a “paradoxical” increment of activation (e.g., quadriceps) or considerable changes in the activation waveforms (hamstring muscles). Note also the absence of the typical burst of RF at the beginning of the swing phase at low simulated gravity levels (Figure 3(a)), consistent with other studies on the effect of body weight unloading [42] and walking speed [43]. It is unlikely that these changes are due to the order of trials or the consequence of learning the hypogravity condition since presentation order of speeds and BWS was randomized across sessions and experiments [28]. Also, the duration of each trial was

1 min, with at least 2 min rest between trials, and a short (

30 s) training period of walking at different speeds was allowed for each simulated reduced gravity level before the actual data collection was begun (the walking patterns typically adapt rapidly to simulated reduced gravity [4, 5]). This reorganization is presumably related to the multifunctional (biarticular) action of these muscles and to the need to repartition the joint torque contributions across different muscles as a function of the changes induced by gravity. At 1 g, the main peak of m. biceps femoris activity occurring before heel-contact serves to decelerate the swinging limb [37]. However, as gravity is decreased, its main activity occurs in mid-stance and late stance, presumably in relation to the need to assist vaulting over an inverted pendulum of the stance limb and swing initiation.


(a)
(b)
(a)
(b) Nonlinear reorganization of muscle activity patterns. (a) An example of ensemble-averaged electromyographic (EMG) activity of lower limb muscles versus the normalized gait cycle is shown for a subject walking at 3 km/h at different simulated reduced gravity levels [28]. (b) Mean EMG activity computed over the gait cycle and averaged across all cycles and subjects (

There might be various factors accounting for the nonlinear reorganization of muscle activity patterns with gravity. To start with, nonlinear scaling also occurs during walking at different speeds at 1 g. For instance, VL and RF activity is quite small at low speeds (less than

3 km/h) but becomes prominent at higher speeds (>4 km/h) (Figure 3(b)), a speed effect consistent with that reported in the literature [28, 43, 45, 46]. Given that, it should be stressed that walking at lower gravity levels at the same speed (Figure 3(a)) corresponds to walking at higher speeds if one uses the Froude number as a dimensionless parameter (e.g., walk-run transition at 0.25 g occurs at

4 km/h, Figure 2(a)), so that “paradoxical” increments of VL and RF EMG activity in Figure 3(a) may reflect higher biomechanical demands on proximal leg muscles at higher dimensionless speeds. Nonlinear reorganization of EMG patterns was also observed when using exoskeleton robotic devices that provide body weight support [42, 47]. Changes in the body reference configuration during stance (slightly flexed posture [48, 49]) may contribute to a greater activity of proximal extensors as well. Finally, there is a differential effect of speed on quadriceps muscle activity at reduced gravity levels: VL and RF activity increases at low speeds (<3 km/h) while it decreases at a high speed (5 km/h) (Figure 3(b)). Potential nonlinear scaling of muscle activity for most whole body movements in microgravity should also be taken into account for exercise countermeasures for astronauts.

5. Different Gaits

Considering complex, high-dimensional, dynamically coupled interactions between an organism and gravitational environment, in principle, one challenging solution is to adopt different coordination patterns and not only an optimal speed of locomotion. Are different gaits possible on other planets?

One approach to study locomotor adaptations is to look at the effect of gravity on gait transitions. A gait has been defined as “a pattern of locomotion characteristic of a limited range of speeds described by quantities of which one or more change discontinuously at transitions to other gaits” [29]. An important aspect of gait transitions is a discontinuous switch that occurs at some point while varying the speed of progression (although some exceptions may exist [50–52]). As already discussed (Figure 2(a)), gravity has a strong effect on the speed at which gait transitions occur (Fr

0.5). Surprisingly, however, we found [18, 19] that at lower levels of simulated gravity the transition between walking and running was generally gradual, without any noticeable abrupt change in gait parameters or EMG bursts (Figure 4(a)). This was associated with a significant prolongation of the swing phase, whose duration became virtually equal to that of stance in the vicinity of the walk-run transition speed, and with a gradual shift from inverted-pendulum gait (walking) to bouncing gait (running). A lack of discontinuous changes in the pattern of speed-dependent locomotor characteristics in a hypogravity environment (Figure 4(b)) is consistent with the idea of a continuous shift in the state of a given set of central pattern generators, rather than the activation of a separate set of central pattern generators for each distinct gait [19].


(a)
(b)
(a)
(b) Smoothness/abruptness of gait transitions at different gravity levels. (a) Soleus (SOL) EMG patterns during slow changes in treadmill belt speed (lower panels) in one representative subject at 0.16 g (left) and 1 g (right). Upper panels: examples of SOL EMG waveforms (left, plotted versus time right, plotted versus normalized cycle) during 5 consecutive strides of both legs around the transition from walking (black lines) to running (gray lines). Dotted curves denote the (transition) stride of the leg in which the swing phase first exceeded 50% gait cycle. Bottom horizontal bars denote stance (black) and swing (white) phases. Lower panels: the color maps represent a sequence of discrete activation waveforms (vertical slices).

