Physical Therapy Korea

The shoulder complex is the most involved region in musculoskeletal disorders, not only in the general population but also in athletes. Moreover, shoulder pathology is commonly associated with limitations in the shoulder range of motion (ROM) [1-4]. Among shoulder joint ROM, shoulder internal rotation (SIR) is a movement that is mainly physically examined or evaluated for shoulder lesions or syndromes [5-7]. The limitations of SIR can be caused by various factors such as tissue alterations, including glenohumeral joint posterior capsule tightness, shortening of the infraspinatus or teres minor, which are the external rotator muscles [8,9]. This limitation the of SIR can result in lesions in the connective tissue around the shoulder, such as the superior labrum anterior to posterior tears, and can also cause symptoms such as rotator cuff tears along with shoulder impingement syndrome [10,11]. Therefore, improving the shoulder joint SIR ROM has been considered crucial for preventing and rehabilitating shoulder joint dysfunctions.

The glenohumeral (GH) joint functions as a ball-and-socket joint with a fixed center of rotation [12]. Högfors et al. [13] defined the GH joint as a singular rotation point, presuming that the rotation center aligns with the center of the humeral head. However, uncontrolled movement of the GH joint during SIR may cause compensatory scapulothoracic movement or anterior translation of the humeral head, interfering with the maintenance of the GH joint center of rotation [8,14]. Harryman et al. [15] noted that tightening the posterior portion of the shoulder capsule causes anterior displacement of the humeral head during internal rotation. Accordingly, controlling the center of rotation may be challenging with restricted SIR ROM.

Sahrmann [16] defined the instantaneous center of rotation (ICR) as the point at which a rigid body rotates at a given instant. In this study, we assumed that the entire arm to be a rigid body in order to investigate the ICR of the GH joint. Furthermore, observing the path of the instantaneous center of rotation (PICR) during active motion may be a valuable indicator for evaluating accurate or balanced movements [16]. However, the ICR of the GH joint cannot be easily quantified because it is located inside the joint or bony surface, and displacement of the humeral head during shoulder movements is very small [14,15]. Therefore, applying the concept of the ICR which is defined as a specific point when a rigid body rotates, and assuming that the entire arm acts as a rigid body, the olecranon, which is an anatomical landmark of the arm may be useful for tracking the center of rotation during SIR. Previous studies have focused on SIR limitations due to factors such as posterior capsule tightness or muscle overactivity, but no studies have been conducted on quantification measurement of PICR. Quantitative measurement of the PICR through the displacement of the olecranon during SIR using a 2-dimensional marker tracking analysis system will be important to understand the dynamics of the shoulder joint.

Therefore, the aim of the study was to compare the displacement of the olecranon to measure the rotation control of the humeral head during SIR in individuals with and without restricted SIR ROM. We hypothesized that the displacement of the olecranon would be greater in the restricted SIR group than in the control group.

1. Participants

Twenty-four participants with and without restricted SIR ROM participated in this study. Twelve participants with restricted SIR ROM and twelve participants without restricted SIR ROM, referred to as the control group, volunteered for the study. The demographic and ROM data for each group are summarized in Table 1. Prior to the study, individuals were measured to identify the limitation of ROM in SIR at 90° abduction. Participants in restricted SIR group were included if the passive SIR ROM was less than 40° and had no history of shoulder pain or injury. Participants in the control group were included if the passive SIR ROM was > 60° and had no history of shoulder pain and injury [8]. The exclusion criteria for both groups were as follows: a) history of shoulder surgery, b) shoulder symptoms requiring medical care within the past year, c) shoulder pain greater than 5 out of 10 using a numerical pain scale, d) shoulder pain at the time of the study, and e) inability to perform the required movements for measurement. All participants signed an informed consent form, and the study was approved by the Yonsei University Mirae Institutional Review Board (approval No. 1041849-202312-BM-234-02).

Table 1 . Characteristics of study participants (N = 24).

Variable	Restricted SIR group (n = 12)	Control group (n = 12)	p-value
Age (y)	33.33 ± 9.32	32.67 ± 7.61	0.849
Height (cm)	171.50 ± 6.02	168.83 ± 9.06	0.405
Body mass (kg)	68.08 ± 11.25	65.50 ± 11.45	0.583
SIR ROM (°)	28.03 ± 7.82	70.56 ± 7.81	< 0.05

Values are presented as mean ± standard deviation. SIR, shoulder internal rotation; ROM, range of motion..

2. Instruments and Measurement

A Smart KEMA motion sensor (KOREATECH Co., Ltd.) was used to measure the kinematics of the passive SIR in the supine position. The Smart KEMA motion sensor outputs shoulder ROM measurements in degrees. The motion sensor comprises a tri-axillar gyroscope, magnetometer, accelerometer, signal converter, and signal transmission sensor. Data were transmitted to an Android tablet (Galaxy Tab S6; Samsung, Inc.) using Smart KEMA software (KOREATECH, Inc.) at a sampling frequency of 25 Hz [17].

