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The Bio Mechanics of Errors

Errors displayed by performers of different ability levels may be identified and placed into one of three categories: segmental movement errors, timing and sequencing errors, and errors owing to lack of strength or power.

Segmental Movement Errors

The position of body segments during a performance often provides a clue to performance errors.

A beginner attempting to perform a kinetic link activity often positions a more distal segment ahead of a more proximal segment. A common example is that of the location of the wrist relative to the elbow. While the wrist is noticeable ahead of the elbow, the beginner may hold the racket or the projectile back by hyperextending the wrist joint.

A second common and easily identified segmental position error, that is frequently in the beginning performer involves the location of the elbow joint relative to a line connecting the shoulders. The elbow is positioned ahead of the shoulders prior to contact or release.

The transverse adduction of the shoulder joint prior to release places the elbow ahead of the shoulder line. In cases of gross errors, an entire throw or strike patter begins and ends with elbow extension from an elbow located directly in front of the same shoulder joint. This error is common to most beginning or immature throwing and striking skills.

Sequencing and Timing errors

Segmental movements are difficult to distinguish from one another without the aid of a video or movie film. The segmental sequencing in any skill that is governed by the kinetic link concept should proceed from proximal to distal, more massive to less massive, fixed end to free end. The initial forward movement of a distal segment should occur at the point of maximum velocity of the preceding segment.

Although seeing velocities and accelerations is impossible a teacher should recognize where in a pattern a proximal segment contributes its acceleration and when the next distal segment motion should begin. As a general rule, the proximal segment uses about one half to two thirds of its range of motion (ROM) to reach its peak velocity.

A second comparator that may be used is a mental image of whiplike timing. Often the beginner moves segments in distinct blocks rather than overlapping each with the adjacent segments. This pattern resembles a pushing pattern in that the segments move simultaneously rather than sequentially.

At the novice level, insufficient relaxation of the muscle groups antagonistic to the desired movement can inhibit the free rotation of one segment relative to another. As the distal links are used, the internal muscular torques that acts to bring those distal segments forward may be ill timed. If adequate lag back of distal segments is not noticeable, the distal segments may not be in a position to elicit the stretch reflex in the muscle tissues nor to make use of the muscles elastic properties.

Sometimes having the player exaggerate each segmental in proper order gives the performer the idea of the proper sequencing. The exaggeration of each segmental movement is particularly helpful to beginners who are having trouble focusing their attention on individual segmental movements.

A second technique is to hold back the contact point, hand, racket, bat or club while the performer attempts to finish the skill. While the endpoint is held the proximal segments move out from under the distal segments. The performer should feel the sequential nature of the skill.

The novice performer typically displays a more simultaneous pattern, with the contact or release point ill timed, too early or too late.) Improper timing not only reduces the projection velocity but also alters the direction of the projectile. On the other hand, and intermediate performer tends to display erratic timing of the segments. Some segmental movements may be initialed too early, some too late, and some just at the right instant.

At the intermediate level of performance the critical analysis of the teacher or coach is necessary for identifying errors. The coach should attempt to focus on the timing of one or two segments at a time.

Errors owing to lack of Power

The skill, strength or power of the performer may alter the location of any skill on the continuum from that of the ideal. A person who lacks the necessary power to sequentially accelerate an object with the distal links carries the object closer to the mid-line of the body, so that a simultaneous pushing motion is observed.

The effect of the size and weight of the equipment on throw-like patterns is observed in a tennis player who does not have the strength to hold the racket in a neutral position at the wrist. Eventually, the weight of the racket causes the wrist to fall into an ulnar-flexed position. In addition, to reduce the inertial resistance of the racket and the torque of the impact, the performer adducts the shoulder joint by pulling the elbow into the side, thus increasing elbow flexion. The increased elbow flexion is an attempt to compensate for the racket head being lowered vertically due to the ulnar-flexed position of the wrist.

The effects of the size and mass of an object or implement on the throw stroke patter has several implications for the selection of equipment for people differing in sternest and abilities. If the player is to demonstrate a mature patter with all of its segmental movements and timing requirements, then the equipment may have to be made smaller or less massive or both.

The stages of throwing reflect an increased use of sequential segmental rotations as the mature patterns develops.

Stage 1. The elbow is located forward of the shoulder joint. The ball is thrown primarily with elbow extension. No rotation of the trunk is visible.

Stage 2. A trunk rotation to the right (RH) accompanies the backward motion of the arm. The throw is initiated by the arm swing forward accompanied by trunk rotation left. The elbow extends at variable times during the forward arm swing.

Stage 3 A step is taken with the ipsilateral (Same side) food. The step is followed by trunk rotation and the forward arm swing. The elbow extension occurs later than in stage 2.

Stage 4. A step is taken with the contralateral foot, the trunk rotates to the left, a transversadduction of the arm and elbow extension ends the force phase.

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