Dr. Kevin Yip

Dr Kevin Yip
Orthopaedic Surgeon
MBBS(UK), FRCS(EDIN), FAM(SING), FHKCOS(ORTHO)

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Throwing

Throwing injuries are of increasing importance in both professional and amateur sports. They affect adults and children alike. Sports prone to throwing injuries include baseball, American football, tennis and other
racket sports, javelin throwing, team handball, cricket, and sports with other overhead motions such as swimming and volleyball.

Throwing mechanism

The basic principle in throwing is the use of a kinetic chain to generate and transfer energy from the larger body parts (legs and trunk) to a smaller (more injury-prone) upper extremity. This kinetic chain in throwing includes the following sequence of motions: stride, pelvis rotation, upper torso rotation, elbow extension, shoulder internal rotation and wrist flexion. This biomechanical motion is a combination of true glenohumeral rotation, trunk hyperextension, and shoulder blade motion against the rib cage. For example, when a ball is released, there has been significant energy and momentum transferred to the throwing arm and ball. After the ball is released, the kinetic chain must decelerate the moving arm.
The throwing mechanism (here using baseball pitching as an example) can be divided into six phases: wind-up, stride, arm cocking, arm acceleration, arm deceleration with ball release, and follow-through.

Wind-up

During wind-up, the shoulder is hyperextended, externally rotated and abducted, and the elbow is flexed to an angle of about 45°. At this stage, the anterior structures of the shoulder are under mild tension. The objective of this phase is to put the thrower in a good starting position.

Stride

In this phase the thrower turns the lead side towards the target. The stance foot is planted, the arm abducts, and the athlete stretches the body. Forward movement is initiated by hip abduction of the stance leg, followed by knee and hip extension. During the stride, the throwing shoulder externally rotates and horizontally abducts. Wrist and finger extensors have very high activity during the stride phase. The stride phase ends when the lead foot contacts the ground.

Arm cocking

Arm cocking begins as the lead foot contacts the ground and ends at maximum shoulder external rotation. The shoulder girdle muscles stabilize the shoulder blade and position the shoulder socket (glenoid). The rotator cuff and other shoulder muscles are also active in stabilizing the glenohumeral joint. The shoulder remains abducted at approximately 90° throughout the cocking phase. At the end of the cocking phase, the shoulder is externally rotated 150–180°. Competitive throwing can stretch out the shoulder capsule and increase shoulder flexibility. A competitive pitcher will often have 10–15° more external rotation in the throwing shoulder than the nonthrowing shoulder.

Arm acceleration

Arm acceleration is the explosive phase when the trunk flexes forward to a neutral position and the ball is released. The throwing shoulder remains abducted at approximately 90° during this phase. The shoulder internal rotator muscles contract to produce maximal internal rotation velocity. The rotator cuff muscles are highly active to stabilize the glenohumeral joint.

Arm deceleration

Arm deceleration is a short phase that begins at ball release. During this phase the shoulder maximally rotates internally to approximately 0°. A shoulder posterior force is produced to resist anterior humeral translation. The teres minor muscle has the highest activity of all the glenohumeral muscles during this phase.

The acceleration and deceleration phases can cause injury to the internal rotator muscles of the shoulder. In adolescents, an injury to the proximal humeral growth zone (epiphysis) can occur. This causes widening and absorption of the epiphysis of the proximal humerus that is similar to a stress fracture.

Follow-through

Follow-through begins at maximal shoulder internal rotation and ends when the arm completes its movement across the body and the athlete is in a balanced position. The kinetic chain during this motion helps to reduce the stress placed on the throwing arm by transferring the body weight and momentum to the lead leg. The posterior shoulder muscles continue to be active throughout the follow-through. The serratus anterior is the most active scapular stabilizer during this phase. In addition, during follow-through the forearm is rotated into pronation. This may cause rotational and shearing forces on the outside (lateral) side of the elbow, as well as compression on the back (posterior) of the elbow.

Treatment

Overuse injuries from excess throwing are common. An athlete who begins to develop signs of overuse from repeated overhead activities must limit the activity to avoid pain and possible further injury. Most of these problems will resolve with rest and a graduated return to activity.

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