Arm Deceleration

The arm-deceleration phase, which only lasts a few hun-dredths of a second, begins at ball release and ends when the shoulder has reached its maximum internal rotation (Fig. 2.1I,J). An eccentric elbow flexion torque of approximately 10 to 35 N-m is produced throughout the arm-deceleration phase to decelerate elbow extension (Fig. 2.2).20,24 Moderate to high eccentric contractions of the elbow flexors have been reported during arm decel-eration.24,26,28 Researchers also have shown that the pronator teres is very active in decelerating elbow extension and pronating the forearm.28,40 The biceps brachii and supinator muscles are responsible for controlling forearm pronation.

A maximum elbow compressive force of 800 to 1000 N occurs just after ball release to prevent elbow distraction (Fig. 2.4).20 This compressive force is greatest when throwing fastball or slider pitches (Table 2.2).31 Compressive forces also have been shown to increase as skill level increases.41 Elbow flexors produce a compressive force, as well as terminate elbow extension, before the olecranon impinges in the olecranon fossa.24 Elbow extension terminates when the elbow is flexed approximately 200.24,25

the olecranon fossa. This impingement leads to osteophyte production at the posterior and posteromedial aspect of the olecranon tip and can cause chondromalacia and loose body formation.23 Figure 2.2 shows that substantial varus torque is generated throughout the arm-cocking and arm-acceleration phases in order to resist valgus torque. During these phases, the elbow extends through a range of approximately 65° (from approximately 85° to approximately 20°).20 This combination of elbow extension and resistance to valgus torque supports

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