Chapter 21. Locomotion: When Suspended and Free of Support

At the conclusion of this chapter, the student should be able to:

• 1. Explain how each of the following influences the action of swinging bodies: weight of the body, length of the pendulum, angular momentum, potential-kinetic energy, centripetal–centrifugal force, and friction.
• 2. Describe how to initiate pendular action, increase the height of a swing, alter the period, change grips, and dismount safely.
• 3. Explain how each of the following influences the flight path of unsupported bodies: angle of projection, vertical velocity, gravity, and angular momentum.
• 4. Describe how to initiate and control rotation of unsupported bodies.
• 5. Analyze the performance of a suspension and a nonsupport movement, following the outline for kinesiological analysis described in Chapter 1.

Climbing, hanging, swinging, and other suspension activities were more commonly engaged in by our early ancestors than by members of more recent generations. The modern version of these brachial activities is seen in the trapeze activities of the aerial artist at the circus, in gymnastics events on the high bar, parallel bars, uneven bars, and rings, and in various forms of hanging on ladders and ropes in the gymnasium and on the playground. Success in suspension activities depends on considerable strength and endurance, particularly of the hand, arm, and shoulder musculature, and the ability to adjust body positions to counteract or take advantage of the forces acting on the body (Figure 21.1). Ladder, rope or rock climbing, and brachial locomotion are modifications of locomotion. Where swinging movements of a suspended body are involved, the principles of a pendulum, angular momentum, and centripetal–centrifugal forces are major considerations.

Principles Related to Hanging and Hand-Traveling Activities

• 1. In hanging activities the muscles of the arm and shoulder girdle must contract to protect the joints. The pull of the body’s weight puts stress on the joints by tending to separatethem.
• 2. Hand traveling is a locomotor pattern governed by the principle of action and reaction. As in walking, force applied against a supporting surface in one direction causes the body to move in the opposite direction. Hand traveling sideward on a bar or rail without swinging is achieved by alternately moving one hand away from the other hand, and then moving the second hand toward the first. As the first hand moves, the second hand pushes laterally against the apparatus. Both hands share the weight equally for a moment, then the second hand is released and brought toward the first hand while the first hand is pulling laterally on the apparatus.
• 3. The action used in hand-climbing activities is essentially a pull-up action. In rope climbing, for instance, either with or without the use of the legs, the body is raised through a forceful pull-up action of the arms. The rock climber uses a similar motion to advance up the rock face. The gymnast using a kip motion to mount a bar or the rings uses this same pull-up action to raise the body toward the hands. This action is then often converted to a push-up type of motion, ending in a straight-arm support position.
• 4. In accord with Newton’s Law of Inertia, sequential hand support movements should be continuous. The momentum of one action contributes to the next action. In the kip movement of the gymnast, there should be no pause between the pulling phase and the pushing phase of the hands and arms. Similarly, the rock climber tries to use the motion produced by the upward pull of one arm to contribute momentum to the next upward pull.

