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The muscles that were stretched were the Latissimus Dorsi, Teres Major, Pectoralis Major and Serratus Anterior muscles and these can be seen more specifically in Figure 4-37.
Now, as the swing develops, the right elbow drops down and tucks in tight against the body making the shape of the letter “L” seen in Figure 4-38. This action is driven by the Latissimus Dorsi and Teres Major muscles which were previously stretched and seen above in Figure 4-37.
The right elbow also supinates in the process which forces the palm of the hand to start turning upward. This is caused by the elbow supinator muscles or more specifically, the Biceps and the Supinator. These muscles can be seen more clearly in Figure 4-39.
As a result of these two actions, the knob end of the bat points forward. See Figure 4-40.
Now, as the body continues it’s rotation toward the pitcher, the right arm applies a short but hard, push forward (Figure 4-41a) which brings the bat-head around (Figure 4-41b) near the contact point with the ball (Figure 4-41c).
This pushing motion of the right arm is caused primarily by the Pectoralis major and Serratus Anterior muscles with help from the Anterior Deltoid and Coracobrahialis muscles. See Figure 4-42.
The elbow also starts to extend in the process, though not completely, just like the left arm and this can be seen in Figure 4-43.
This movement is caused by the elbow extensor muscles, which are the Triceps and Anconeus muscles, and can be seen again in Figure 4-44.
In the next section we will talk about the fifth and last noticeable movement during the launching phase, which is the guiding action of the hands on the bat, and identify the muscles involved with it.
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