Arm Cocking

The arm-cocking phase, which lasts from 0.10 to 0.15 second, begins at lead foot contact and ends at maximum shoulder external rotation (Fig. 2.1E-H). "Arm cocking" is a more accurate description of this phase than "cocking," because only the arm is cocked during this entire phase.24 Some parts of the body, such as the pelvis and lower extremities, accelerate or decelerate during this phase. Shortly after the arm-cocking phase begins, the pelvis and upper torso rotate to face the batter.

Elbow joint forces and torques are generated throughout the arm-cocking phase. A low to moderate flexion torque of 0 to 32 N-m is produced at the elbow throughout the arm-cocking phase (Fig. 2.2).20 Consequently, the elbow flexors demonstrate low to moderate activity, primarily during the middle third of the arm-cocking phase.24,26,28

The forearm produces a large valgus torque onto the upper arm at the elbow; the pelvis and upper torso rotation and rapid shoulder external rotation contribute, in part, to this valgus torque. To resist the valgus torque, the upper arm generates a maximum varus torque ranging from 52 to 76 N-m (mean, 64 N-m) onto the forearm shortly before maximum shoulder external rotation (Fig. 2.3).20 The flexor and pronator muscle mass of the forearm displays moderate to high activity, which helps to contribute to varus torque (Table 2.1).28 Because these muscles originate at the medial epicondyle, they contract to help to stabilize the elbow. Large tensile forces on the medial aspect of the elbow result from the valgus torque placed on the arm. Repetitive valgus loading eventually may lead to injury to the ulnar collateral ligament (UCL). Furthermore, inflammation of the medial epicondyle or adjacent tissues may occur (i.e., medial epicondylitis).

As indicated, an in vitro study by Morrey and An showed that the UCL contributes approximately 54% of the resistance to valgus loading in the flexed arm posi-

FC MER REL MIR

FIGURE 2.2. Torques applied to the forearm at the elbow in the flexion (F) and varus (V) directions. The instants of foot contact (FC), maximum external rotation (MER), ball release (REL), and maximum internal rotation (MIR) are shown. (Adapted from Fleisig et al.20)

FC MER REL MIR

FIGURE 2.2. Torques applied to the forearm at the elbow in the flexion (F) and varus (V) directions. The instants of foot contact (FC), maximum external rotation (MER), ball release (REL), and maximum internal rotation (MIR) are shown. (Adapted from Fleisig et al.20)

Maximum Voluntary Torque Elbow Flexors

FIGURE 2.3. Shortly before maximum external rotation is achieved, the first critical instant occurs. At this instant, the arm is externally rotated to 165°, and the elbow is flexed to 95°. Among the loads generated at this time are 64 N • m of varus torque at the elbow and 67 N • m of internal rotation torque and 310 N of anterior force at the shoulder. (Adapted from Fleisig et al.20)

FIGURE 2.3. Shortly before maximum external rotation is achieved, the first critical instant occurs. At this instant, the arm is externally rotated to 165°, and the elbow is flexed to 95°. Among the loads generated at this time are 64 N • m of varus torque at the elbow and 67 N • m of internal rotation torque and 310 N of anterior force at the shoulder. (Adapted from Fleisig et al.20)

tion.11 Assuming that the UCL produces 54% of the 52-to 76-N-m maximum varus torque that an elite pitcher generates, the UCL provides approximately 30 to 40 N-m of varus torque. This is similar to the 32-N-m failure load that Dillman et al. reported; thus, during baseball pitching, the UCL appears to be loaded near its maximum capacity.12'20 However, this result is only an approximation of the UCL's contribution in throwing because the cadaveric research does not account for muscle contributions. Muscle contraction during this phase may reduce the stress seen on the UCL by compressing the joint and adding stability.24