Principles Related to Swinging Movements

• 1. The movement of a pendulum is produced by the force of gravity. This statement presupposes a starting position in which potential energy is present. In other words, the pendulum must be moved from its resting position before the force of gravity can make it swing downward.
  • The initial problem of the child on the swing and of the gymnast on the rings is that of being given potential energy. Without the help of an assistant, the swinger must find a way to be put into a position with potential energy. This is usually done in one of three ways. The performer may move the apparatus to some position other than its normal position of rest before being suspended (i.e., the swing is pulled back and up); the performer may initiate the swing with a push of the feet against the floor or with small running steps; or a pumping action may be used to get started. For the gymnast swinging on the rings, the last alternative involves bending the legs up in front of the body and then extending them as high as possible. The range of the arc of motion may be increased by the repetition of this procedure on the forward-upward phase of the swing. The child in the swing accomplishes the same result by inclining the trunk backward and raising the feet forward while pulling the ropes toward the body and pressing forward against the seat.
• 2. As the pendulum swings downward, gravity causes its speed to increase; as it swings upward, gravity counteracts its speed, diminishing it until the zero point is reached. Hence the pendulum’s speed is greatest at the bottom of the arc and least (zero) at each end of the arc.
  • Caution must be exercised to maintain a firm grip on the supporting surface (rings, bar, or ropes), particularly at the bottom of the swing where the tendency to fly off is the greatest.
• 3. The upward movement of a pendulum is brought about by the momentum developed in the downward movement. The swinging body moves through an arc, first in one direction, then in the reverse direction (one-half the arc’s distance is called the amplitude). Thus it undergoes partial rotation about a center of motion. Because this rotation takes place in a vertical plane, the influence of gravitational pull must be taken into consideration. Whereas the force of gravity produces the downward swing, it opposes the upward swing. Nevertheless, it is indirectly responsible for the latter, inasmuch as the upward swing is caused by the momentum that was built up in the preceding downward swing.
  • This relationship is especially important in skills performed in gymnastics when long, swinging actions are involved. The higher the position from which the downswing is initiated, the higher will be the upswing. The range of a swing depends on the height from which the movement is initiated.
• 4. The potential energy of the pendulum is greatest at the height of the swing and least at the bottom. Conversely, kinetic energy is greatest at the bottom and zero at the top. In a perfect mechanical system, a pendulum would continue to swing with a constant amplitude with the required energy supplied by the perpetual conversion from potential to kinetic energy. Most of the potential energy is converted to kinetic energy in the downswing and reconverted on the upswing. Some is lost, however, to friction and air resistance, and a pendulum left to its own devices will gradually decrease in amplitude and stop.
  • To make up for the lost mechanical energy, supplemental energy in the form of properly timed muscular action of the swinger must be used. Maximum repetitions of a pendulum left to its own devices will occur if the initial height is maximum and friction is minimized.
• 5. The centripetal–centrifugal force in pendular movements increases as the mass or velocity increases and decreases as the radius increases. The centripetal force is greatest at the bottom of the swing, where the velocity is the greatest, and decreases to zero at the height of the swing. However, bodies develop proportionately more centripetal force and need more strength to maintain a grasp and counteract the opposing centrifugal force than do those of less weight. When centripetal force ceases to act on a swinging performer, the body will obey Newton’s first law and will fly off tangent to the arc of the swing at that instant.
  • The grip must be the most secure at the bottom of the swing, where the tendency to fly off is the greatest. Centripetal force has been calculated to be four to five times body weight as the body swings under the bar in a giant swing. To resist the equal and opposite outward-pulling centrifugal force, strong hands and arms are essential. The bottom of the swing is the point where the beginner or the novice is most likely to lose contact with the bar.
  • Although actions requiring release and regrasp of a bar or ring should be done at the peak of the swing, the same is not always true of dismounts. Dismounts and cutaways should be timed to occur at a point where the tangent of the arc at the instant of release coincides with the desired line of flight.
• 6. When a pendulum reaches the end of its arc, just before it reverses its direction, it reaches a zero point in velocity. At this precise moment the force of gravity is momentarily neutralized by the upward momentum.
  • The performer can take advantage of this situation by using this moment to perform position changes such as changing grips, reversing direction, and performing dismounts and cutaways. A release move on the high bar is best executed if the grip is released and the regrasp occurs at the top of the swing, during the instantaneous period of weightlessness.