-axis indicates the number of the gait cycles (corresponding to the appropriate timing of the trial),

-axis indicates normalized gait cycle (from touchdown to another touchdown), and color indicates EMG amplitude. The white line indicates when toe off occurred. Vertical dashed lines indicate walk-to-run (W-R) and run-to-walk (R-W) transitions. Note abrupt changes in the relative stance duration and muscle activation patterns at gait transitions at 1 g and no obvious distinction in these parameters at the transition from walking to running at 0.16 g. (b) Schematic representation of the smoothness of gait transitions as a function of gravity. The orange curve symbolizes the dimensionless walk-run transition speed consistent with the theory of dynamic similarity (Fr

Interestingly, the smoothness of gait transitions is accompanied by a gradual shift from inverted-pendulum gait to bouncing gait, resulting in a “paradoxical” inverted-pendulum running in the vicinity of run-walk and walk-run transitions [18]. The swing phase may have more influence on gait than it was previously thought. For instance, relatively slower swing and longer foot excursions (tilted BWS condition, Figure 2(c)) may raise questions about optimality or comfort of walking and could account for potentially different preferred gaits, such as loping on the Moon observed in Apollo astronauts (though the Lunar suit limits the range of motion in the leg joints and may also contribute to the loping gait on the Moon [9]). The resulting changes in the intersegmental and interlimb coordination may in turn affect the COM motion. Overall, the results support the idea of looking for new forms of locomotion (both bipedal and quadrupedal) in a heterogravity environment [54] based on the interplay between stance and swing leg dynamics, altered interlimb coupling, and altered center of mass movements.

Other significant influences of gravity on short-term and long-term gait adaptations may be related to its effects on the body reference configuration [48, 49] and anticipatory mechanisms of limb and body movements [55, 56]. For instance, the basis of habitual human posture is postural tone of the skeletal muscles and microgravity elicits substantial changes in muscle tone and posture [48, 49]. Based on clinical observations, it has been recently argued that any reflection on the nature and choice of preferred gait (e.g., bipedal versus quadrupedal) should include a consideration of the mechanisms determining the choice of unconscious habitual posture [57]. Also, in analogy with the results based on upper-limb movements related to time-to-contact [55] or movement planning [58], anticipatory postural and locomotor adjustments for lower limb movements (e.g., for the control of heel strike or accurate foot placement) should take gravity into consideration. Therefore, altered gravity conditions may also affect locomotor-related tasks such as the negotiation of stationary and moving obstructions during walking or gait initiation/termination [56, 59, 60].

Finally, the repertoire of known gaits can be expanded to a variety of animals. For instance, on Earth only a few legged species, such as water strider insects and some aquatic birds and lizards, can run on water. For most other species, including humans, this is precluded by body size and proportions, lack of appropriate appendages, and limited muscle power. However, if gravity is reduced to less than Earth’s gravity, running on water should require less muscle power. Recently, Minetti et al. [53] used this hydrodynamic model of Glasheen and McMahon [61] to predict the gravity levels at which humans should be able to run on water and tested the hypothesis in the laboratory using a reduced gravity simulator (Figure 5). The results showed that a hydrodynamic model of Basilisk lizards running on water [61] can also be applied to humans, despite the enormous difference in body size and morphology. Particularly, 22% of Earth’s gravity is the maximum at which humans can run on water, when assisted by a small rigid fin (Figure 5) [53]. It is also worth noting the limitations for our musculoskeletal system for producing force/power (endurance) for instance, the stride frequency in humans is limited to about 2 Hz, whatever the planet is. On Earth, the biggest animal that can run on water is likely Western Grebes, and even these birds can run only for several seconds since the force production is basically anaerobic (participants in [53] could run at simulated “Moon” gravity only for

10 s). In contrast, at reduced gravity (Moon), these birds could run on water in a charming manner for much longer time.


(a)
(b)
(a)
(b) Running on water at simulated reduced gravity. The blue curve represents the net vertical impulse available to run on water, as predicted by the model used by Minetti et al. [53]. Bars represent the number of subjects, out of 6, capable of avoiding sinking at different simulated gravity values. Both variables show that 22% of Earth gravity (

6. Clinical Implications

Reduced gravity also offers unique opportunities for adjusting the basic patterns to altered locomotor conditions for gait rehabilitation. Body weight support systems coupled with robotic devices or pharmacologic treatments are now often used in the rehabilitation practice to assist physical therapy of individuals with neurological disorders. We will not review any detailed analysis of clinical outcomes for ambulation when using locomotor training with body weight support systems and refer to other reviews [64]. Nevertheless, it is worth emphasizing a facilitatory effect of the lack of gravity on rhythmogenesis and its potential for gait recovery.

