MECHANICS OF THE JUMP APPROACH
A Manuscript by Irving Schexnayder, University of Southwestern Louisiana
Importance of the Jump Approach
When projected into flight, the center of mass of any object (including the human body) follows a predetermined, predictable, unalterable parabolic curve. Thus, establishment of the proper flight path is totally dependant upon proper force application during ground contact. Since all takeoff forces, including eccentric forces, are applied while still in contact with the ground, any resultant rotations are produced during ground contact as well. These rotations continue into flight and assist or interfere with efficient landings and clearances. In light of these facts, it is obvious that proper force application at takeoff is the only way to produce good performances in the jumping events.
Although good takeoffs produce good jumps, there are prerequisites to the execution of correct takeoff mechanics. Consider an athlete executing an approach run in a jumping event. A person running at relatively low speed can demonstrate greater accuracy in aligning the body into positions favorable for efficient takeoffs, because prior mechanical errors can be easily corrected.
However, when dealing with the high velocities we see in competitive jumping, the correction of errors occurring during the run is limited due to time constraints, and is often impossible without great compromises. Also, at these higher velocities, reflexes play a much greater part in the pattern of movement, again minimizing the chances to correct earlier errors in body positioning.
The execution of any part of an athletic endeavor is largely dependant upon the proper execution of prior parts. In cyclic tasks such as running, the correct execution of one cycle is prerequisite to the correct execution of the next cycle.
It is then obvious that in competitive situations, an error during the approach run will cause the takeoff, and thus the jump, to be compromised to some extent. The resultant jump is simply a modification of running mechanics, for better or worse. Considering that good execution of the run is prerequisite to proper takeoff and flight mechanics, it is logical that much time and effort be devoted to teaching this technical component, developing it into a contributing, rather than a negating factor.
Overview of the Locomotive Process
In humans, the goal of the locomotive process is to displace the body while maintaining stability. Human locomotion is produced by the creation of force and its application to the ground so that the ground reaction forces produce displacement without permanent sacrifice of stability.
Establishing the Base of Support
Consider one goal of locomotion, that being displacement. Horizontal momentum gained earlier in the locomotion process would be an aid in continuance of movement in that direction, so the conservation of that momentum is desirable. Mechanics that cause the foot to be moving backwards with respect to the forward moving center of mass at contact would reduce braking forces and help us to conserve this momentum. The further back touchdown occurs with respect to the body’s center of mass, the more attainable this goal would be.
At the same time another goal of locomotion is maintenance of dynamic stability. The further back the contacts occur with respect to the center of mass, the more likely the body is to topple over. Touchdowns must be sufficiently advanced to insure stability.
It follows that these two strategies are clearly at odds with each other. There exists therefore a tradeoff between the attainment of the goals of stability and conservation of horizontal momentum. The ideal touchdown point maximizes the benefits gained in light of both goals. For any given point in the acceleration process, the point of touchdown is predetermined if this tradeoff is to be optimized.
When force is applied to any elastic body which is not properly aligned and stabilized, the force intended to cause movement will instead cause distortion within the body. Improper alignment may also cause forces to act eccentrically, causing excessive rotation rather than maximum displacement.
Postural integrity is essential for this reason. During running, forces must be applied to and from a stable base to produce efficient movement. Postural concerns in running and jumping involve proper alignment of the head, spine and pelvis along with stabilization by appropriate musculature to establish this base. At the same time, this aligned unit must be kept in place and moved predictably as a unit so that forces may be applied appropriately. Erratic movements make force application difficult.
The head, throughout the process of acceleration and sprinting, must be kept in its natural alignment with the cervical spine, so that associated musculature is not overused and vestibular function is not disturbed. The spine must be stabilized in its correct alignment as to best accept the loading it receives from the forces produced by the legs and gravity. The pelvis should be stabilized with a slight upward tilt, in order to best facilitate force production and proper leg movements. A neutral alignment of the pelvis effects slight compromise, while a downwardly rotated pelvis and its associated lordosis offers great force production compromises and impairs proper function of the legs. Training implications for muscle groups responsible for pelvic alignment are profound.
Jump takeoffs, as stated earlier, are a modification of the mechanics of running being performed prior to preparation and takeoff. Extremely high levels of coordination are necessary to perform these modifications efficiently at high velocities, The optimal final velocity of the approach is then somewhat less than maximal velocity. We shall call this value the maximal desired velocity.