• 7. The height of a swing may be increased by lengthening the radius of rotation on the downswing and decreasing it on the upswing (Figure 21.2). Shortening the radius on the upswing decreases the moment of inertia and therefore increases the angular velocity. This occurs because the angular momentum is conserved and, because angular momentum equals Iω, a decrease in I requires an increase in ω. The increased angular velocity is accompanied by an increase in kinetic energy and consequently more height results. The increased height for the start of the downswing and the lengthened radius of rotation produce a gain in angular momentum on the downswing because gravity has a longer time to act on the body before it reaches the bottom of the swing. When the shortening of the radius occurs for the next upswing, the increased momentum is conserved and, once again, there is an increase in angular velocity. The equation for angular momentum, Iω, also shows that the greater the angular velocity on the downswing, the less the body has to be shortened to decrease I to reach its desired height on the upswing.
  • A swinging performer may shorten the radius of rotation by moving body parts toward the axis of rotation. This may be done by flexing at the hips, hunching the shoulders by depressing the shoulder girdle, or arching the back. A performer in the giant swing depresses the shoulder girdle and flexes at the hips on the upswing and may also arch the back slightly. In a circle from a front uprise, the head is thrown back to shorten the radius on the upswing.
• 8. To increase height, the decrease in radius should be initiated at the moment the center of gravity of the body is directly under the axis of rotation. In the movement of a pendulum, speed is greatest at the bottom of the arc. Shortening the radius at this point accelerates its angular velocity more than at any other position.
  • In the backward hip circle on the uneven parallel bars, for example, the movement is started with the body extended and the hips on the bar. This gives the longest possible radius of rotation on the downswing. At the moment the legs pass under the hands (the axis of rotation), the hips are flexed to shorten the radius on the upswing, thereby decreasing the moment of inertia and increasing the angular velocity.
• 9. The time taken by the pendulum to make a single round-trip excursion (known as its period) is related to the length of the pendulum. The longer the pendulum, the more slowly it swings. Specifically, the period of the pendulum is proportional to the square root of its length.
  • When the hands are the axis of rotation, such as in the preparatory moves for a giant swing, the period is longer than in swings about a hip axis or a knee axis.
• 10. The period of the pendulum is not influenced by its weight. A heavy body will swing no faster than a lighter one, and vice versa. This is consistent with the behavior of freely falling bodies.
  • Trapeze artists take advantage of this principle. A performer swinging from one trapeze knows that an empty trapeze of the same length, swung from an opposite platform of the same height and at the same time as the trapeze artist’s, will have the same period. Consequently the performer also knows that it is safe to let go of the original bar at the height of its swing and turn around in midair, because the empty bar will be right there to be grasped, also at the height of its swing.
• 11. When a body consisting of two segments reaches the vertical with the proximal segment leading on the downswing, the distal segment will accelerate relative to the other segment and precede it into the upswing. If, on the other hand, the distal segment reaches the vertical first, the reverse will occur. The distal segment will decelerate and the proximal segment will lead into the upswing (Figure 21.3).
  • Action at the bottom of a swing that forces the hips through in front of the feet increases the acceleration on the upswing owing to hip flexion in shortening the radius. Such swings are called “beat” swings. Beat swings are used to set up release–regrasp skills on apparatus such as the uneven and even parallel bars, the high bar, and the still rings.
• 12. The rotation of the hands about a bar is opposed by frictional forces. This accounts for some loss in swinging energy as it is converted into heat energy. Evidence of this is the appearance of friction blisters and skin irritations on a gymnast’s hands. These friction forces tend to strengthen the gymnast’s grip when the swing is in the direction the palms are facing and weakens it when the swing is in the reverse direction.
  • Gymnasts reverse their grasps when the swing is reversed. This is usually done at the peak of the swing at the point of “weightlessness,” when the pressure on the hands is at a minimum. Gymnasts also sand the bar before each performance to ensure its smoothness and to minimize the friction on the hands.
• 13. In all mounting exercises involving swinging, the center of gravity must be brought as near as possible to the center of rotation. This is usually done when the center of gravity is directly under the bar on the downswing.
  • In the glide kip on the unevens, for instance, the performer swings the body forward and back, alternately flexing and extending to gain momentum. At the end of the glide the gymnast flexes fully at the hips so that the feet are near the bar. This sharp flexion increases the angular velocity by decreasing the moment of inertia. As the center of gravity passes under the bar on the downswing, the hips are extended vigorously. This movement raises the body and places the center of gravity close to the bar, thus further decreasing the moment of inertia and enabling the body to rotate until it is in a front support position with the center of gravity located at the fixed point of the grip.
• 14. In support swings the center of gravity should be at the point of support. Hip circles forward or backward require the center of gravity of the body to be close to the hand supports. In this position it takes less effort to keep the body against the bar while turning because the torque between the center of gravity of the body and the axis of rotation is kept at a minimum.

