MECHANICS OF THE JUMP APPROACH (Part 2 of 2)
A Manuscript by Irving Schexnayder, University of Southwestern Louisiana
Click here for Part I of Mechanics of the Jump Approach
Elastic Energy Conservation and Steering
As stated previously, the existence of an undulating path of the center of mass and hip oscillation serve as a great elastic energy producing mechanism. It is logical that we would want to continue to enjoy the benefits of this elastic energy gain throughout the takeoff. Thus, undulation and oscillation must continue to exist in some form throughout final part of the run and the takeoff process.
Often in jumpers, the production of elastic energy through undulation and hip oscillation is disrupted during the later stages of the approach, as a result of bracing in anticipation of takeoff forces. In addition to the loss of elastic energy available for takeoff, this disruption causes decreased stride length, disrupts location of the base of support with respect to the center of mass, and causes disturbances in target tracking. Inaccurate approaches result. Since stride lengths are decreased as elastic energy is diminished and target tracking occurs, the approach usually falls short of the target.
Amplitude of Motion and the Action of the Hip Joint
The hip extension movement is the prime locomotor in acceleration and sprinting. During acceleration, other joints are involved to greater degrees than at maximal velocities. At the onset of acceleration, due to the low center of mass caused by forward lean, most of the hip extension movement occurs during support. As acceleration progresses and the body angle increases, the center of mass rises and more of the hip extension movement occurs prior to touchdown.
During the approach run, the femur in elite horizontal jumpers, moves through a range of motion of approximately 100 degrees. At the onset of acceleration, the femur moves from a position of flexion parallel to the long axis of the body, to a position of extension forming an angle of about 80 degrees to the trunk. This progresses to a range of motion during maximum velocities that exhibits 10 degrees of hyperextension to a position of flexion parallel to the surface. The occurrence of hyperextension only at maximal velocities suggests that hyperextension is a result of angular momentum of the thigh.
Sufficient amplitude of motion in the hip assures that as extension occurs, the foot will be moving backwards with respect to the body’s center of mass at contact to minimize braking forces, but still contacting somewhat in front of the center of mass to insure dynamic stability. Proper femoral path of movement is also prerequisite to correct penultimate mechanics. In addition, great amplitude produces elastic energy gains. In light of these benefits of large amplitude in the hip joint, ramifications for mobility training are profound.
It is of value to consider the femur as an elastic pendulum (the femoral pendulum model). Some properly timed, voluntary hip flexion is combined with the elastic energy generated by the hip flexors to raise the femur. On the other hand, well timed voluntary contractions of the hip extensors, along with elastic energy generated by their pre-stretch produces the hip extension movement. Thus, voluntary contractions enhance the elastic contractions if they are properly timed.
Causes of Incorrect Path of the Femur
If at any time the voluntary contractions are improperly timed, or too much voluntary involvement by any muscle group exists, the elastic energy generation of the entire system is diminished and efficiency is reduced. The possibility of injury due to co-contraction is increased as well. When the hip flexors are over-involved, typically the femur reaches its normal upward location, but does not reach the desired range of extension, reducing stride length and elastic energy benefits. This is why a sitting type posture often results when knee lift is overemphasized.
On the other hand, when excessive extension of the hip, knee, or ankle joint occurs, the hip extends too far, extended beyond desirable ranges. This makes recovery difficult. Although correct amplitude may exist, that angle is misaligned, with the excessive extension lowering the femur’s upper limit of motion.
When this trailing of the femur results, it is generally because extension of the leg is taking too long and toe-off is occurring too far back. It must then be determined which joint is overextending and causing the delay in recovery. One possibility is that the neural message to stop extending the hip is being sent too late, allowing the hip to overextend before recovering it.
Overextension of the knee may be the cause. This is seldom the case in early stages of acceleration, but at maximum velocity the knee does not have enough time to fully extend without disrupting recovery of the femur. Finally, the ankle may be overextending, approaching extreme degrees of plantar flexion.
A final consideration is timing the sending of the neural signals. It must be remembered that messages starting or stopping the flexion or extension of any joint must be sent before the action is desired to occur, because it takes time for the message to travel from the brain to the musculature initiating the action. This fact must be considered when devising cue systems.