For a certain event, the maximal desired velocity for an approach is dependant on (1) the athlete’s maximal velocity, (2) the athlete’s coordination levels, and (3) the freedom of movement the athlete demonstrates at high velocities. Training these qualities can improve optimal final approach velocity for an individual athlete.
Development of Momentum and Velocity
The development of horizontal momentum is of the utmost importance in the jump approach. Momentum is developed by impulse generation. Impulse is defined as the product of force applied and the time over which the force is applied. This means that increasing force production and/or lengthening the time spent in force application will increase impulse.
Consider the process of maximal velocity sprinting. At high velocities, due to the cyclic nature of running, the duration of each ground contact is short, and the distance the center of mass traveled while the foot is in contact with the ground is very small. A greater percentage of the total distance traveled is due to the flight phase. Since these low ground contact times correlate directly to high velocity, minimizing contact time at maximum velocity is a worthwhile goal. In any sprint or jumping event, development of high horizontal velocity creates greater potential for optimal performances.
Unfortunately, these short ground contact times give us little opportunity to generate impulse and greatly change velocities and momentum values. The distances used in approach running are necessarily short due to energy system and coordination limitations, offering further limitation to momentum development. Impulse generation requires large force applications and long ground contact times, conditions that are clearly at odds with the high velocities that are desirable at takeoff.
At the initiation of the acceleration process, the cyclic process is slower and the body is moving at lower velocities, meaning longer ground contact times are appropriate. These velocities allow force to be applied for a longer period of time. More recovery opportunity exists as well, making it possible to more easily adjust the location of the base of support. It follows that the majority of the acceleration process should be accomplished at the initiation of the approach run, and that certain momentum values are prerequisite to proper execution of mechanics at high velocities. It is then necessary to devise some type of mechanism to develop momentum early in the approach.
The long ground contact times during the initiation of acceleration supply the opportunity for long periods of force application. To facilitate these long periods, the center of mass should travel a long path while the body is in motion and force is being applied to the ground. Since the optimal point of touchdown with respect to the center of mass is predetermined by the tradeoffs mentioned earlier, the only opportunity for lengthening contact times is to use a strategy that allows the foot to stay in contact with the ground until the center of mass is well past, allowing toe-off to occur far behind.
The only way to accomplish these rearward toe-offs during initial acceleration while maintaining postural integrity is to establish a forward body lean. This forward lean is the tool chosen by sprinters and jumpers to develop momentum early in the acceleration process. Generally the more hurried the acceleration process, the greater the initial lean, until a point is reached where the low initial body angle allows insufficient flight time for the correct execution of recovery mechanics.
Since displacement gains without additional energy expenditures clearly indicate efficiency, it is desirable to optimize the flight path associated with each stride to achieve maximal effective stride length. The most obvious method of increasing stride length, advancing the location of foot touchdown, subjects us to the tradeoffs mentioned earlier, and would cause corresponding decreases in the length of succeeding strides.
An appropriate strategy for maximizing the flight phase without compromising successive strides is to idealize the undulation path of the body’s center of mass. The body’s center of mass, during the running action, follows a sinusoidal curve. The peaks of this curve occur during the flight phase, while the low points occur during the support phase. Through proper joint extension and stabilization techniques, we can adjust this path as a unit, keeping the center of mass an optimal distance above the surface, maximizing the distance of the flight phase while still optimizing the yield from the tradeoffs involved in selecting touchdown location.
The Acceleration Process
These mechanical alterations (body lean, long contact times, rearward toe-offs, and maximizing displacement) give us our mechanism for efficient acceleration and momentum development in the approach run. As the acceleration process continues, velocities increase, and changes must occur. Until maximal desired velocity is reached, ground contact times must decrease so that the base of support can be rapidly readjusted and dynamic stability can be preserved. Toe-offs must advance until they are occurring only slightly behind the center of mass at maximal desired velocity. Touchdowns must constantly occur somewhat in front of the center of mass to insure stability. (In the pole vault, the mass of the pole must be considered as part of the postural unit). Failure to accelerate in accordance to these principles must result in inefficient force applications and postural alignments.
As toe-offs advance, initial body lean must decrease until maximal desired velocity is reached, when erect postures result. The body angle, from the onset of acceleration until the desired maximum velocity is reached, must progressively become larger. Sufficient advancement of the foot at touchdown and proper joint extension, changing the body’s alignment as a unit, is the proper vehicle for this process. Attempts at volitional lifting of the upper body without adjustment in the location of the base of support result in a pelvis misaligned with respect to the spine.