Suspension Analysis Example

Half-Turn Flying Hip Circle with Hecht Dismount—Uneven Parallel Bars

Gymnasts who perform on the uneven parallel bars have numerous combinations of movements from which to choose. Each routine starts with a mount and concludes with a dismount. In between the gymnast selects from a series of movements, including those characterized as circling, swinging, and flight elements. Within these broad categories turns around the long axis, grip changes, releases, and saltos should be included. Difficulty is rated from A to G. In the nine difficulty elements, each of the five element categories should be included. The moves shown in Figure 21.4 and described here are B-level moves (Fédération Internationale de Gymnastique, 2006). The gymnast starts in a handstand position on the high bar (a). As she begins to swing downward she performs a half-twist so that she faces in the direction of the swing (b, c). At the bottom of the swing she begins to flex at the hips in preparation for continuing in a back hip circle around the low bar (d, e). Toward the end of the hip circle she extends at the hip joint and dismounts forward over the low bar in a move called a hecht (f, g). She lands with her back to the bars (h).

Mechanical Essentials

In starting the downward swing from the handstand position, the gymnast pushes against the bar. Because of action–reaction, the bar pushes back in the opposite direction. The give of the bar adds to the reactive force and thus increases the total energy of the system. The push of the gymnast backward and the accompanying stretch increase the distance between the center of gravity and the axis of rotation and thus increase the torque on the downward swing. The time for gravity to act on the downward swinging body is also increased. The half-twist is performed at the beginning of the downswing when the forces acting on the hands are minimal and the grip change can be accomplished with ease (Figure 21.4b). At the bottom of the swing the gymnast flexes at the hips, thus shortening the radius of rotation and increasing her angular velocity on the upswing. This piked position also enables her to “wrap” around the low bar as she moves into the backward hip circle (Figure 21.4d). There should be no pause between one movement and the next. Any slowing down would require more energy to pick up again (Law of Inertia) and fluidity would be lost. As the gymnast’s hands leave the high bar, she pushes up and forward, causing the reactive force to add to the downward trunk rotation (Figure 21.4e).

The hip circle should be accomplished with the hips close to the bar so that the center of gravity is near the axis of rotation. Otherwise, centrifugal force would tend to pull the body away from the bar. Toward the completion of the hip circle the gymnast forcefully extends by lifting the upper trunk and arms forward and upward (Figure 21.4f). This action causes several things to happen. In reaction, the legs also extend so that the whole body is straight. The moment of inertia about the transverse axis is increased, and the rate of rotation decreases. The body pushes down and back against the bar causing the bar to push the body forward and up. All of this results in the body rotating up and away from the bar for the dismount. Once the body has left the bar, the back should be arched to again decrease the radius slightly and aid in the rotation of the body, allowing the feet to move slightly ahead of the center of gravity at the moment of landing (Figure 21.4h). The success of the hecht dismount depends on suf-ficient angular momentum being developed in the downswing and hip circle and a forceful, properly timed extension of the body at the end of the hip circle.

Anatomical Essentials

The anatomical essentials for successful performance in this movement series are strength and flexibility. Strength of shoulders, arms, and hands is especially important for swinging movements. Grip strength is essential, and until one is sure of such strength, certain swinging movements should not be attempted without the assistance of a spotter. Arm and shoulder strength can be checked by testing oneself with push-ups and pull-ups. Strong abdominal and back extensor muscles are also important. These muscle groups play a significant role in maintaining and changing the trunk position in the downward swing, the hip circle, and the hecht dismount as shown here. Range of motion is also needed for these movements, particularly in the shoulder during the swing, and in the hip and lower back during the pike of the hip circle. In the absence of EMG data, these general statements must be sufficient. Any joint-by-joint anatomical analysis would be based on implied muscle actions deduced from joint motions and forces.

The unsupported body moves through the air along a pathway determined prior to the beginning of flight. The flight path of the body may be primarily horizontal, as in a long jump, mostly vertical, as in springboard diving or high jumping, or somewhere in between, as in vaulting and tumbling events. Principles governing the flight path of the body relate to those of the projectile. Additional principles explaining the effect of the body’s lean at the moment of takeoff and the twisting movements in the air are derived from Newton’s third law, the Law of Action–Reaction and the Conservation of Angular Momentum.