Valgus torque also can cause high compressive forces on the lateral elbow, which can lead to lateral elbow compression injury.20 Specifically, valgus torque can cause compression between the radial head and humeral capitel-lum.30 According to the in vitro study by Morrey and An, joint articulation supplies 33% of the varus torque needed to resist the valgus torque that the forearm applies.11 Thirty-three percent of the 52- to 76-N-m maximum varus torque generated during pitching is 17 to 25 N-m. If the distance from the axis of valgus rotation to the compression point between the radial head and the humeral capitellum is approximately 4 cm, then the compressive force generated between the radius and humerus to produce 17 to 25 N- m of varus torque is approximately 425 to 625 N-m.20 Muscle contraction about the elbow or loss of joint integrity on the medial side of the elbow can cause this compressive force to increase. Excessive or repetitive compressive force can result in avascular necrosis, osteochondritis dissecans, or osteochondral chip fractures of the radiocapitellar joint.30

In addition to a varus torque, the upper arm applies a maximum 240 to 360 N of medial force onto the forearm to resist lateral translation of the forearm at the elbow (Fig. 2.4). This force is significantly greater during a fastball or curveball pitch than during a change-up or slider pitch (Table 2.2).31 The greater medial force during arm cocking in the curveball pitch compared with other off-speed pitches (e.g., change-up and slider) may be related to medial elbow injuries. Further research is needed to address this issue. The forearm is supinated more during the arm-cocking phase for a curveball pitch than for a fastball pitch, which also may be related to elbow injuries.32,33

Other forces also are produced at the elbow during arm cocking. The upper arm applies a maximum anterior force of 80 to 240 N onto the forearm to resist posterior translation of the forearm at the elbow.20 Similarly, the upper arm applies a maximum compressive force of 150 to 390 N to the forearm to resist elbow distraction.20

The elbow achieves a maximum flexion of 85° to 105° approximately 30 ms before maximum shoulder external rotation (Fig. 2.5).20,24 The triceps muscle appears to control maximum elbow flexion, which shows moderate activity during the last third of the arm-cocking phase.24,28 This hypothesis is supported by Roberts's data, which

Direction Curveball Rotation
FIGURE 2.4. Forces applied onto the forearm at the elbow in the medial (M), anterior (A), and compressive (C) directions. The instants of foot contact (FC), maximum external rotation (MER), ball release (REL), and maximum internal rotation (MIR) are shown. (Adapted from Fleisig et al.20)

TABLE 2.2. Comparison of elbow biomechanics with different pitches

Fastball

Curveball

Change-up

Slider

Arm cocking

Elbow medial force (N) 280 280 240 240

Arm acceleration Elbow extension velocity (degrees/second) 2400 2400 2100 2500

Arm deceleration

Elbow compressive force (N) 790 730 640 780

Adapted from Escamilla et al.31 and Escamilla et al.3

show that if a radial nerve block paralyzes the triceps muscle the elbow "collapses" and continues flexing near its limit (approximately 145° of elbow flexion).34 This collapse is caused by a centripetal flexion torque at the elbow that the rapidly rotating upper torso and arm create. The triceps muscle apparently contracts eccentrically and then isometrically in resisting the centripetal elbow flexion torque that occurs during late arm cocking. At approximately the time that the elbow reaches maximum elbow flexion (i.e., approximately 30 ms before maximum shoulder external rotation), the elbow flexors become inactive, and the triceps contracts concentrically to aid in elbow extension.20,24 Figure 2.5 shows the interactions among muscle activity, elbow joint torque, and elbow extension.

Cure Tennis Elbow Without Surgery

Cure Tennis Elbow Without Surgery

Everything you wanted to know about. How To Cure Tennis Elbow. Are you an athlete who suffers from tennis elbow? Contrary to popular opinion, most people who suffer from tennis elbow do not even play tennis. They get this condition, which is a torn tendon in the elbow, from the strain of using the same motions with the arm, repeatedly. If you have tennis elbow, you understand how the pain can disrupt your day.

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