Chapter 18. Moving Objects: Throwing, Striking, and Kicking

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

• 1. Classify activities involving sequential throwing, kicking, or striking patterns according to the nature of the force application.
• 2. Name and discuss anatomical and mechanical factors that apply to representative throwing, kicking, or striking activities.
• 3. Perform a kinesiological analysis of someone engaging in a sequential throwing, kicking, or striking skill under each of these force application conditions: momentary contact, projection, continuous application.

A baseball pitcher throws a baseball across the plate and the batter hits it to center field, an elderly man pitches horseshoes, a young person spikes a volleyball, a student practices driving a golf ball while a college athlete practices punting a football. Once more, as is the case with pushing and pulling, a widely diverse set of activities has a common denominator. Each of these activities involves sequential movement of the body segments resulting in the production of a summated velocity at the end of the chain of segments used. During the summation of velocity, it is important to note that changes are the result of an acceleration imparted through an unbalanced force. The magnitude of the unbalanced force as well as the time over which this force acts will determine the final velocity. This concept of the force and the time over which it acts effecting final velocity is called impulse (Chapter 12). Recall that the change in momentum is equivalent to the product of the force and time over which that force acts. The path produced by the end point of this chain of segments is curvilinear in nature.

Sequential segmental motions are most frequently used to produce high velocities in external objects. Depending on the objective of the skill, speed, accuracy, distance, or some combination, modifications in the sequential pattern may be made. Greater or fewer numbers of segments may be involved, larger or smaller ranges of motion might be used, and longer or shorter lever lengths may be chosen. Regardless of the modifications, the basic nature of the sequential throwing, striking, or kicking pattern remains the same.

Joint Action Patterns

Broer was the first to call attention to the similarity of movement patterns used in seemingly dissimilar activities such as the baseball pitch, the badminton clear, and the tennis serve (Broer and Zernicke 1979). Objective evidence of such similarities between throwing and striking activities within each of the three major upper-extremity patterns (overarm, sidearm, and underarm) was originally revealed by the Broer and Houtz EMG investigations (1967) and confirmed by Moynes and colleagues (1986).

Atwater (1980) distinguished between the over-arm and sidearm throwing patterns in terms of the direction in which the trunk laterally flexed. When lateral flexion occurred away from the throwing arm, an overarm pattern was used; lateral flexion toward the throwing arm indicated a sidearm pattern. The underarm pattern is distinguished by motion predominantly in a sagittal plane with the hand below the waist.

Each pattern involves a preparatory movement referred to as a backswing, or windup, followed by the establishment of a base of support prior to the initiation of the force phase and ending in a follow-through. The base of support in the direction of the force application (forward-backward) is a distinguishing feature of skill level. It has been well documented that more highly skilled individuals have longer strides. Once the base has been established, the more proximal segments begin the force application phase while the more distal segments complete the backswing. The purpose of the backswing is to place the segments in a favorable position for the force phase.

There are two primary benefits to including a backswing or windup phase in a sequential motion. The first of these is to take advantage of the impulse-momentum relationship. The backswing will increase the time over which the force can be applied, thereby increasing momentum. The second major benefit is to place the segments into a favorable position for a full range of motion force phase. The backswing lengthens the muscle groups that will be the agonists for the force phase, the stretch part of the stretch-shortening cycle (Grezios et al. 2006). Initiation of the concentric force phase utilizes the stored elastic energy to assist in the development of the contraction. This also evokes a phasic stretch reflex, further enhancing the force phase contraction.

Representative joint actions for each throwing pattern are as follows.

Overarm Pattern

This kind of throw or strike is characterized by rotation at the shoulder joint. In the backswing, or preparatory phase, the abducted arm rotates laterally, and in the forward, or force, phase the arm rotates medially. Some elbow extension, wrist flexion, and spinal rotation occur in the force phase. These movements are accompanied by rotation of the pelvis at the hip joint of the opposite limb, resulting in medial rotation of the thigh (Figures 18.1 and 18.2). An immature pattern (Figure 18.3) may be identified as using fewer segments, working more simultaneously rather than sequentially, and involving a more limited range of motion. This description is easily documented by referring to the developmental literature.

