29.職業棒球投手之慣用手與非慣用手肩關節內轉與外轉活動度之差異之原因為：①後面旋轉肌 緊縮 ②後關節囊緊縮 ③肱骨後傾（retroversion）角度增大 ④中盂肱韌帶（middle glenohumeral ligament）緊縮
(A)僅①②③
(B)僅①②④
(C)僅②③④
(D)僅①③④

肱骨後傾(Humeral retroversion) 角度

定義：肱骨頭近端和遠端手肘軸線的角度，角度差異可以從-6~60度之大
Humeral retroversion is generally defined as the angular difference between the orientation of the proximal humeral head and the axis of the elbow at the distal humerus.

The retroversion angle of the humeral head has a wide range of variability, from –6° to 60°.

會影響瞬時旋轉中心的位置、關節的穩定性以及ER/IR的角度
correct retroversion of the humeral component is important because it affects the position of the instant center of rotation, the stability of the joint, and the amount of internal or external rotation.

然後平均角度大約30度

參考資料
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5435663/