Another consideration when adjusting the range of motion of the femur to its proper position is the position of the pelvis. A pelvic girdle with a slight upward tilt is favorable for effective force application and gaining the proper range of motion of the femur. A pelvis with a downward tilt however skews the appearance of the femoral path. Although the femur may appear to be trailing because of its appearance with respect to the trunk or the surface, it may indeed be moving correctly with respect to the misaligned pelvis.
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The primary mechanical concern of the recovery phase is to reduce the effective radius of the recovery leg. This is accomplished by flexion at the knee joint. The degree of flexion present is a function of velocity. Shorter segments are able to generate greater angular velocity, therefore a tightly flexed knee with a high heel recovery path are desirable at maximum velocities. During the acceleration process, since velocities are submaximal, commensurately lesser degrees of knee flexion should be seen during recovery. At maximal velocities, maximal knee flexion should be seen as the femur reaches a position perpendicular to the ground, since it is at this point that the other leg is commensurately at maximum extension.
The knee flexion seen in recovery is primarily the natural result of well timed flexion of the hip. When the direction of the femur is changed, the prior angular momentum of the lower leg results in knee flexion. Volitional knee flexion plays a very small part.
Efficient recovery assures that the next foot contact will occur in the correct location to insure stability and proper contact time. Efficient recoveries also insure that the thigh will have sufficient time to reach its proper upward range of motion, maintaining freedom of movement and elasticity in the musculature of the hip joint. However, it should be noted that poor recoveries are generally symptoms of incorrect firing orders.
Recovery and Dynamic Stability
As shown earlier, improper firing orders may exist in the form of overuse or overextension in one or more joints. Improper firing orders cause inefficient recovery mechanics. Overextension in the hip, knee, or ankle joint will delay toe-off and compromise stability. The recovery mechanics are corrupted and hurried in an effort to restore stability.
Since toe-off has been delayed, these corrupted recovery mechanics generally take the form of delayed flexion of the knee during recovery, resulting in lower heel recovery path. In extreme cases, there may not be enough time available to allow the knee to reach its ideal degree of flexion before preparing for the next ground contact. In either case, the compromised recovery generally results in a hip that does not flex sufficiently. Injury due to co-contractions may result, since the hip extensors may still be in contraction while the hip flexors start to contract (or vice versa). This causes severe compromise of elastic energy gains and impairs free movement in the hip joint.
As stated earlier, efficient movements in the hip and spinal area during running are primarily a result of elastic energy formed by stretch reflexes and well timed voluntary contractions of proper duration and magnitude. Since improper voluntary contributions can over-stabilize and prevent the development of stretches, obviously there exists a limit on the magnitude and duration of even well timed voluntary contractions, beyond which elasticity suffers. Therefore, there exists a maximum frequency for each stride in the acceleration process for any given individual. To attempt to achieve a frequency greater than that maximum frequency is possible, but at the expense of inefficiency due to the corresponding loss of elastic energy. The development of frequency must be gradual and progressive, with the resultant frequency of each stride not exceeding that maximal frequency. The resonance of the oscillating system must be maintained, and excessive volitional energy being fed into the system creates artificial frequency.
The first requisite for creating artificial frequency is a base from which to apply force. The strategy generally chosen to accomplish this goal is excessive stabilization of the torso and pelvic regions, which (1) disrupts elastic energy production in that region, (2) causes a lowering of the center of mass, (3) causes a decrease in stride length because oscillation and freedom of movement have been compromised and (4) causes alterations in the location of’ touchdown with respect to the center of mass. In light of these considerations, the wisdom of a strategy that employs a frequency increase in the final few strides of the approach (in excess of the normal associated increase in frequency) should be questioned.
Kinetics of the Knee Joint
At the onset of acceleration, during support at the initiation of the hip extension movement, the knee joint is stabilized by its associated musculature so that force generated by hip extension may be efficiently transmitted to the ground. However, as the hip continues to extend, the knee’s function shifts from stabilizer to force generator. The knee extends, contributing to force application through a stabilized ankle joint. As stated earlier, knee flexion occurs during the recovery process. When the flexion of the hip is stopped and extension occurs, the angular momentum possessed by the leg causes some knee extension to occur prior to ground contact.