Underarm Pattern

This pattern consists of a forward movement of the extended arm in the sagittal plane, usually starting from a position of hyperextension and ending in a forward reach. The basic joint action of the arm is flexion. The actions of the wrist, spine, and pelvis are the same as those observed in the overarm pattern (Figure 18.4).

Sidearm Pattern

In this pattern the basic movement is medial rotation of the pelvis on the opposite hip with the arm usually in an abducted position. The arm is moved forward in a horizontal plane because of the pelvic action and spinal rotation. The spine also laterally flexes toward the throwing arm. The range of the upper-extremity movement may also be enlarged by the addition of horizontal adduction at the shoulder. The elbow is maintained in extension or is extended from a slightly flexed position, depending on the nature of the skill in question (e.g., basketball throw for distance, tennis forehand drive, or batting). Wrist flexion may also be part of the action in some techniques (Figure 18.5).

Kicking Pattern

Another form of sequential movement pattern involves use of the lower extremities and is referred to as kicking. This pattern is used, for example, when punting a football (Figure 18.6) or initiating a soccer kick. Kicking is a modification of a locomotor pattern in which force is imparted to an external object during the forward swing of the non-weight-bearing limb.

To initiate the kicking motion, the left foot (of a right-footed kicker) is stabilized. The pelvis is fixed over the thigh and rotated toward the left. The right non-weight-bearing limb lags behind, resulting in thigh abduction and hyperextension at the hip joint. The right thigh then begins to flex at the hip joint followed by knee extension. This sequential segmental movement pattern results in the summation of forces and allows for maximum linear velocity of the foot at ball contact. The specific point of ball contact depends on the placement of the supporting foot and motion of the free-swinging limb during the backswing.

Nature of Force Application

Momentary Contact

Movements such as striking and kicking are characterized in sequential movement patterns by a momentary contact made with an object by a moving part of the body or by an implement held by or attached to a moving segment of the body. Baseball batting, golf, tennis, and soccer kicking are all examples of force imparted through momentary contact.

Projection

In sequential patterns, an object is given some velocity as the end point of a part of the body or as an implement attached to the body. At the desired point in the curvilinear path, the object is released to become a projectile. Throwing is an example of this type of projection, as are the projection of balls by implements as in lacrosse or jai alai.

Anatomical Principles

Regardless of which movement pattern is used, the following factors must be considered when imparting force to an object through the sequential use of body segments. To ignore the anatomical principles of sequential motion is to risk injury because sequential motion can generate extremely high velocities, as well as impart high braking forces.

• 1. Muscles contract more forcefully if they are first put on a stretch, provided they are not overstretched. This principle suggests the function of the windup in pitching and of the preliminary movements in other sport skills.
• 2. Unnecessary movements and tensions in the performance of a motor skill mean both awkwardness and unnecessary fatigue; hence they should be eliminated.
• 3. Skillful and efficient performance in a particular technique can be developed only by practice of that technique. Only in this way can the necessary adjustments in the neuromuscular mechanism be made to ensure a well-coordinated movement.
• 4. The most efficient type of movement in throwing and striking skills is ballistic movement. Skills that are primarily ballistic should be practiced ballistically, even in the earliest learning stages. This means that from the beginning the emphasis should be placed on form rather than on aim. Accuracy of aim will develop with practice. If the emphasis is placed on accuracy in the learning stages, the beginner tends to perform the skill as a “moving fixation” or as a slow, tense movement. Once this pattern of movement is established, it is extremely difficult to change it later to a ballistic movement.
• 5. When there is a choice of anatomical leverage, the lever appropriate for the task should be used—that is, a lever with a long resistance arm for movements requiring range or speed, and a lever with a long effort arm for movements requiring strength. For example, kicking is an effective way to impart force to a football because the leg provides a lever with a long resistance arm but, for the same reason, it is a poor way to move a heavy suitcase along the floor.

Mechanical Principles

Although many of the mechanical principles for throwing, striking, and kicking are similar to those cited for pushing and pulling, the applications of these principles are different. Whereas force application in push-pull patterns is maximized through using large numbers of segments simultaneously, force in throwing, kicking, and striking patterns is maximized through sequential transfer of momentum from large segments to less massive segments. To illustrate this, the mechanical principles involved in sequential movement patterns are given as they apply both to throwing and to striking.

