Elsevier

Journal of Biomechanics

Volume 34, Issue 12, December 2001, Pages 1643-1647
Journal of Biomechanics

Technical note
Shoulder joint movement of the non-throwing arm during baseball pitch—comparison between skilled and unskilled pitchers

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Abstract

The shoulder of a non-throwing arm during a baseball pitch must be in a constant position while the shoulder of the throwing arm moves in a nearly circular path around it. However, it has not been investigated whether a skilled pitch requires less shoulder-joint movement. It was hypothesized that pitchers with less shoulder movement of the non-throwing arm can be considered to have higher skill and to attain higher initial ball velocity. Nine baseball pitchers were used as subjects. The coach classified them into a skilled and an unskilled group. The pitching motions were recorded using two high-speed cameras. The time series of three-dimensional landmark coordinates of the shoulder joint of the non-throwing arm during the baseball pitch were calculated using the direct linear transformation method. The shoulder-joint movement (SJM) index, which expresses the movement (displacement) of the shoulder joint of the non-throwing arm quantitatively, was proposed to compare the SJM at different skill levels and investigate the relationship between SJM and initial ball velocity. The SJM of the skilled pitchers was smaller than that of the unskilled pitchers, and the smaller value of the SJM led to faster initial ball velocity. The data suggest that the less SJM of the non-throwing arm is required to attain a skilled pitch and higher initial ball velocity.

Introduction

Less kinematic differences are desirable to attain an effective pitch (Barrentine et al., 1998; Escamilla et al., 1998). For several phases of throwing (stride foot contact, arm cocking, arm acceleration, and ball release), Fleisig et al. (1999) found kinematic and kinetic parameters of baseball pitching that reached statistically significant differences among competition levels and suggested that they might be used to learn proper pitching mechanics and protect a pitcher from potential injuries. The shoulder of the non-throwing arm stayed in an almost steady position while the shoulder on the throwing arm moved in a nearly circular path around it (Feltner, 1989). This observation shows that the shoulder of a non-throwing arm is important as a pivot or fulcrum of the trunk or throwing arm to rotate about.

On the basis of this observation, the movement of the shoulder joint of the non-throwing arm may be important to attain a skilled pitch and higher initial ball velocity. However, the observation has not been demonstrated quantitatively. We know of no studies that report on the kinematics of the shoulder of the non-throwing arm, compare it between skilled and unskilled pitchers, and relate it with initial ball velocity. The purpose of this study was to verify the hypothesis that a pitcher with less movement (displacement) of the shoulder of the non-throwing arm can attain higher ball velocities. We also attempted to examine whether the shoulder-joint movement (SJM) of the non-throwing arm was less for the skilled than for the unskilled pitchers.

Section snippets

Methods

Six baseball pitchers playing for a business corporation and three high-school baseball pitchers, ranging in age from 18 to 25 were used as subjects (Table 1). Their mean height (±S.D.) and mean weight (±S.D.) were 175.6±8.9 cm and 72.5±10.3 kg, respectively. They were all in good health and did not have injuries. They were not recovering from an injury either, and they had not undergone surgery in the recent past. Each subject had at least three years’ experience throwing two types of pitches

Results

The mean initial ball velocity of fastball pitches for the skilled group was faster than that for the unskilled group (Table 2). The mean initial ball velocity of curveball pitches did not differ between the skilled and unskilled group (Table 2). These data validate the classification of all subjects into the skilled and unskilled groups. The mean SJM, SJMx, and SMJy tended to be higher for the unskilled pitchers than for the skilled pitchers (Table 3). It was verified that the movement of the

Discussion

A larger torque produces faster motion of the throwing arm and consequently leads to faster initial ball velocity (Feltner and Dapena, 1986). The shoulder of the non-throwing arm was in a roughly constant position while the shoulder of the throwing arm moved in a nearly circular path around it (Feltner, 1989). The shoulder of the non-throwing arm acts as a pivot point or fulcrum for the upper trunk and throwing arm to rotate about. Therefore, the role of the shoulder of the non-throwing arm may

Acknowledgements

The author would like to thank Hirokazu IWASE for his assistance.

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