Principles Related to Nonsupport Activities

• 1. The path of motion of the body’s center of gravity in space is determined by the angle at which it is projected into space, the force of the projection, and the force of gravity. Nothing a diver or a gymnast can do will alter the pathway of the center of gravity once the feet have left the push-off surface. The horizontal velocity of the body remains constant and the path of the center of gravity is a parabola determined by the initial force and gravity. Without the application of an external force, this path cannot be altered. For this reason the initial path must be suited to the desired result. The diver must allow for clearance of the board and sufficient air time for stunts while the gymnast must either produce a vertical path to land on a trampoline, balance beam, or vaulting horse or must produce sufficient air time for stunts on the floor.
• 2. The time a body remains unsupported depends on the height of its projection, which is governed by the vertical velocity of the projection (Figure 21.5). The more complicated or lengthy the stunt a diver or gymnast wishes to perform, the higher the peak of projection must be. A diver must emphasize vertical rather than horizontal distance.
• 3. Most rotary movements are initiated before the performer leaves the supporting surface. To possess angular momentum during a nonsupport activity, an eccentric force must be used for takeoff. In a front somersault, for example, the center of gravity must be in front of the feet at the moment of takeoff for torque to exist. In a somersault with a twist (Figure 21.6), the push must be sideways as well as diagonally backward as the upper body leads the twist in the opposite direction. In keeping with Newton’s third law, the direction of the twist is opposite to the direction in which the feet push against the supporting surface.
• 4. The angular momentum(Iω) of an unsupported body is conserved. It cannot be increased or decreased. A performer executing a somersault may increase the angular velocity by decreasing the moment of inertia. This is done by decreasing the radius of rotation, that is, by increasing the amount of tuck. A tuck somersault has a smaller moment of inertia than a pike somersault, and a pike somersault has less than a layout. To stop rotation in anticipation of landing or entry into the water, the moment of inertia is increased, thereby decreasing the angular velocity.
  • If a somersault starts with a twist from the push-off surface, the twist can be increased by decreasing the arch or the pike of the spin and by moving the arms closer to the body. Each of these moves decreases the moment of inertia and increases the angular velocity of the twist.
• 5. When a body is free in space, movement of a part in one direction results in movement of the rest of the body in the opposite direction. In a back dive in the pike position, the diver should wait until the legs have rotated past the vertical before the pike is opened. As the trunk and arms move back, the legs will react in the opposite direction and therefore move back to the vertical position.
  • A reverse layout dive with a one-half twist (Figure 21.7) may be performed by initiating the twist in the air rather than from the board. The diver will cause the body to twist to the opposite direction by moving one arm across the chest after the body is in a layout position in the air. The arm must then be moved overhead, keeping it close to the body so that another equal and opposite reaction does not occur. This method for initiating a twist is slow and is not usually used for more than a half-twist.
• 6. A performer who is rotating about a horizontal axis in the air may initiate a twist about a vertical axis by tilting the body to one side. In any twisting somersault, a diver can tilt to the side by moving one arm from the side horizontal to a position over the head and the other arm sideward downward across the body. This happens because of action–reaction. As the arms move in one direction in the frontal plane, the body moves in the other direction. The twist will occur in the direction of the raised arm and be directly proportional to the spin. That is, the faster the performer is rotating forward or backward, the faster will be the twist (Figure 21.8).

Nonsupport Analysis Example

The Reverse Layout Dive with One-Half Twist

A twisting dive is one in which the body makes at least one-half turn about the vertical axis. Twisting dives may be combined with somersaults in the tuck, pike, or layout positions. Divers may face forward and leave the board with a forward-spinning action or a backward-spinning action (reverse spinning dives). They may also initiate dives from a backward stance and spin backward or forward (inward spinning dives). In the reverse layout dive with a one-half twist pictured in Figure 21.7, the diver starts the dive facing the end of the board. He performs a reverse spinning somersault about a transverse axis in a layout position completing a one-half revolution. At the same time, he twists one-half a revolution about the vertical axis.

Mechanical Essentials

In starting a reverse dive the diver must end the forward approach by pushing backward toward the back end of the board with the feet. The rebounding board will push back with an equal and opposite reaction, causing the legs to move forward. This action of the board causes the reverse spin to develop in the body as the feet and legs move forward and the head and upper trunk rotate backward. It also causes the entire body to move forward away from the board. Because the feet push back against the board in reverse dives and force the body forward, less lean is needed on takeoff than with forward dives, where the feet push forward toward the tip of the board. The amount of rotation (spin) about the transverse axis must be determined in large measure at takeoff of this dive because usual methods of controlling the rate of spin by decreasing the moment of inertia through tucking or piking are not permitted. The angular momentum of the dive is determined and conserved at takeoff and cannot be altered, although some control may be exercised by varying the arch in the back in the initial layout. An increase in the arch will increase the rate of rotation.