Throwing

The efficiency of imparting force to a ball is judged in terms of the speed, distance, and direction of the ball after its release. The purpose of the throw determines which of these is given the greater emphasis. Both the speed and distance of the thrown ball are directly related to the magnitude of the force used in throwing it and to the speed of the hand at the moment of release. The speed the hand is able to achieve depends on the distance through which it moves in the preparatory part of the act and the summed angular velocities of the contributing body segments. Hence, the longer the preparatory backswing and the greater the distance that can be added by means of rotating the body, shifting the weight, and perhaps even taking a step, the greater the opportunity for accelerating. Approximately 50 percent of the ball speed is obtained from the forward step and body rotation. The remaining speed is contributed by the joint actions in the shoulder, elbow, wrist, and fingers. This is why the technique of a baseball pitcher is designed to allow maximum time and distance over which to accelerate the ball before its release. In addition, if the ground reaction force is to be maximal, the surface against which the thrower pushes must be firm and there must be no sliding between the ground surface and the foot. The more the direction of the body thrust is backward, the more important the friction becomes and the more the value of the cleated shoes is appreciated. If distance is a major objective of the throw, the angle of projection and the effects of gravitational force and air resistance must also be taken into consideration.

The accuracy with which a ball is thrown depends on accurate judgment of the distance and direction of the throw’s target. The ball will leave the hand in the direction it is moving at the instant of release and continue in that direction except for modifications because of gravity and air resistance. Therefore, the effect of gravity, wind, and spin must be considered, as well as the release direction. If the hand is traveling in an arc at release, the ball will follow a path tangent to the arc, and the timing of the release is highly critical. Flattening the arc of the ball’s path prior to release increases the margin of error by allowing more time over which the ball can be released in the desired direction. This may be done by taking a step and shifting the weight forward while flexing at the forward knee. The rotation of the pelvis and spine also helps.

The follow-through of any throwing motion is critical to the successful performance of a throw. In addition to the application of the impulse to the windup, impulse is employed during the follow-through as well. In the acceleration phase of a throw, the velocity of the ball is increased from zero to maximum velocity at ball release. Next, the arm must be decelerated to slow down the arm. The shorter the follow-through, the more force is required from the external rotators of the shoulder, particularly the rotator cuff muscles (especially the infraspinatus and teres minor).

When the object being thrown is other than a small ball, such as a javelin, discus, bowling ball, or horseshoe, the general principles are the same but must be modified according to the nature of the object and the regulations of the sport.

Principles

• 1. The object will move only if the force is of sufficient magnitude to overcome the object’s inertia. The force must be great enough to overcome not only the mass of the object but also all restraining forces. These include (a) friction between the object and the supporting surface, (b) resistance of the surrounding medium (e.g., wind or water), and (c) internal resistance. Warming up helps prevent injury as well as decrease internal resistance. Increasing range of motion through training is also important and effective.
• 2. The pattern and range of joint movements depends on the purpose of the movement. If maximum force magnitude is not needed, the optimum movement pattern should be changed with respect to the range, speed, and number of joint actions until it is the most efficient for the task. Throwing a ball to a small child 10 feet away does not require the same pattern as does pitching a baseball.
• 3. Force exerted by the body will be transferred to an external object in proportion to the effectiveness of the counterforce of the feet (or other parts of the body) against the ground (or other supporting surface). This effectiveness depends on the counterpressure and friction presented by the supporting surface. The force applied to a ball thrown while in midair or while treading water is less than that applied to it while pushing against the ground.
• 4. Linear velocity is imparted to external objects as a result of the angular velocity of the body segments. The linear velocity at the end of any segment is the product of the angular velocity and the length of the segment. Thus, for a given angular speed, the linear speed imparted to a tennis ball with a tennis racket is much greater than that of a ball hit with the hand. On the other hand, more force is needed to move the longer of the two levers.
• 5. Optimum summation of internal force is needed if maximum force is to be applied to move an object. For maximum velocity of each contributing body segment to occur at release of, or impact with, the external object, the slower or heavier segments must start to move first and the lightest and quickest ones last. It is preferable for the slower segments to begin their forward movements while the faster segments are still completing the backswing. This timing pattern facilitates use of the stretch reflex (see Chapter 4).
• 6. For a change in momentum to occur, force must be applied over time (impulse). If maximum force is desired in projection-type activities, maximum muscle torques should be applied over as long a time as possible.
• 7. Force applied in line with an object’s center of gravity will result in linear motion of the object, provided the latter is freely movable.
• 8. If the force applied to a freely movable object is not in line with the latter’s center of gravity, it will result in rotary motion of the object.

