The glenohumeral joint (shoulder) is the primary joint of focus during the baseball pitching motion and is the location of many overuse injuries. Thus, much of the literature has focused on the multiple quandaries that arise about the shoulder from baseball pitching (15,21,26,27). The biomechanics of the baseball pitch is a total body motion. It is of particular interest to perform biomechanical analysis of joint motion in not only the shoulder but also in the preceding joints in the body. This would serve as a foundation for greater understanding of the baseball pitch.
The kinematics and kinetics of the baseball pitching motion have been well documented (4-8,13,25). Baseball studies in the literature have primarily focused on mechanics and typical pathomechanics of the pitching motion. However, to the investigators' knowledge, there have been no studies that have examined a particular proximal aspect, such as the trunk, of the movement and its relationship to a more distal point, such as the shoulder, in the movement. Putnam (20) has described that the appropriate timing and transfer of momentum through the larger more proximal segments (i.e., the trunk) have to be summated by the speed principle. Throughout the pitching motion, the more distal segment (lower arm) initiates its movement when the adjacent proximal segment (upper arm) reaches its maximum angular velocity. When examining activities such as kicking, jumping, and the tennis serve, the trunk segment has been shown to have the largest contribution to the body's total angular momentum (3,4,19,20).
It has been reported that the trunk contributes as much as 50% of the kinetic energy and force production during the entire throwing motion (24). Failure of the kinetic chain could potentially be a detriment to not only performance but also exacerbate the propensity for injury. Many researchers have discussed the importance of the kinetic chain (3,4,14-16,19,20,28); however, none have examined the relationship of a proximal segment (such as the trunk) to a more distal segment (such as the shoulder) throughout the pitching cycle. Thus, it was the purpose of this paper to examine the kinematics of the trunk and pelvis and determine their relationship to the kinematics of the shoulder. We hypothesized that trunk and pelvic kinematics would have a positive relationship with shoulder kinematics during the baseball pitch of high-school pitchers.
Experimental Approach to the Problem
A single group, repeated-measures design was used to collect data describing pelvis, torso, and shoulder kinematics throughout the stride, arm-cocking, arm acceleration, and arm deceleration phases of the pitching motion. Data describing the kinematics of the movement were collected at the points of foot contact, maximum shoulder external rotation, ball release, and maximum shoulder internal rotation. The data in the current study were collected in a manner such that subjects threw a series of maximal effort fastballs to a catcher located the regulation distance from the pitching mound, and those data from the fastest pitch passing through the strike zone were analyzed (11,22). After test trials, kinematic data were analyzed using a series of descriptive statistics to identify outliers and determine the nature of the distribution before testing for the presence of relationships between the various parameters.
Once the data were determined to be distributed normally, testing for relationships was conducted by calculating the Pearson product moment correlation coefficients to examine the relationships between both pelvis and torso kinematics, and shoulder kinematics. In the current design, data describing pelvis and torso kinematics were the independent variables, whereas the kinematic data describing movements at and about the shoulder were the dependent variables. These variable assignments are consistent with the opinion that the pitching motion functions as a product of the kinematic chain and that alterations in the movements of proximal segments may result in alterations in the movements of distal segments.
Twelve high-school male pitchers (age: 16.3 ± 1.1 years; height: 176.6 ± 7.8 cm; and mass: 76.4 ± 7.4 kg) regardless of throwing arm dominance volunteered to participate in the current study. All subjects had recently completed their competitive spring baseball seasons and were thus deemed appropriately conditioned for competition. Additional criterion for participant selection included recommendation of their respective coaching staff, multiple years (up through the current season) of pitching experience, and freedom from injury throughout their recently completed competitive baseball season.
Data collection sessions were conducted indoors at the University of Arkansas Health, Physical Education, and Recreation building and were designed to best simulate a competitive setting. All testing protocols used in the current study were approved by the University of Arkansas Institutional Review Board. Before participation, the approved procedures, risks, and benefits were explained to each participant and their parents who then signed the appropriate paperwork to provide consent for testing.
Subjects reported for testing before engaging in resistance training or any vigorous activity that day. Once on site, subjects were prepared so that kinematic data could be collected using The MotionMonitor™ electromagnetic tracking system (Innovative Sports Training, Chicago IL). Subjects had a series of electromagnetic sensors attached to the medial aspect of the torso and pelvis at the C7 and S1 locations, respectively (17), and the distal/posterior aspect of the throwing humerus (Figure 1). Sensors were affixed using double-sided tape and then wrapped using flexible hypoallergenic athletic tape. After the attachment of the electromagnetic sensors, a fourth sensor was attached to a stylus and used to digitize the palpated position of various bony landmarks (18). To accurately digitize the selected bony landmarks, subjects stood in the neutral anatomical position while digitization was being completed.
Throwing kinematics for right-handed subjects were calculated using the standards and conventions for reporting joint motion recommended by the International Shoulder Group of the International Society of Biomechanics (29,30). Briefly, raw data regarding sensor orientation and position were transformed to locally based coordinate systems for each of the respective body segments. Euler angle decomposition sequences were used to describe both the position and orientation of the both the pelvis and trunk relative to the global coordinate system (29,30). The use of these rotational sequences allowed the data to be described in a manner that most closely represented the clinical definitions for the movements reported (18). Angle decomposition sequencing for the pelvis and torso, and definitions of the movements they describe are shown in Table 1. Throwing kinematics for left-handed subjects were calculated using the same conventions; however, it was necessary to mirror the world Z-axis so that all movements could be calculated, analyzed, and described from a right-hand point of view (29,30).
