Relationships between Shoulder Injuries during Throwing Motions and the Range of Motions in Junior and Senior High School Baseball Players ()
1. Introduction
Throwing motions are performed by using every part of the human body. It has been reported that the flexibility of the whole body including the lower limbs and the trunk is important for preventing pitching-related injuries (Hamada et al., 2009; Garrison et al., 2012) . Shoulder injuries during throwing motions after epiphyseal closure often include symptoms of shoulder impingement syndrome (Morihara et al., 2013; Walch et al., 1992) , which is a typical shoulder joint injury caused by throwing motions. It has been reported that impingement between the rotator cuff and the articular lip result from excessive external rotation and horizontal abduction of the shoulder joint (Burkhart & Morgan, 1998) during the late cocking phase of throwing motions. Keep throwing after repeated impingement causes tears of the rotator cuff and damage to the articular lip. Therefore, early detection and prevention are required.
It has been considered that the range of motions could be one of the causes of joint impingement syndrome. Comparing the range of motion between non-injured athletes and injured athletes has suggested that limited internal rotation of the shoulder joint might cause injuries to the shoulder and elbow joints (Shanley et al., 2011) . Other studies have reported no correlation between internal and external rotation, or the horizontal adduction angle and injuries to the elbow (Sweitzer et al., 2012; Wilk et al., 2014) . To date, there is no agreement on this issue.
This study assessed the range of motions of the upper and the lower limbs in junior and senior high school baseball players to examine relationships between the range of motions and shoulder injuries caused during throwing motions. Moreover, the effects of factors other than the range of motions, such as height, weight, age, years of baseball experience, and the age of starting baseball were taken into consideration.
2. Methods
2.1. Participants
Junior high school baseball players (N = 134, 34 pitchers, and 100 fielders) and senior high school baseball players (N = 309, 97 pitchers, and 212 fielders) that participated in a medical examination participated in the survey in off-season. The number of participants differed depending on the type of measurement.
2.2. Evaluating Pitching-Related Injuries
The Hyper External Rotational Test (HERT; Meister et al., 2004 ) was conducted by orthopedists to diagnose pitching-related injuries. Participants were classified into the shoulder joint impingement syndrome group and a control group based on the diagnosis. The player with pain of lower extremities and the lumbago excluded it from subject.
2.3. Evaluation of Physical Functions
The external and internal shoulder joint rotation angles (shoulder joint abduction to 90 degrees and the elbow joint flexion to 90 degrees), the shoulder joint horizontal adduction angle, the external and internal the hip joint rotation angle (hip joint and knee joint flexion to 90 degrees), the heel buttock distance (HBD), and long seat type body anteflexion were used to assess the range of motions.
The assessment was conducted by four physical therapists, following the measurement methods adopted by the Japanese Association of Rehabilitation Medicine (The Japanese Orthopaedic Association Disability Committee, 1974) as well as the Kibler’s method (Kibler et al., 1998) . Among the four physical therapists, one fixed the body of a participant, one moved it, one measured the angles and distances, and one checked the axis of the goniometer and appearance of compensatory movements. The angle was measured using the Todai-type goniometer in degrees, and the distance was measured using the measuring tape in millimeters.
2.4. Analysis
First, the number and the percentage of injured players that participated in the medical check were identified. Next, differences in mean values in joint motions range, height, weight, age, years of baseball experience and age of starting baseball between the control and the injured groups were compared using an unpaired t-test. Before conducting this study, an explanation about the study was made to the participants and their parents, and their informed consent was obtained. Furthermore, the approval of the ethics committee of the Kyoto Prefectural University of Medicine was obtained in advance (approval number: RBMR-C1197-2).
3. Results
The number of baseball players diagnosed as having injuries among the 135 junior high school was nine (7.2%). On the other hand, 31 (10.0%) were positive to the throwing injuries test among 309 senior high school baseball players (Table 1 & Table 2). Tests of the proportion of the population indicated a significant difference between the groups (p < .05).
Significant differences in the range of motion were not shown for any of the items between the injured and the healthy groups of junior or senior high school students. The height, weight, age, years of baseball experience, and the age of starting baseball indicated a significant difference only for the weight in the injured junior high school group. Significant differences were not shown for any other variables.
4. Discussion
The possibility of the collapse of the kinetic chain among upper extremity, lower limbs, and the trunk increases when flexibility declines. Consequently, it was expected that throwing motions would place high stresses on the upper extremity, which causes pitching-related injuries. However, the results of this study did not indicate a correlation between the range of motions and shoulder injuries.
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Table 1. Physical functions of junior high school baseball players.
*p < .05.
The reasons for this could be as follows. Firstly, the maximum shoulder external rotation angle during pitching motion reaches 180 degrees (Diliman et al., 1991) and it requires high mobility. This angle is achieved by the involvement of the trunk and scapula, i.e. the glenohumeral joint (72.4%), scapular posterior tilt (17.2%), and thoracic spine extension (6.9%), among others (Miyashita et al., 2009) . It is a complex movement that includes the trunk, and which cannot be achieved only by the glenohumeral joint. The throwing motion could be performed by compensatory movements of other joints if there is a restriction in the mobility of a single joint. Therefore, a correlation might not have been observed
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Table 2. Physical functions of senior high school baseball players.
between the range of motions and shoulder injuries. Secondly, previous studies have indicated that muscle strength (Ellenbecker et al., 1997; Noffal, 2003) , pitching forms (Whiteley, 2007; Matsuo et al., 2002) , and energy transfer when using an appropriate kinetic chain (Setoguchi et al., 2008) are also important factors related to pitching-related injuries. Moreover, other factors such as height, types and the speed of pitches, and the training time might be related to pitching-related injuries. The mutual effects of the above factors and factors that have an antecedent effect are not clear. Moreover, causal relationships among these factors might interact in a complex manner (Matsui et al., 2014) .
The above findings indicate the necessity to examine not only the range of motions in pitching-related injuries but also the causal relationships between such injuries and physical functions. Moreover, complex factors in pitching-related injuries such as muscle strength must be considered from a comprehensive perspective that includes athletes’ primary physical data and the degree and the content of training, among others. When they rehabilitated the shoulder injuries players, this study results thought that it was necessary to perform it in consideration of many elements including the muscular strength as well as flexibility. Furthermore, longitudinal studies are required to investigate the causal relationships in pitching-related injuries by considering the extent to which joint functions of the whole body in addition to the shoulder joint could be compensated, as well as the timing of injuries.