Kinematics and electromyographic analysis of three types of internal rotation in the shoulder

PRESS RELEASE
The Open Orthopaedics Journal
NEWS RELEASE: 17-07-2026

The study, "Kinematics and Electromyographic Analysis of Three Types of Internal Rotation in the Shoulder," was carried out by Daisuke Segawa, Junichiro Hamada, Hiroshi Karasuno, Kunio Yoshizaki, Kazuhiro Endo, and Ryo Sahara in The Open Orthopaedics Journal.

Why Shoulder Internal Rotation Matters More Than Most People Realise

Internal rotation of the shoulder—the inward rotation of the arm—is essential for many everyday tasks. Reaching behind the back to tuck in a shirt, fastening a bra strap, retrieving a wallet from a back pocket, all depend on a specific form of internal rotation in which the hand moves upward behind the back toward the spine. Clinicians commonly assess this capacity in patients with adhesive capsulitis, rotator cuff tears, and after shoulder arthroplasty using three clinical tests: internal rotation at the side (IRs), with the elbow at the side and the forearm swinging inward; internal rotation at 90°, abduction (IRa) with the arm abducted and rotated downward; and internal rotation to the posterior (IRp), in which the hand reaches upward behind the back. These tests are often used together in clinical practice, yet little research has examined whether they measure the same underlying motion or different ones. This distinction matters when clinicians encounter patients whose IRp is severely limited while IRs appears near normal, or when IRp deficits prove resistant to rehabilitation.

Methods
 

To address this question, we recruited ten healthy young men at Kuwano Kyoritsu Hospital (Koriyama, Japan). Participants were instrumented with reflective markers at anatomical landmarks on the thorax, scapula, and upper limb. A ten‑camera three‑dimensional motion‑analysis system recorded joint kinematics at the humerothoracic and scapulothoracic interfaces. Simultaneously, fine‑wire electrodes were inserted into each rotator cuff muscle—the subscapularis, supraspinatus, infraspinatus, and teres minor—to record muscle electrical activity throughout each test.

Results
 

Kinematic analysis showed that the three tests produce substantially different ranges of motion. IRa produced the largest total internal rotation angle (72.4°), IRs was intermediate (56.6°), and IRp produced the smallest angle (31.6°). Unlike IRs and IRa, which rotated about a relatively fixed axis, IRp involved a continuously shifting rotation axis as the arm simultaneously extended and abducted, making its mechanics considerably more complex. Scapular motion contributed minimally to all three tests, remaining below 14°, indicating that differences among the tests were driven primarily by humerothoracic motion rather than scapular compensation.

Electromyographic recordings revealed distinct muscle activation patterns. During IRs, all four rotator cuff muscles showed relatively steady, low‑level activity consistent with a smooth rotation about a fixed axis. During IRa, the infraspinatus and teres minor—muscles typically associated with external rotation—were highly active at movement onset and progressively decreased activity as the movement continued, while the subscapularis gradually increased its contribution. IRp elicited the greatest demand: activity in all four muscles rose steadily throughout the movement, with the subscapularis reaching a peak of 29.8% of maximum voluntary contraction at end range, significantly higher than in IRs (20.3%) or IRa (18.2%). The internal rotation component of IRp was completed early in the gesture; the remainder of the motion that brings the hand higher along the spine was carried mainly by arm extension and anterior tilting of the scapula, suggesting that true humeral internal rotation reaches its structural limit well before the hand attains its highest vertebral level.

Discussion and Clinical Implications
 

These results resolve a longstanding clinical puzzle: IRs, IRa, and IRp are not interchangeable measures of a single motion but are mechanically and neuromuscularly distinct. IRp is a more demanding and mechanically complex movement than IRs or IRa, requiring greater subscapularis effort, involving a shifting rotation axis, and reaching its internal rotation limit early in the movement. Consequently, limitations in IRp may reflect structural constraints or complex multi‑planar mechanics rather than isolated muscle weakness or simple capsular tightness. This distinction has direct implications for assessment and rehabilitation: interventions that focus solely on strengthening or straightforward capsular stretching may fail to restore IRp if they do not address the specific mechanical and neuromuscular demands of that movement. Rehabilitation programs should therefore be tailored to the particular test and functional task being targeted.

The authors note important limitations: the sample comprised only ten young, healthy men; skin‑mounted markers introduce measurement error; and fine‑wire EMG could not isolate distinct functional sub –regions within individual muscles. The authors recommend future studies using standing open MRI or CT to obtain more precise joint‑level measurements and expanding participant samples to include women and middle‑aged individuals.

Article title: Kinematics and Electromyographic Analysis of Three Types of Internal Rotation in the Shoulder

DOI: 10.2174/0118743250471973260716071544

Read the published article here: https://bit.ly/4gfBw1R