Striking, Hitting, and Kicking

As in the case of throwing, the effectiveness of striking, hitting, and kicking is judged by the speed, distance, and direction of the struck ball. All the factors that apply to these aspects of a thrown ball apply similarly to a struck ball. There appear to be five major factors that apply to the speed of a struck ball. These are:

• 1. The speed of the oncoming ball and the striking implement.
• 2. The mass of the ball and the striking implement.
• 3. The coefficient of restitution (elasticity) between the ball and the striking implement.
• 4. The direction the ball and the implement are moving at the time of impact.
• 5. The point of impact between the ball and the implement.

The speed of the approaching ball and the speed of the striking implement may be further analyzed into secondary factors. For instance, the speed of the striking implement is determined by the magnitude of the force exerted, and the magnitude of the force depends on the distance of the preparatory backswing and the speed of muscular contraction. The distance of the preparatory backswing further depends on the range of motion in the joints and the timing of the swing. Furthermore, the effectiveness of the force exerted by the body depends completely on a strong grip and a firm wrist for transmission of the force from the body to the striking implement, and a firm ground support for transmission of the normal reactive force to the body.

Objects that have been hit through their centers of gravity will have linear motion, whereas those that have been hit off center may curve because of a resultant spin. The more the striking force is eccentric in its application, the less is the linear speed imparted to the ball. Furthermore, balls that hit a striking surface at an angle to the surface will rebound at an angle approximately equal and opposite to the striking angle. Some examples of ball and striker velocities before and after impact are presented in Table 18.1.

The force to bring about the changes in velocity noted in Table 18.1 can be rather large, as in this example using a baseball hit from a tee:

Therefore, if the time the bat contacts the ball is 0.00125 second, the bat impacts the ball with a force of 996.25 pounds (4431 N).

Principles

Those mechanical principles cited as pertaining to throwing patterns are also related to striking patterns. In addition, the following factors also must be applied when analyzing striking patterns.

• 1. The direction in which the object moves is determined by the direction of force applied to it. If the force consists of two or more components, the object will move in the direction of the resultant of those components. After being projected or struck, the direction will be modified by gravity and air resistance.
• 2. Momentum is conserved in all collisions. The momentum before impact must equal the momentum after impact, provided that none is lost through friction or other forces.
• 3. Any change in the momentum of colliding objects is related to the force and duration of the collision. The greater the impulse (Ft), the greater will be the change in momentum. This is why the follow-through is important. The longer the missile can be carried along by the striking implement at its greatest velocity, the greater will be the change in velocity of the missile.
• 4. The greater the velocity of the approaching ball, the greater the velocity of the ball in the opposite direction after it is struck, other things being equal.
• 5. The greater the velocity of the striking implement at the moment of contact, the greater the velocity of the struck ball, other things being equal. Obviously, a full-powered swing will send the ball farther and faster than will a bunt. Both increasing the length of the lever and striking with a good follow-through help increase the velocity of the striking implement and therefore increase the force of impact.
• 6. The greater the mass of the ball, up to a point, the greater its velocity after being struck, other things being equal. A hard baseball will travel farther and faster than a softball. Nevertheless, an iron ball would offer too much resistance for the average batter using an average bat.
• 7. The greater the mass of the striking implement, up to a point, the greater the striking force, and hence the greater the speed of the struck ball, other things being equal. A good baseball player uses a heavier bat. Too heavy a bat, however, is inadvisable because of the difficulty in swinging it with sufficient speed and control.
• 8. The higher the coefficient of restitution (the elasticity) of the ball and of the striking implement, the greater the speed of the struck ball, other things being equal.
• 9. The direction taken by the struck ball is determined by four factors: (a) the direction of the striking implement at the moment of contact; (b) the relation of the striking force to the ball’s center of gravity (an off-center application of force causes spin, and spin affects direction); (c) degree of firmness of grip and wrist at moment of impact; and (d) the laws governing rebound (see Angle of Rebound).

