Relationship between glenohumeral horizontal abduction angle and scapular internal rotation angle at maximum shoulder external rotation during baseball pitching

Chapter 1

Relationship between glenohumeral horizontal abduction angle and scapular internal rotation angle at maximum shoulder external rotation during baseball pitching

Shoulder injuries in baseball pitchers generally occur at the point of maximum external rotation during pitching. Walch et al reported that insertions of the supraspinatus and infraspinatus were compressed between the greater tuberosity and the posterior-superior border of the glenoid at maximum external rotation. This phenomenon is called internal impingement. It has been theoretically recognized that excessive stresses associated with shoulder horizontal abduction at maximum external rotation may result in internal impingement.

Background: Increased scapulothoracic internal rotation at maximum external rotation during baseball pitching can decrease glenohumeral horizontal adduction, leading to excessive glenohumeral horizontal abduction. However, few studies have examined this direct relationship, and none have investigated the scapulothoracic internal rotation angle at stride foot contact and maximum external rotation. This study investigated the relationships between glenohumeral horizontal adduction and scapulothoracic internal rotation angles at maximum external rotation and between scapulothoracic internal rotation angles at stride foot contact and maximum external rotation.

Methods: We recruited fifteen asymptomatic collegiate baseball pitchers and assessed the scapulothoracic internal rotation angle at stride foot contact and maximum external rotation and glenohumeral horizontal adduction angle at maximum external rotation during pitching using an optical motion tracking system. Pearson's correlation coefficients determined the relationships between glenohumeral horizontal adduction and scapulothoracic internal rotation angles at maximum external rotation and between scapulothoracic internal rotation angles at stride foot contact and maximum external rotation.

Results: The glenohumeral horizontal adduction angle at maximum external rotation was significantly related to the scapulothoracic internal rotation angle at maximum external rotation (r equals negative zero point five eight, P equals point zero two two). The scapulothoracic internal rotation angle at stride foot contact was significantly related to the scapulothoracic internal rotation angle at maximum external rotation (r equals zero point five three, P equals point zero four five).

Conclusions: The glenohumeral horizontal adduction angle at maximum external rotation is associated with the scapulothoracic internal rotation angle at maximum external rotation in asymptomatic collegiate baseball pitchers, and the scapulothoracic internal rotation angle at stride foot contact is related to the scapulothoracic internal rotation angle at maximum external rotation. Thus, the scapula and humerus should be controlled from stride foot contact to maximum external rotation during pitching to reduce internal impingement risk in asymptomatic baseball pitchers.

Mihata et al showed that excessive glenohumeral horizontal abduction and scapulothoracic internal rotation at maximum external rotation of simulated pitching caused internal impingement in vitro. This study specifically demonstrated that internal impingement occurs when the glenohumeral joint is horizontally abducted beyond thirty degrees. Therefore, baseball pitchers should avoid excessive glenohumeral horizontal abduction at maximum external rotation, known as "hyper-angulation." The reduction of glenohumeral horizontal abduction and scapulothoracic internal rotation at maximum external rotation plays an important role in the normal biomechanics of baseball pitching and the prevention of internal impingement.

The movement of the shoulder joint during baseball pitching is accomplished by the combined motion of the scapulothoracic and glenohumeral joints. During pitching, the motion of the scapulothoracic joint may affect the motion of the glenohumeral joint. In the sagittal plane,

decreased scapulothoracic posterior tilt at the maximum external rotation is responsible for excessive glenohumeral external rotation. In the horizontal plane, it is generally believed that an increase in scapulothoracic internal rotation during the late cocking phase induces compensatory excessive glenohumeral horizontal abduction. Recent studies have demonstrated that coordinated motion of the glenohumeral and scapulothoracic joints on the transverse plane helps prevent excessive glenohumeral horizontal abduction (angle less than thirty degrees) at maximum external rotation in asymptomatic elite baseball pitchers. However, few studies have examined the direct relationship between the scapulothoracic internal rotation angle and glenohumeral horizontal abduction angle at maximum external rotation. Thus, it is not clear whether asymptomatic pitchers with increased scapulothoracic internal rotation experience excessive glenohumeral horizontal abduction at maximum external rotation. A detailed analysis of the relationship between the scapulothoracic internal rotation angle and glenohumeral horizontal abduction angle at maximum external rotation is essential for understanding the kinematic factors contributing to internal impingement mechanisms during baseball pitching.

In baseball pitchers, the kinematics in the early phase of the pitching motion is linked to the kinematics in the latter phase, because pitching involves the sequential action of body segments. Previous studies have shown that early trunk rotation at the point of stride foot contact increases shoulder horizontal abduction at maximum external rotation. Although the mechanisms of internal impingement during the late cocking phase have not been entirely confirmed, the scapula in pitchers with early trunk rotation at the stride foot contact may be improperly positioned. Therefore, an increased scapulothoracic internal rotation angle at the stride foot contact may be associated with an increased scapulothoracic internal rotation angle at maximum external rotation. However, no data exist on the relationship between the scapulothoracic internal rotation angle at stride foot contact and the scapulothoracic internal rotation angle at maximum external rotation. A focused examination of the relationship between the scapulothoracic internal rotation angle at stride foot contact and at maximum external rotation is critical for optimizing scapular motion during the late cocking phase in baseball pitching.

We aimed to investigate the relationship between the glenohumeral horizontal abduction and scapulothoracic internal rotation angles, respectively, at maximum external rotation. In the sagittal plane, pitchers with a decreased scapular posterior tilt at maximum external rotation may have excessive glenohumeral external rotation. In the transverse plane, we hypothesized that pitchers with an increased scapulothoracic internal rotation at maximum external rotation would experience excessive glenohumeral horizontal abduction. We also aimed to investigate the relationship between the scapulothoracic internal rotation angle at stride foot contact and the scapulothoracic internal rotation angle at maximum external rotation. We hypothesized that pitchers with increased scapulothoracic internal rotation at stride foot contact would also exhibit increased scapulothoracic internal rotation at maximum external rotation, because baseball pitching involves the sequential action of body segments.

Materials and methods

Participants

Fifteen male college baseball pitchers (age, twenty point two ± one point nine years; height, one point seven six ± zero point zero five meters; body mass, seventy-three point three ± six point seven kilograms; throwing arm, four lefts and eleven rights) participated in this cross-sectional study. All pitchers used an overhand style during pitching. Participants were required to have more than three years of experience in baseball pitching. Pitchers were excluded if they had pain in the upper or lower extremities and trunk at the time of testing or had a history of upper or lower extremity and trunk surgery at least three years before the day of data collection.

All participants signed a written informed consent form before participating in the study. The study protocol was approved by the Sapporo Medical University Ethical Committee and the Hokusho University Ethical Committee. The ethical aspects of the study conformed to the principles of the Declaration of Helsinki.