The knee is then stabilized by the quadriceps in preparation for the next ground contact. As velocities increase, changes occur. The knees function during support shifts from stabilizer and force producer to stabilizer. At maximal velocities, the knee does not display full extension at toeoff.
Excessive extension by the knee joint would delay recovery excessively. This decreased contribution by the knee joint at high velocities results in a very efficient kinesiological arrangement. The musculature of the knee joint is primarily slow twitch in nature, and is called upon to stabilize, while the fast twitch rich gluteals act as locomotors.
As stated earlier, the degree of knee flexion during recovery increases as velocity increases. As greater angular velocities develop during recovery, more knee extension results when the flexion of the hip is reversed. As the hip starts to extend, the quadriceps stabilizes the knee at a certain degree of extension. This stabilization prior to and during the support phase provides the opportunity for elastic energy generation by stretch shortening cycles set up in the musculature.
This energy is produced without excessive degrees of extension.
The degree of knee extension that occurs prior to ground contact differs greatly among individuals. Those who show large degrees of knee flexion at touchdown are more able to use the hip extensors as producers of large forces. Those who exhibit great degrees of knee extension at touchdown keep the center of mass at a higher point throughout the process of sprinting, locating the undulating path of the center of mass higher and maximizing displacement and the flight phase.
Related Article by Boo: Development of Speed in the Horizontal Jumper
Position and Kinetics of the Ankle Joint
Ankle position during acceleration and maximum velocity sprinting is important in two realms, joint stability and force production. A dorsiflexed position of the ankle is superior in both aspects.
Anatomically, the dorsiflexed ankle is more stable due to the skeletal construction of the joint, contraction of the plantar flexors, and the pre-stretched gastrocnemius and soleus. This stability makes the ankle joint resistant to collapse upon ground contact, making short ground contact times at high velocities achievable. Elite sprinters and jumpers show ankle angles during the approach which seldom vary greatly from 90 degrees.
The pre-stretch on the gastrocnemius and soleus offer opportunity for necessarily quick elastic force generation through tendon reflexes upon ground contact, without introducing great degrees of plantar flexion upon push-off which would delay toe-off and hamper recovery efforts.
Also, since the gastrocnemius originates above the knee, keeping it on stretch by dorsiflexing the ankle allows it to assist in any voluntary knee flexion during recovery, allowing the hamstrings to remain relaxed.
Stability of the ankle is of the utmost importance in acceleration, sprinting, and jumping. A stable ankle provides a solid base through which the hip and knee joints may apply force to the ground.
During acceleration, touchdown occurs on the ball of the foot, and excessive collapse in the ankle joint means force will be absorbed rather than transmitted to the ground. As velocities increase, the ankle is stabilized in a very slightly plantar flexed position, to facilitate this force application by the ball of the foot through the stable ankle. In either case, the ankle remains stable and the ball of the foot acts as a fulcrum and point of force application as the tibia pivots forward, rather than allowing the ankle to collapse and the tibia to pivot forward at the heel. During the final stages of support, a quick force contribution occurs by way of the tendon reflex. The fact that many quadruped mammals use the ankle in a similar fashion, can give us insight into the correct function of the ankle joint during locomotion.
Conversely, a plantar flexed ankle position results in joint instability and creates longer contact times, delaying toe-off and disrupting the location of the base of support with respect to the center of mass, Because of this instability, some collapse of the ankle joint upon contact is inevitable, and a compensatory plantar flexion occurs as a substitute force producer. Another compensating strategy involves the ankle joint stabilizing upon contact by firing early, out of sequence.
It is important that any contribution by the ankle be quick, elastic, and reflexive in nature. Excessive contribution by plantar flexion at toe-off results in delayed recovery, because the plantar flexors are primarily composed of slow twitch muscle fiber. An unstable ankle joint also delays hip and knee extension because establishment of a firm base of support occurs too late, resulting in poor pushoffs and takeoffs.
Because of time restrictions and reflexes that occur at high speeds, it is not possible to recruit ankle stabilizers at selected points. Proper ankle positioning must be constantly maintained.
Action of the Arms
During acceleration and sprinting, the function of the arms is to counter force applications by the legs, thus aiding in the maintenance of stability and posture, While small oscillations of the shoulder axis do occur in opposition to the oscillation of the hip axis, most of this countering is accomplished by the arms themselves, as they work in opposition to the legs.
