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Eight active and former collegiate softball pitchers were studied with
ASMI's motion analysis system and force plates. To simplify the interpretation
of data, the pitching motion was separated into four phases: windup, stride,
delivery and follow-through (Figure 4). The wind up phase was defined
as the time of initial movement until lead foot toe off (Figure 4a-c).
During the windup phase, the arm was hyperextended at the shoulder. The
stride phase was defined as the time from toe off to complete contact
of the lead foot with the ground Figure 4d-f). During this phase forward
momentum was generated and the trunk (pelvis, upper torso) was rotated
away from home plate to position the body for delivery. The delivery phase
was defined as the time from lead foot contact to release of the ball
(Figure 4g-j). During this phase the ball was accelerated forward with
a combination of trunk (pelvis and upper torso) rotation and arm (flexion
and internal) rotation. The final phase was the follow-through which occurred
from the instant of ball release until forward motion of the throwing
arm stopped (Figure 4k-l). During follow-through, the elbow continued
to flex as the arm and forearm were decelerated. Compared to baseball
pitching, softball pitching had a significantly longer stride from the
rubber (110 %height vs. 84 %height) since the softball pitchers (illegally)
jump forward from the rubber before throwing. While softball pitchers
generated less internal rotation speed that baseball pitchers (4600°/s
vs. 7600°/s), softball pitchers also generated great shoulder flexion
(5300°/s). Large magnitudes of superior force (98 %weight) and extension
torque (9.8 %weight*height) were produced during the delivery phase to
stabilize the shoulder. These magnitudes are considerable and imply that
underhand pitching is stressful on the arm.
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