The twist in this dive is accomplished through the use of two methods of initiating twists. As the diver leaves the board, he not only pushes back to create the reversed spin, but he also pushes slightly to the right to initiate a twist to the left. Once the diver is in the air, the principle of action–reaction is used to complete the twist. As the diver twists to the left, the left arm is brought across the chest, causing his body to move in the opposite direction (left) in reaction. He continues to aid the twist by circling the arm down next to the body, close to the axis of rotation. Keeping the head in line with the body and the right arm overhead during the twist decreases the moment of inertia of the rotating body and increases the speed of the twist. Turning the head left toward the water helps in the turn and the diver’s orientation. The diver’s twist is completed so that he enters the water with the back to the board and at the point where the center of gravity was predetermined to land the instant the feet left the board. In spite of the spin of the body about both a vertical and a horizontal axis, the body’s center of gravity follows a perfect parabolic curve controlled only by the velocity and angle of projection and the downward acceleration of gravity.

The entry of the dive must consider that the forward rotation of the dive will continue because of the Law of Conservation of Momentum. The stretched position of the body with the arms overhead creates the greatest moment of inertia possible and therefore the slowest rotation, but there is still some rotation. For this reason the diver should continue to turn underwater in the same direction. The angle of entry of the dive should be a natural continuation of the parabolic path of the center of gravity of the body. For reverse dives the entry would be almost vertical.

Anatomical Essentials

The difficulty of instrumentation has precluded EMG studies of the muscles involved in springboard diving. With no definitive data on specific muscle action in diving, it is best to deal with the anatomical essentials in a more general manner. As with all dives from a springboard, the reverse layout dive with a half-twist requires good strength, flexibility, and control, particularly in the trunk and legs. Good range of motion in the ankle joint is essential because the feet contact the end of the board, dorsiflex as the board is depressed, and then ride it upward and extend until the toes leave the board. Strength in the extensors of the hip and knee is also important in lifting the body from the board and maintaining the extended position in the air. Strength of the abdominals and back extensors is of prime importance to divers. Vertical alignment in the air relies on control by these muscles, as does the entry position. The abdominals also are important in the initiation and control of rotations and in the prevention of overarching the back. Contrary to popular belief, extreme extension mobility of the back is not desirable in dives and should be avoided. In the dive described here, overarching would decrease the height of the dive and slow the twist, as it causes an increase in the moment of inertia about the vertical axis. Flexibility of the back and hips for full flexion in tuck and pike positions is desirable, however.

Free Fall

Although all unsupported bodies follow the law of falling bodies, most return to earth in a very short time, having performed the desired stunts on the way. Yet there are times when the human body is in a state of free fall for much longer periods of time. Skydiving and parachuting are examples of human bodies in free fall. The human body will reach terminal velocity between 100 and 200 mph, depending on the surface area presented to the airflow (Brancazio 1984). In a nose dive, or head down, position it takes about 20 seconds to reach terminal velocity, close to 200 mph. Increasing the surface area presented to the airflow by assuming a “spread-eagle” position reduces the terminal velocity to 100 mph, reached in about 15 seconds. By manipulating the shape of the body in this way, the rate of descent can be partially controlled. Direction can also be modified by flexing and extending the extremities on one side of the body. Arm and leg flexion will decrease drag on that side of the body, producing a sideslipping or sideways motion. Reducing drag at one point of the body will produce rotation (Figure 21.9).

Weightlessness

Once human beings ventured into space, dealing with a gravity-free environment became a matter for serious study. With the planning and construction of orbiting space stations, more and more people will be confronted with the necessity of living and working in weightless conditions. Students of movement in the first few decades of the twenty-first century will most likely be the generation that devises exercise programs and sports for a three-dimensional, gravity-free environment. With this in mind, the concepts of mechanics take on a whole new meaning.