The action of the arms at the shoulder joint should be elastic in nature. Stretch reflexes are set up which, along with well timed voluntary contractions, produce the movement at the shoulder joint in each direction. Very slight outward flare of the elbows helps to set up elasticity in the shoulder joint. Too much voluntary muscular involvement results in a decrease in elastic energy generation and decreased efficiency. Attempts at frequency increase often produce this involvement.
There should be a commensurance between the length of the firing order and the length of the arm as an effective lever. Longer ground contact times should be associated with more obtuse angles of the elbow. This means elbow angles should progressively decrease in size as acceleration progresses.
There should also be a commensurance between the length of the arm as an effective lever and the rate of force generation of the joints involved in locomotion. As the hip initiates its extension, the angle of the opposite elbow should be somewhat large, since the acceleration produced
during this particular pushoff is not yet maximized. As the knee and ankle contribute, the body has accelerated somewhat, thus the elbow angle should be decreased somewhat. This means that for any stride, as the arm moves forward, the elbow angle should be decreasing in correspondence to the acceleration occurring during that pushoff and the size of the primary joint contributing.
The wrists should be stabilized in a normal extended alignment to create minimal muscular involvement, and to allow the effective length or the arm to be accurately controlled by elbow flexion and extension. A loosely cupped position of the hand creates minimal muscular involvement and the most elastic situations in the arms and shoulders.
Importance of the Start
As previously stated, in elastic situations and at high velocities it becomes very difficult to correct previous errors in body positioning. Thus, it follows logically that many of these mechanical variables should be established correctly at the start, since chances for later corrections are slight. Because of the large number of elements that must be correctly established at the start, it also follows that as little extraneous movement as possible be involved in the start. This implies that standing or rocking starts give better control of these variables, and that walking or jogging starts may cause difficulty in adjusting vital mechanical parameters. In any start, any preliminary movements and/or the position reached immediately prior to pushoff should establish (1) positioning of the front-side femur near its ideal upward range with respect to the torso, (2) positioning of the backside arm high enough and extended enough to correctly counter the force of pushoff, (3) positioning of the front-side arm in correct opposition to the other arm, (4) dorsiflexion of both ankles, (5) proper location of the center of mass with respect to the base of support (front foot) so that an ideal degree of initial lean can be established when pushoff occurs, (6) adequate height of the center of mass so that the initial flight phase will be long enough to permit recovery and a correct successive foot strike, (7) some pre-stretch on the hip flexors of the rear leg so that as the rear leg comes forward, adequate range of motion can be established, and (8) proper postural alignment of the pelvis, torso, and head.
The pushoff should establish, (1) a correct summation of force generation by the front leg (well timed hip, knee, and ankle extension in correct quantities), (2) correct degree of initial body lean (3) sufficient dynamic stability, (4) correct postural alignment of the pelvis, torso, and head, (5) repositioning of the arms to counter the next pushoff, (6) slight force application by the rear leg, (7) partially elastic lifting of the rear femur to its correct upper position, (8) establishment of correct amplitude of motion in both femurs, and (9) sufficient flight time to execute recovery correctly.
With the exception of the obvious limitations that arise from arm length and positioning, the block start should not deviate from any of the aforementioned parameters.
It should be noted that many elite performers deviate from one or more of the guidelines,
principles, and suggested optimal methods stated above, Some of these differences result in somewhat compromised efficiency, yet are not harmful enough to prohibit performance at high levels by talented individuals.
Other deviations from the above stated principles are compensating maneuvers. Some elite performers employ seemingly stylistic maneuvers which are actually corrections of prior errors.
When one or more of the above principles are violated, some strategy must be adopted to compensate, and often this compensation is sufficient to allow high levels of performance. An example would be a high jumper who starts incorrectly; allowing toe-off’s to occur too far behind the center of mass and instability to result. A high, bounding skip in the middle of the run may be a strategy employed to buy time to correct the location of the base of support with respect to the center of mass.
The purpose of this manuscript has been to examine the kinetics and kinematics of certain mechanical parameters of the jump approach, explain the behavior of these parameters, and examine any special considerations for jumpers that would affect these parameters, It is the hope of the author that this manuscript will suggest new avenues for research and new parameters for examination in biomechanic reports.
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