Once all initial setup and pretesting had been completed, subjects were allotted an unlimited time to perform their own specified precompetition warm-up routine. During this time, subjects were asked to spend a small portion of their warm-up throwing from the indoor pitching mound to be used during the test trials. After completing their warm-up and gaining familiarity with the pitching surface, each participant threw a series of maximal effort fastballs for strikes toward a catcher located the regulation distance from the pitching mound (18.44 m). For the current study, those data from the fastest pitch passing through the strike zone were selected for detailed analysis (11,22).
Data analysis for the current study was conducted using the statistical analysis package SPSS 11.5 for Windows (SPSS, Chicago, IL, USA). For the fastest strike thrown by each participant, mean and SD for all kinematic parameters were calculated. After the calculation of the central tendency measures was completed, a series of descriptive statistics were employed to identify the nature of the distribution for each parameter. Once the data were deemed to be normally distributed through the calculation of Shapiro-Wilk statistic (W-statistic, p > 0.05), Pearson product moment correlation coefficients were calculated to identify the relationships between both pelvis and torso kinematics and shoulder kinematics. For the current study, although the data were derived from the same group of subjects at multiple intervals, the level of significance was retained at p ≤ 0.05 as each phase of the pitching motion was analyzed as an independent interval. With regard to the observation of correlation power for the current study, significance was indicated by an observed power ≥0.80.
Pelvis and Trunk Kinematics
Results of pelvis, torso, and shoulder kinematics are displayed in Table 2. Throughout the pitching motion, both the pelvis and the torso remained tilted laterally toward the glove hand and rotated forward toward the plate with the pelvis rotating just ahead of the torso. In addition, both peak axial rotation velocity and minimum axial rotation velocity for the pelvis preceded those of the torso.
Throughout the pitching motion, the throwing shoulder was abducted to an angle near 100° ± 17 during the stride and cocking phases, and remained there through the duration of the movement. It also progressed through a long arc of horizontal abduction (plane of elevation), moving from an angle of −38° ± 13 at foot contact, to an angle of 44° ± 13 at maximum internal rotation. Finally, throughout the stride and cocking phases, the throwing shoulder was externally rotated an average of −82° ± 16 before being internally rotated throughout the remainder of the pitching motion.
Results of the Pearson product moment correlation analyses are shown in Table 3 (trunk kinematics/shoulder kinematics) and Table 4 (dependent variable interclass correlation coefficients). These results indicate that for several parameters, the actions at and about the shoulder are strongly related to the actions of the pelvis and torso throughout the pitching motion. Most of the relationships observed between the trunk and the shoulder were inverse in nature. In fact, only the relationships between the angle of lateral tilt for both the pelvis and torso and the angle of shoulder external rotation were positive.
The results of the current study indicate that although pelvis and torso kinematics throughout the pitching motion are inversely related to both shoulder elevation and the plane of shoulder elevation, only the rate of axial torso rotation is significantly related to these shoulder parameters. More importantly, the rate of axial torso rotation is significantly related to these shoulder parameters in a way that may help explain the high rate of shoulder injury in high-school pitchers.
At the point of foot contact, a significant inverse relationship was observed between the rate of axial torso rotation and the plane of shoulder elevation. In this relationship, the rate of axial torso rotation was shown to explain 60% of the variance in the plane of shoulder elevation (r = −0.774; r2 = 0.599). Because it relates to overuse injury in high-school baseball pitchers, this relationship, and especially its timing, becomes very important. Three studies (1,10,22) have indicated that hyperangulation of the shoulder may be a product of both early and rapid trunk rotation during the stride phase. The results of this study support that notion and provide a quantitative measure of the relationship between these parameters.
In addition to this, another important relationship identified in the current study was the relationship between the rate of axial torso rotation and the angle of shoulder elevation at release. This relationship (also an inverse relationship) may help explain some of the increased shoulder kinetics commonly observed in high-school pitchers. Throughout the pitch cycle, the rate of axial torso rotation peaked before release and then began to decrease. This decrease may be related to the increase in the angle of shoulder elevation observed in the latter stages of the acceleration phase as the rate of axial torso rotation at release was shown to explain 67% of the variance in the angle of shoulder elevation at release (r = −0.821; r2 = 0.674). Previously, it has been shown that an increased angle of abduction of the shoulder may contribute to an increased elbow varus torque in pitchers (16). This increased varus torque could ultimately contribute to the high incidence of elbow injury now being observed in high-school baseball pitchers.
The findings from the present study indicate that the rate of torso rotation is strongly related to shoulder parameters in high-school baseball pitchers. If torso rotation influences what is happening at the shoulder, then more training focus should be on the torso. When we refer to this training of the torso, our focus is on the core musculature. Strength training should focus on developing a strong, stable core (that includes the gluteal musculature) in attempt to control the rate of torso rotation during the pitch. By participating in practice and competition with unconditioned core musculature, high-school baseball pitchers may exhibit a decrease in their ability to control of torso rotation. Subsequently, by increasing conditioning levels of the core musculature, these same pitchers may increase their ability to control the rate of torso rotation and ultimately decrease the likelihood of suffering injury related to an increased horizontal abduction and/or an increased angle of abduction. Previously, authors (2,9,10,23) have introduced practical core strengthening programs that could be advantageous to the high-school baseball pitcher. In addition, the findings to the current study have implications for further research addressing core control and determining if increased core strength may help prevent the typical overuse injuries that occur in the shoulder and are so often linked to uncontrolled torso rotation.
The authors would like to thank Bob Carver and his support in the throwing biomechanics research being conducted at the University of Arkansas, Priscilla Dwelly, Hiedi Hoffman, and Jackie Booker for their aid in data collection, and the coaches, the players, and their parents without whose participation, this study would not have been possible. No authors received financial support for this study.
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Keywords:© 2010 National Strength and Conditioning Association
torso rotation; pitching; motion analysis; core control