The time and/or distance required to perform and complete these mechanical changes varies from event to event. Part of the process of distribution involves allowing this process an ideal length of time for a particular race or event. Proper distribution insures that momentum prerequisites are met, insuring top performance and preventing injury. Rushing the process too much results in compromised momentum values, while spending too much time in the process eventually causes the base of support to fall behind, requiring a corresponding frequency increase to maintain stability. This increase is facilitated by corrupted posture and faulty recovery mechanics.
Coaches should exercise caution in using rhythmic cues while teaching distribution and acceleration. Rhythmic cues can be interpreted within many mechanical modes, some of which are at odds with proper momentum development.
Muscles can create much greater forces when contractions are preceded by a pre-stretch of the muscle and its tendons. The rebounding effect that occurs creates elastically produced energy. The fact that this energy creates no metabolic fatigue, yet produces greater forces than purely concentric contractions means that the development of elastic energy is a highly desirable goal in a variety of athletic endeavors.
The processes of acceleration and sprinting occur much more efficiently when elastic energy is developed. The processes associated with elastic energy gains during running are spinal engine function, hip oscillation, undulation of the center of mass, and establishment of proper amplitude of motion.
The spinal engine theory resulted primarily from observance of the vital function of the spine in producing locomotion in lower vertebrates. It seems logical that some of the gait producing function of the spine and its associated musculature would have survived the process of evolution, and that in man the spine does perform some locomotive function. While traditional models of locomotion assume the vertebral column simply rests upon the moving pelvis, the spinal engine theory hypothesizes that the spinal musculature actually participates in the generation of gait. The action of the pelvis and legs are an amplification of this movement.
During the process of running, the pelvis viewed from above rotates clockwise somewhat with each stride taken by the left leg, and rotates counterclockwise with each stride taken by the right leg. We shall call this repeated movement in the transverse plane hip oscillation. This oscillation produces stretches in the local musculature and connective tissue, creating great elastic energy gains. Hip oscillation also results in increased stride length without metabolic energy cost.
As stated earlier, during running the center of mass follows an undulating path. The low points of this curve occur during support. These periods of amortization offer opportunities for stretch shortening cycles to be developed in the pelvic stabilizing and leg extensor muscles, again producing elastic energy. Care must be taken so that the amortization process is not excessive and that foot contacts occur in the correct location, so that the undulatory path of the center of mass may be located for maximal displacement and elastic energy producing benefits.
Finally, large amplitude of motion in the hip joint, as evidenced by a long angular path of the femur, sets up elastic energy gains. Sufficient amplitude must be developed since stretches occur only near the limits of motion. Proper elastic operation of the hip joint will be discussed shortly.
Postural Integrity and Elastic Energy Generation
The process of postural integrity previously discussed is complex. The pelvis must be stabilized in an ideal alignment so that forces applied produce displacement without excessive distortion and rotation. However, the pelvis must be stabilized in a way such that the development of elastic energy is not impaired by restricting movement. Proper posture during the acceleration process should not be associated with total rigidity. This idea of stability while allowing elastic movementis consistent with the spinal engine theory.
The degree of elastic energy generation during the locomotive process is dependant upon dynamic stability and posture. Excessive instability and/or postural misalignment cause posturalmuscles to overwork to compensate for this instability and to maintain balance. This restricts their ability to function in elastic fashion.
Ramifications of Elastic Movements
The large involvement of elastic energy generating reflexes in athletic movement, along with the high velocities involved mean the execution of one part of the task is dependant upon execution of prior parts. Many errors of technical execution are simply reflexive actions set up by previous errors, so backtracking to earlier movements is often a very effective troubleshooting tool. Also, because of the reflexes involved, the value of technical learning exercises and drills which occur at velocities insufficient to evoke these reflexes must be questioned.
Summation of Forces
The musculature of different joints differ in their ability to produce force, and differ in the rate of development of force as well. The production of large forces by the body is dependant upon the ideal timing of the stabilizations and extensions of each individual joint involved in the motion. The force generating characteristics of the musculature of each joint determine the most appropriate point for that joint to contribute to the entire motion. Generally proximal muscles with great force producing ability and slower rates of force development contribute early, while distal muscles with less force generating capability and faster rates of force development contribute later. We will refer to a particular timing of stabilizations and extensions of joints as a firing order.
