Relationship between shoulder external rotation strength in zero-position and horizontal abduction angle and glenohumeral aliment on MRI in baseball players

This study aimed to examine the relation between Zero ER strength, shoulder horizontal abduction angle, and glenohumeral alignment. We hypothesized that people who cannot exert Zero ER strength may adopt a horizontal abduction position and find maintaining a centripetal position difficult for the glenohumeral alignment.

This study evaluated 18 professional baseball players (age: 26.3 ± 3.8 years, height: 180.3 ± 4.7 cm, weight:85.6 ± 5.1 kg). A handheld dynamometer was used to measure their Zero ER strength. The limb positions for measurement included the shoulder joint at zero-position, the elbow joint at 90° flexion, and the forearm at mid-position, with the distal forearm serving as the resistance site. Isometric shoulder external rotation muscle strength in this limb position was measured, and the weight ratio of the average value of the three trials was calculated. Horizontal abduction angle, anterior glenohumeral distance (AGHD), and posterior glenohumeral distance (PGHD) were measured from the MRI pictures in the abduction external rotation position. AGHD and PGHD were defined as the vertical distances between the humeral head and the anterior and posterior edges of the glenoid cavity, respectively. The glenohumeral distance ratio was defined as the ratio of AGHD to PGHD. Coefficients of Pearson’s product-moment correlation and Spearman’s rank correlation were used for statistical analysis to examine the association between zero-position external rotator strength and anterior glenohumeral distance, posterior glenohumeral distance, and glenohumeral distance ratios. A p-value of 0.05 indicated a statistical significance.

Zero ER strength and horizontal abduction angle (r = −0.61, p = 0.006) and glenohumeral distance ratio (ρ = −0.58, p = 0.01) demonstrated a significant negative correlation.

Zero ER strength and shoulder horizontal abduction angle and glenohumeral distance ratio in professional baseball pitchers demonstrated a significant negative correlation. Athletes who cannot exert Zero ER strength may have a large horizontal shoulder abduction and glenohumeral distance ratio on MRI.

Improving Zero ER strength will lead to keep glenohumeral joint alignment on the scapular plane, contributing to the prevention and treatment of throwing disorder.