Analysis of the Overarm Throw

To give the reader a better understanding of the coordination pattern referred to as sequential segmental actions, the force phase of the overarm pattern is analyzed as applied in a forceful throw such as in pitching. This analysis includes joint actions, muscle activity, and mechanics for the upper extremity only.

The purpose of the backswing, or preparatory, motion is to place the segments in a favorable position for the force phase. There are many ways to execute the backswing while taking advantage of the stretch-shortening characteristics of muscle. Although some favor one method over another, any windup is appropriate that places the segments in the most advantageous position to maximize the number of segments and the time over which force can be applied. Such a windup, allowing the joints to be placed in an optimal position to involve the greatest number of segments, includes pelvic and trunk rotation in the opposite direction from the intended throw (right for a right-hander), horizontal abduction, and lateral rotation at the shoulder joint with elbow flexion and wrist hyperextension. This sequence of arm action continues as a forward step is taken with the opposite foot. This hand–foot opposition permits the greatest range of motion in the trunk and pelvis. It also allows for a large base of support over which the force phase can be applied while preserving balance.

No pause occurs between the windup and the force phases. The windup ends with a forward stride using the opposite leg. Establishment of a base of support is followed immediately by pelvic and then trunk rotation, accompanied by lateral flexion to the left (right-handed pitcher). The trunk motion causes increased horizontal abduction with continuing lateral rotation at the shoulder joint. Elbow joint extension is followed by the initiation of rapid medial rotation at the shoulder joint, forearm pronation at the radial–ulnar joint, and then flexion and often ulnar flexion at the wrist joint. The force phase ends with the release of the ball. The angle at the elbow joint at release is approximately 105 degrees. The follow-through includes the actions from ball release until the momentum developed in the arm can be safely dissipated as the arm continues across the body in a downward direction. In addition, a forward step is also often used.

These actions proceed from the proximal (more massive) to the distal (lighter) segments. In characteristic sequential patterns, subsequent segments lag behind those preceding them. The subsequent segment is carried along, lagging behind until the more proximal segment attains its maximum angular velocity. The distal segment then accelerates because of a combination of the series elastic components, stretch reflex, and muscular contraction. This combination results in the reaction or slowing down of the preceding segment. Because of conservation of momentum, momentum (mv) is transferred from the more massive (proximal) to the less massive (distal) segments, significantly increasing their velocity. Therefore, the linear velocity at the end of the chain—that is, the ball at release—often can be over 90 miles per hour.

The legs play an important role in any successful throw. They provide the stable base over which the trunk and other segments act. In addition, the thrust they provide contributes significantly to the force of this movement. The transfer of momentum from proximal to distal, however, focuses the attention on the musculature of the upper extremity. Because this phase emphasizes the application of the force accelerating the limb, muscle contractions are mainly concentric preceded by a short eccentric phase because of the lagging of the distal behind the proximal segments.

Muscular Analysis of Upper Extremity (Table 18.2)

The lateral rotation preceding the medial rotation of the right shoulder joint is controlled by the eccentric contraction of the medial rotators of the humerus followed by the concentric contraction of the same muscles, including the subscapularis, pectoralis major, and latissimus dorsi muscles. During the acceleration phase, the height of the humerus was controlled by a static contraction of the middle deltoid. The deltoid and supraspinatus muscles contract concentrically during the backswing to position the upper arm, and eccentrically during the follow-through to help decelerate the arm. The infraspinatus muscle was also very active (eccentric contraction) to assist in decelerating the arm during the follow-through.

The biceps brachii muscle activity reaches its peak as the elbow joint is flexed late into the backswing and at the beginning of the force phase. Marked activity again appears during the follow-through to protect the elbow joint in decelerating the forearm. The latissimus dorsi muscle, active during medial rotation, remains active eccentrically contracting during the follow-through to assist in controlling the arm motion across the body.

It is clear that the stretch reflex is an important facilitating mechanism in helping accelerate the lagging distal segment at the appropriate time. The more rapid the stretch (eccentric contraction), the greater will be the facilitating effect on the resulting concentric contraction of the same muscle. In this way, forces can be summated more appropriately. To gain the greatest facilitation from the stretch reflex, there should be no pause between the windup and force phases.

Because the trunk rotates under the stationary head (eyes focused on the target), the tonic neck reflex may facilitate the strong acceleration occurring during the force phase. The asymmetric tonic neck reflex facilitates the shoulder abductors and elbow extensors on the chin side. This is precisely the arm position at release.