Consider a single force application (push-off) during a certain point in the acceleration process. The force created and applied to the ground can be thought of as a summation of the forces generated by the extension of the hip, knee, and ankle joints. The hip extends initially while the knee and ankle stabilize, Later, as the hip continues to extend, musculature of the knee shifts from stabilizer to force generator, extending the knee and applying force through a stabilized ankle. Finally, as the hip and knee have nearly completed their extensions, the ankle contributes.
Effective push-off forces during acceleration and sprinting are the result of appropriate firing orders. It follows that this ideal firing order of extension requires a certain unique amount of time, thus a certain ground contact time is prerequisite for this ideal process. Different firing orders are ideal at different points in the acceleration process, because of the varying amounts of ground contact time available.
During the onset of acceleration, firing orders which take longer yet involve more musculature and produce more force are more appropriate. This is consistent with the idea of early impulse generation and momentum development, Such firing orders effectively act as a low gear, a mechanical mode for efficiently accelerating the body from rest, which again points to the importance of using these firing orders early in the approach.
Summation of Forces and Stability
Given a certain velocity, the degree of dynamic stability of the body determines the time available to apply force to the ground. If touchdown occurs too far back with respect to the body’s center of mass, dynamic stability will be sacrificed and insufficient time will be available. The next stride must come quickly as the body topples forward. This means the extension will be rushed or incomplete, joints must fire simultaneously or fail to contribute, and an inefficient summation results.
If the foot contact is too far advanced with respect to the center of mass, braking forces will be encountered and too much contact time will be available. Two strategies are often employed to conserve horizontal momentum in this situation. One involves overworking the initial force generators (the hip extensors). The body must be pulled over its base of support. A certain degree of acceleration by the hip extension movement must occur prior to the extension of other joints for an efficient summation to take place. By the time the hip extends, other joints can no longer contribute because the time required would place toe-off too far back, resulting in excessive instability. This means the remaining extension will be rushed or incomplete, joints must fire simultaneously or fail to contribute, and an inefficient summation results.
The other strategy involves collapsing the knee and/or ankle joint, allowing the body to continue forward. This eliminates the existence of a stable base through which force can applied to the ground, and an inefficient summation results.
Facilitation of Steering
Steering refers to the adjustments in stride length made in order to takeoff from a desired location, whether it be from a takeoff board, a certain spot on the runway or apron, etc. Although it is desirable to develop accuracy in the approach through rehearsal, it should be realized that steering is a useful, desirable tool for making certain necessary adjustments, and even to prevent injury in some instances. Steering ability exists to varying degrees in different individuals, but is a trainable ability.
The steering process is primarily dependant upon visual sensory input, It follows that visual location of the target should occur as early as permitted by body alignment, at the onset of the run if possible. This visual contact should be maintained as long as possible. Usually visual contact must eventually become peripheral, and later aborted or readjusted in order to keep the head in proper postural alignment. This should not present a problem. By establishing visual contact early and maintaining it as long as possible, the jumper has developed a sense of his velocity and the rate at which the target seems to be approaching, so that this process need only be cognitively continued (target tracking) to execute an accurate takeoff.
Because sensory input throughout the run is essential to the jumper establishing a sense of his relative location and rate of approach, it is necessary that desired mechanical changes included in the run are performed gradually. Any abrupt changes in mechanical execution cause abrupt changes in sensory information fed to the steering mechanism, disrupt target tracking, andinaccurate approaches result. This implies that changes in body angle and the location of the base of support with respect to the center of mass found in sound acceleration patterns must be progressive and gradual.
Often abrupt changes in body angle and the location of the base of support with respect to the center of mass are performed late in the approach. These are usually last second attempts to correct these factors and get into efficient jumping postures, but because of their lateness usually result in inaccurate takeoffs and short fouls.
Often, as changes (especially postural changes) are made in the takeoff mechanism over time, the steering mechanism must be retrained for accuracy. Perceived distances from the target can be greatly changed by differing postural alignments, even if they are improved alignments.
Finally, the steering mechanism works to put the jumper in position to accurately execute an anticipated takeoff mechanism. Steering to inappropriate locations may occur when a mechanically improper takeoff is expected. These expectations usually arise due to past experience, incorrect prior positioning of the body, or both. An example would be a high jumper who senses inadequate development of centrifugal force during the approach, steering to a pointtoo close to the bar as a compensating strategy. Improving the takeoff mechanism often solves the problem of training an athlete to takeoff from a correct location………
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