Novel pharmacological strategies [65] and electromagnetic stimulation techniques [62, 66–68] are being developed aimed at modulating spinal activity and restoring the locomotor function. The spinal central pattern generator (CPG) circuitry can be easily activated in healthy humans in a gravity neutral position by applying tonic central and peripheral sensory inputs. To minimize interference with the ongoing task of body weight and balance control, stepping movements are elicited during air-stepping in the absence of gravity influences and external resistance. Figure 6 illustrates examples of nonvoluntary rhythmic movements of the suspended legs induced by electrical stimulation of peroneal nerve [62] and during hand walking [63]. It has been suggested that functional multisensory stimulations and a functional neural coupling between arm and legs can inspect CPG access by sensory and central activations and entrain locomotor neural networks and promote gait recovery. Such investigations may contribute to the clinical development of central pattern generator-modulating therapies and neuroprosthetic technologies [65, 69].


(a)
(b)
(a)
(b) Eliciting nonvoluntary limb stepping movements in simulated weightlessness (gravity neutral) conditions. (a) An example of nonvoluntary rhythmic movements of the suspended legs induced by electrical stimulation (ES) of peroneal nerve from the study of Selionov et al. [62]. Note the absence of ankle joint rotations during evoked air-stepping. (b) An example of evoked rhythmic leg movements during hand walking in one subject from the study of Sylos-Labini et al. [63]. RF, rectus femoris, BF, biceps femoris, TA, tibialis anterior, LG, lateral gastrocnemius, FCU, flexor carpi ulnaris, BIC, biceps brachii, DELTa, anterior deltoid, ST, and semitendinosus. Hand and foot denote anterior-posterior displacements of the left hand and foot.

7. Concluding Remarks

This perspective outlines an interdisciplinary approach to extend our knowledge on adaptation of human locomotion to a hypogravity environment, including biomechanical, neurophysiological, and comparative aspects, effective exercise countermeasures for astronauts, and even exobiology of new forms of locomotion on different planets. The tools and techniques used for hypogravity simulation and their effects on human locomotion provide new insights into our understanding of the physiological effects of gravity. The beneficial effect of weightlessness on rhythmogenesis would further enhance the utility of this approach and developments of innovative technologies for gait rehabilitation.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by the Italian Health Ministry, Italian Ministry of University and Research (PRIN project), and Italian Space Agency (DCMC, CRUSOE, and COREA grants).

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Copyright

Copyright © 2014 Francesca Sylos-Labini et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Zero-gravity experiments

Researchers from Louisiana Tech University will be floating high above the Gulf of Mexico this month to conduct zero-gravity testing of an experimental DNA analysis instrument developed at Tech that could benefit future NASA astronauts.

Dr. Niel Crews, assistant professor of mechanical engineering, and Collin Tranter, a graduate student with the Institute for Micromanufacturing (IfM) say the instrument could be used to monitor the health of astronauts exposed to cosmic radiation beyond Earth's protective atmosphere.

"Our goal is to understand how the system behaves under conditions similar to actual deployment in space missions," said Crews. The Louisiana Tech-developed devices are beneficial to NASA because they are small, consume less power and require little to no human operation.

The Louisiana Tech researchers will subject themselves to extreme conditions in order to conduct sensitive testing of the miniature device. NASA has used these same flights to train their astronauts.

The instrument attracted the attention of NASA scientists for possible use on the International Space Station, during inter-planetary travel and even for unmanned missions to search for life within the Solar System.

"We hope that by working with NASA, one of our DNA analysis devices will be sent into orbit to study the effects of space environments on living things, first studying DNA then cells," said Tranter. "Some further testing has to occur first, such as making sure the device works properly in low-gravity conditions. This will be done on a parabolic aircraft flight hopefully before the end of the year."

The tests will take place on a NASA airplane operating out of Ellington Field at Johnson Space Center in Houston. The flight pattern will consist of forty steep dives and climbs over the Gulf of Mexico. A controlled dive of nearly 10,000 feet in less than one minute will result in approximately 20 seconds of weightlessness for the researchers and the payload onboard. An abrupt climb back to the starting altitude will create a gravitational force twice the normal amount.

Even Hollywood has gotten into the act, using these flights to depict weightlessness on the silver screen. All of the zero-gravity scenes in the movie Apollo 13 were filmed during these flights. The alternation between zero gravity and 2G forces can so disorienting that NASA astronauts call the aircraft the "vomit comet."

NASA recently selected this system for a week-long series of flights as part of their Facilitated Access to the Space Environment for Technology (FAST) program, which focuses on expanding new technologies to be used in space flight applications.