The extensor thrust reflex may act on both the upper and lower extremities. Increasing pressure on the palmar side of the hand as the arm is being accelerated forward may facilitate the arm extensor muscles. Similarly, as the weight is transferred to the forward foot, the pacinian corpuscles are stimulated by the increased pressure, resulting in a facilitation of the lower-limb extensor muscles. When accuracy is a factor, knee flexion must be maintained to flatten the arc through which the hand is carried. This flattening permits the tangential flight path of the ball to have a broader margin for error in the timing of the release while arriving at the same target point following release.

Analysis of the Forehand Drive in Tennis

Description

The forehand drive (Figure 18.7) is one of the fundamental strokes of tennis. Its objective is to send the ball over the net and deep into the opponent’s court close to the baseline.

Starting Position

The player faces the net with the feet about shoulder-width apart and the weight on the balls of the feet. The racket is held with an eastern or shake-hands grip.

Backswing

The player pivots the entire body so that the shoulder and hips of the nonracket side are toward the net. At the same time, the racket is taken back at shoulder level in either a straight or a circular manner, with the head of the racket above the wrist and its face turned slightly down. The weight of the body is over the rear foot (racketside).

Forward Swing and Follow‐Through

The player flexes at the knee joints to drop the racket and racket arm below the intended contact point (still keeping the racket head above the wrist with its face turned down) and steps toward the ball with the nonracket foot. The pelvis and spine rotate so the trunk faces forward, and the weight is shifted to the forward foot as the racket is swung forward and up. The racket face is perpendicular to the court at ball impact, thus imparting topspin to the ball as it swings through and up. The follow-through continues toward the intended target, with the racket arm swinging across the body and up toward the chin.

Anatomical Factors

The action in the forehand drive is ballistic in nature and, as such, is initiated by muscular force, continued by momentum, and finally terminated by the contraction of antagonistic muscles. The chief lever participating in the movement consists of the arm, trunk, and racket together with the fulcrum located in the opposite hip joint (see Figure 13.19), the point of force application at a point on the pelvis that represents the combined forces of the muscles producing the movement (mainly the gluteus medius and minimis and the adductor magnus), and the resistance point at the center of gravity of the trunk–arm–racket lever. At the moment of impact, however, the resistance point may be considered to be the point of contact of the ball with the racket face. The additional lever actions that are due to the rotation of the spine, the horizontal adduction at the shoulder, and flexion at the wrist, if present, should also be recognized.

Another important anatomical factor is the strength of the muscles responsible for keeping the arm abducted and for assisting in the forward swing as the arm is carried along by the rotating spine and pelvis. Those whose muscles lack the strength to swing the outstretched arm with speed are likely to flex the arm at the elbow or adduct the arm, bringing the racket closer to the trunk and thus shortening the resistance moment arm. Among the muscles that were tested by Van Gheluwe and Hebbelinck (1986) and found active in the forward swing of the forehand drive were the anterior deltoid, pectoralis major, latissimus dorsi, biceps, brachioradialis, and triceps, the latter in two short bursts, the first at the moment of impact, and the second during follow-through (Table 18.3). The erector spinae, external oblique, and rectus abdominis are also active, with the erector spinae most active (Knudson and Blackwell 2000).

Mechanical Analysis

The purpose of the forehand drive is to return the ball so that it will not only land within the opponent’s court but also will land in such a place and manner that it will be difficult to return. For the player to achieve this requires both high speed and expert placement of the ball. Hence, imparting maximum speed to the ball and, at the same time, placing it with accuracy are the two major skills that the player seeks to develop.

The force of impact is determined by the speed of the racket at the moment of contact with the ball, and maximum velocity can be obtained only when maximum distance is used for accelerating. The function of the backswing is to provide this distance. There are two types of backswing, the straight and the circular. The straight backswing has the advantage of greater ease in controlling the direction of the racket and in timing the movement but the disadvantage of necessitating the overcoming of inertia to reverse the direction from the back to the forward swing. On the other hand, the circular backswing permits the arm to move in one continuous motion over a longer path, thereby providing more than twice the distance for building up momentum. For the more skillful player who is able to control both the direction of the racket and the timing of the entire movement, it is the more efficient method.