One critical fact must always be remembered in dealing with human motion in weightless conditions: The laws of motion still apply. Objects still possess inertia based on mass or momentum, acceleration is still directly related to force and mass, and every action has an equal and opposite reaction. The only difference is that the force of gravity no longer provides a constant downward acceleration. In a weightless environment the terms “down” and “up” are relatively meaningless.

The implications of a gravity-free environment deserve some attention in any study of human motion. In such an environment a force equal to or greater than the mass of an object or body is still required to start that object in motion. A force equal to the product of mass and acceleration is also required to stop a body in motion. A ball thrown in space requires the same throwing force and will strike the catcher’s hand with the same force as on earth. However, because of a lack of gravity pulling downward, it will travel in a straight line, not a parabolic path.

Of great concern in manned spaceflight is the loss of bone mineral that occurs with prolonged weightlessness. In an environment with gravity such as that experienced on earth, humans must constantly resist the effects of gravity. In a weightless environment, this stress is removed. Because bone reacts to the stresses placed on it, a reduction of stress produces a reabsorption of bone material. For this reason, researchers and other space scientists are working to devise exercise programs that will continue to stress the musculoskeletal system in a weightless environment. A major obstacle to providing adequate exercise programs is the lack of adequate resistance. In space, if the astronaut pushes against something, such as a treadmill in running, there is still an equal and opposite reaction. Because the astronaut is not subject to the pull of gravity, rather than push off into a running step, the astronaut will push completely off the treadmill. Likewise, lifting weights is as likely to move the astronaut down as to move the weight up. A number of devices have been tested or are being considered to enable astronauts to do quasi-weight-bearing exercise in space. Treadmills with elastic tethering systems provide some resistance and hold the astronaut against the treadmill. Centrifugal acceleration systems that use centrifugal force to produce a simulated “gravitational” environment are also a potential mechanism for combating the disuse atrophy that is a result of space travel (Clement and Pavy-Le Traon 2004; di Prampero and Narici 2003).

Because of the lack of gravity, the concept of up and down has little meaning in space. Once a position is assumed through concentric muscle contraction, the segment or body remains in the new position unless moved by another force, such as further muscle contraction, or through contact with an external force. Reflexes are greatly affected by weightlessness. The extensor thrust reflex is less important because no deep pressure is experienced by the pacinian corpuscles. Also, in the absence of gravity, information from the proprioceptors is confusing, resulting in disorganization within the system. Until the individual adjusts, this disorganization often results in nausea, a serious problem in space travel.

Microgravity

Not all space travel involves weightless conditions. At this writing, humans are in the process of considering travel to Mars and well as the moon. Working and living on these bodies will subject astronauts to microgravity conditions. Most space travel as we know it takes place in microgravity. There will be some gravity, but it will be only a fraction of the gravity on earth, based on the size of the planet or moon. In microgravity normal human motion will be modified. As an example, it has been predicted that it will be more economical to run than to walk on Mars, with walking speed being 30 percent slower and the walk-to-run transition occurring sooner (Hawkey 2005).

• 1. Hang motionless from a rope or a pair of rings. Now start swinging without the help of anyone else. How was the swing accomplished? Explain.
• 2. Swing on a pair of rings without attempting to get much height. Perform the following movements. Explain the results by stating the underlying principles. Be sure to have mats placed under the swinging area.
  • a. Reverse the grasp of one hand. At which point in the swing is this accomplished most easily?
  • b. Dismount at various positions along the arc of the swing. Note the effect on the body.
• 3. Perform the following movements in the water and explain the actions by stating underlying principles. (Except for the factor of increased resistance, the actions simulate those of an unsupported body.)
  • a. Lie on the side and swing both legs forward vigorously, keeping the knees straight. Note the effect on the trunk.
  • b. Lie on the back with the arms in a side horizontal position. Keeping the elbows straight, swing both arms in a clockwise direction across the surface of the water in a 90-degree arc as you roll to the right into a prone float. Note the direction in which the feet now point compared with the starting position.
• 4. Perform a complete kinesiological analysis of a selected swinging or airborne activity, taking particular note of the purpose of the motion, its classification, and the nature of the motion. Include also muscle participation, neuromuscular considerations, mechanical factors, and applicable principles and violations. Conclude the analysis with a prescription for improvement, including suggested methods for change and the elimination of violations of principles.