Tranter is pursuing a Ph.D. in Nanosystems Engineering at Louisiana Tech and will continue to work with Crews on the project. He says they hope to learn very soon if their device can stand up to space environments.

"Low gravity can cause all kinds of unpredictable problems," Tranter said. "Eventually, I hope our system can reveal more about space radiation effects on DNA and cells, leading to options for safe space travel and exploration by humans. Our lab has studied some effects of radiation on DNA, such as UV exposure, but nothing on Earth compares to the environments we hope to study outside of the Earth's atmosphere."


Muscle atrophy and osteoporosis

One of the major effects of weightlessness that is more long-term is the loss of muscle and bone mass. In the absence of gravity there is no weight load on the back and leg muscles, so they begin to weaken and shrink. In some muscles degeneration is rapid, and without regular exercise astronauts may lose up to 20 percent of their muscle mass within 5-11 days.

Due to lack of mechanical pressure on the bone, bone mass is lost at a rate of one and a half percent in just one month in a zero-gravity environment, compared to about three percent a decade in a healthy person in a normal environment. The mass loss mainly affects the lower vertebrae of the spine, the hip joint and the femur. Due to the rapid change in density, bones may become brittle and exhibit symptoms similar to those of osteoporosis.

Even destruction and construction processes of bones change when in space. On Earth, bones are destroyed and renewed regularly using a well-balanced system of bone destroyer cells and bone building cells. Whenever some bone tissue is destroyed, new layers take their place these two processes are coupled to one another. In space, however, an increase in activity of bone destroyer cells is seen, due to the lack of gravity, and the bones decompose into minerals that are absorbed into the body.

Studies on mice have shown that after 16 days in zero gravity there is an increase in the number of bone destroyer cells and a decrease in the number of bone building cells, as well as a decrease in the concentration of growth factors known for their ability to help create new bone. The increase in calcium levels in the blood from the disintegrating bone causes a dangerous calcification of soft tissue and increases the potential of kidney stone formation.

Astronauts show an increase in bone destroyer cell activity, particularly in the pelvic area, which usually carries most of the load under normal gravity conditions. However, unlike patients with osteoporosis, astronauts who remained in space for three to four months, regain their normal bone density after a period of two to three years back on Earth.


Electromagnetic field induced biological effects in humans

Exposure to artificial radio frequency electromagnetic fields (EMFs) has increased significantly in recent decades. Therefore, there is a growing scientific and social interest in its influence on health, even upon exposure significantly below the applicable standards. The intensity of electromagnetic radiation in human environment is increasing and currently reaches astronomical levels that had never before experienced on our planet. The most influential process of EMF impact on living organisms, is its direct tissue penetration. The current established standards of exposure to EMFs in Poland and in the rest of the world are based on the thermal effect. It is well known that weak EMF could cause all sorts of dramatic non-thermal effects in body cells, tissues and organs. The observed symptoms are hardly to assign to other environmental factors occurring simultaneously in the human environment. Although, there are still ongoing discussions on non-thermal effects of EMF influence, on May 31, 2011--International Agency for Research on Cancer (IARC)--Agenda of World Health Organization (WHO) has classified radio electromagnetic fields, to a category 2B as potentially carcinogenic. Electromagnetic fields can be dangerous not only because of the risk of cancer, but also other health problems, including electromagnetic hypersensitivity (EHS). Electromagnetic hypersensitivity (EHS) is a phenomenon characterized by the appearance of symptoms after exposure of people to electromagnetic fields, generated by EHS is characterized as a syndrome with a broad spectrum of non-specific multiple organ symptoms including both acute and chronic inflammatory processes located mainly in the skin and nervous systems, as well as in respiratory, cardiovascular systems, and musculoskeletal system. WHO does not consider the EHS as a disease-- defined on the basis of medical diagnosis and symptoms associated with any known syndrome. The symptoms may be associated with a single source of EMF or be derived from a combination of many sources. Reported symptoms associated with electromagnetic fields are characterized by the overlapping effect with other individuals with these symptoms exhibited a broad spectrum of clinical manifestations, related to exposure to a single or multiple sources of EMF. The phenomenon of electromagnetic hypersensitivity in the form of dermatological disease is associated with mastocytosis. The biopsies taken from skin lesions of patients with EHS indicated on infiltration of the skin layers of the epidermis with mastocytes and their degranulation, as well as on release anaphylactic reaction mediators such as histamine, chymase and tryptase. The number of people suffering from EHS in the world is growing describing themselves as severely dysfunctional, showing multi organ non-specific symptoms upon exposure to low doses of electromagnetic radiation, often associated with hypersensitivity to many chemical agents (Multiple Chemical Sensitivity-MCS) and/or other environmental intolerances (Sensitivity Related Illness-SRI).



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