In considering the force involved in the forehand drive, it is important to distinguish between the force applied to the lever and the force applied by the lever to the ball. Whereas the force applied to the lever is muscular force, the force applied to the ball is the force of momentum. It is determined by both the mass and velocity of the implement that makes contact with the ball. These, in turn, are related to the distance of the point of contact from the fulcrum—in other words, the length of the temporary resistance arm of the lever (“temporary” because the distance from the fulcrum to the point of contact with the ball constitutes the resistance arm of the lever only for the brief moment of impact). In addition to the rotary movement of the trunk–arm–racket lever, the linear motion produced by the forward movement of the body (because of weight shift) adds to the force that meets the ball.

Starting with the pelvic rotation and weight shift and working out toward the racket, each movement in turn gets under way before the next one commences. If the timing is correct, the cumulative effect of these movements is to produce maximum velocity. If any of the movements is added to the preceding one either too early or too late, the potential velocity will not be realized.

Other important factors that contribute to the force applied to the ball, and therefore to the speed of the ball on its return flight, include the following:

• 1. The use of the arm in an almost fully extended position increases the length of the lever, thereby giving greater linear velocity to the racket head than would be the case if the upper arm were close to the body. This is only true, however, if the racket can be moved with the same angular velocity in both positions.
• 2. The effort needed to resist the force of the ball hitting the racket is less when the racket lever arm is shortened.
• 3. It takes less time to swing a shortened racket lever into the striking position than it does a fully extended one. (These first three related factors help explain why beginners and children bend the forehand elbow or choke up on the racket handle.)
• 4. The concentration of mass at the level of the shoulders moving forward at the moment of impact ensures maximum speed for striking.
• 5. A skillful player tends to use a relatively heavy racket because, other things being equal, the greater the mass of the striking implement, the greater the striking force (momentum) and, hence, the greater the speed of the struck ball.
• 6. A new ball and a well-strung racket ensure a good coefficient of restitution (elasticity), thereby increasing the speed of the struck ball.
• 7. The bending of the knees at the beginning of the forward swing followed by the extension of the legs and shift of weight as the racket is swung forward and up increases the ground reactive force imparted to the body and thus to the ball.
• 8. It has been generally accepted that a firm wrist and grip are essential for maximum impulse to be applied by the racket to the ball.
• 9. Placement of the ball is a matter of direction. It will be recalled that the direction taken by a struck ball is determined by four factors:
  • a. The direction of the striking implement at the moment of impact.
  • b. The relation of the striking force to the ball’s center of gravity—in other words, the control of spin.
  • c. Firmness of grip and wrist at the moment of impact.
  • d. Angle of incidence.

The first of these factors is obvious. The beginner may be less aware of the importance of the other three. For successful placing of the ball, an understanding of the effect of spin and the skill of imparting the desired spin to the ball are essential. Firmness of grip depends on wrist and finger strength and is closely related to the angle at which the racket face makes contact with the ball. Because the angle of rebound equals the angle of incidence (actually slightly less than this in the case of tennis balls because of their compressibility), it will be seen that firmness of grip is therefore an important factor in the direction taken by the struck ball.

• 1. Throw a tennis ball or baseball for distance:
  • a. Standing still, facing in the direction of the throw.
  • b. Standing with the left side toward the direction of the throw, with the feet apart and the weight evenly distributed, using a full arm swing and body twist with the throw.
  • c. Same as in b, except with the weight on the right foot to begin with, shifting to the left as the ball is thrown.
  • Compare the three methods for distance. Explain in terms of length of backswing, speed at moment of release, and total distance used in applying force to ball before releasing it.
• 2. If possible, observe a small child or an untrained youth and then a trained boy or girl throw a small ball as forcefully as possible at a target 20 or 30 feet away. Analyze the motions of each with reference to the pathway of the hand immediately preceding, at the moment of, and following the release. Explain the factors differentiating the good throws from the poor.
• 3. Observe slow-motion films of throwing, striking, and other forms of giving motion. Look for the application of the principles stated in this unit or for the lack of such application.
• 4. Perform a complete kinesiological analysis of a selected throwing, kicking, or striking skill, taking particular note of the purpose of the motion, its classification, the simultaneous–sequential nature of the motion, and its component parts. 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.