A smartphone (Galaxy Z flip 4, Samsung Inc.) with video recordings (1920 × 1080 pixels at 60 fps) was used to capture the marker on the olecranon during SIR. All recorded videos were analyzed using the open-source motion analysis software Kinovea (ver. 0.8.15, Kinovea). Each frame was calibrated using a marker width 20 mm. The paths and displacements were exported to an XML file and then opened using spreadsheet software (Microsoft Excel ver.14, Microsoft) [18]. The maximum and minimum values of the x and y coordinates were extracted using a spreadsheet software. The total distance (cm) of horizontal displacement (HD) and vertical displacement (VD) was calculated (Figure 1). The initial reference points for both the horizontal and vertical axes were established of coordinates 0 and 0, respectively. The average HD and VD values were calculated by averaging the measurement from three SIR trials with the scapula fixed.

Figure 1. Shoulder internal rotation control test. The total distance (cm) of (A) horizontal displacement and (B) vertical displacement.

3. Procedures

The participants were measured passive SIR ROM in the supine position with the dominant arm abducted at 90° and the elbow flexed at 90°. The scapula was stabilized, and the coracoid process was palpated to confirm compensatory movement and prevent anterior translation of the humeral head. The examiner passively internal rotated the extremity of the participant until the end feel was perceived. The Smart KEMA motion sensor was attached to the wrist, and the end range of the SIR ROM was collected. Each participant was measured twice, and the data were averaged.

To analyze the motion of rotation control during SIR, the participants performed a shoulder internal rotation control test (SIRCT) and all videos were recorded. The camera was set at a distance of 150 cm and a height of 1 m from the participant, and the focus of camera was set on the grid lines. The test was initiated with the participant in a position of shoulder at 90° external rotation and the elbow flexed at 90° facing forward. The participants were instructed to perform three sets of SIRs, starting from a 90° external rotation position and ending at a 45° internal rotation position, while seated (Figure 2). The examiner fixed the scapula to prevent compensatory movements. The target bar was positioned to indicate full external rotation of the shoulder and at 45° internal rotation. A maker with a diameter of 20 mm was attached to the bony landmark of the olecranon.

Figure 2. Shoulder internal rotation control test (SIRCT). (A) Starting position and (B) end position of the SIRCT.

4. Statistical Analysis

The data were analyzed using IBM SPSS Statistics ver. 23.0 (IBM Co.). Means and standard deviations were calculated. Data normality was examined using the Shapiro–Wilk test. An independent t-test was performed to identify statistically significant differences in displacement of olecranon between the two groups. The level of significance was set at p < 0.05.

The results of the independent t-test indicated a significant difference in the VD of the olecranon between individuals with and without restricted SIR ROM (p < 0.05) (Figure 3). However, there was no significant difference in the HD of the olecranon (p > 0.05) (Table 2).

Table 2 . Comparison of displacement of olecranon between two groups (N = 24).

Displacement of rotation control	Restricted SIR group (n = 12)	Control group (n = 12)	p-value	Effect size
HD (cm)	3.74 ± 1.50	3.76 ± 1.56	0.982	0.01
VD (cm)	7.77 ± 1.75	3.17 ± 0.99	< 0.001*	3.24

Values are presented as mean ± standard deviation. SIR, shoulder internal rotation; HD, horizontal displacement; VD, vertical displacement. *Independent t-test..

Figure 3. Comparison of displacement of rotation control between two groups. HD, horizontal displacement; VD, vertical displacement; SIR, shoulder internal rotation. *p < 0.05.

The purpose of this study was to investigate alterations in the control of the center of rotation among individuals with limited SIR ROM. The results of this study demonstrate that individuals with restricted SIR ROM have significant positional instability in shoulder PICR, implying challenges in rotational control during SIR. The significant difference in the VD of the trajectory between the restricted SIR group (145%) and the control group is apparent from the data presented in Table 2. It is observed that the restricted SIR group experienced a more substantial shift in the VD of the trajectory compared to the control group. However, the anteroposterior movement of the ICR of the participants in the two groups did not differ significantly.

Multiple studies have demonstrated that the scapula compensates for GH joint movement during SIR. Proper positioning of the scapula is essential to achieve optimal alignment and centralization of the humeral head during shoulder movement [19,20]. Comerford and Mottram [8] defined scapular compensation movement as an uncontrolled motion and demonstrated that the scapula may tilt anteriorly and elevate to compensate for the lack of internal rotation. Furthermore, Borich et al. [21] reported that during GH internal rotation at 90° abduction, the scapula was significantly tilted anteriorly in individuals with GH internal rotation deficit (GIRD).

To limit the compensatory movement of the scapula, we fixed the scapula and performed SIRCT. During this test, individuals with limited SIR experience difficulty or discomfort when attempting SIR without assistance from compensatory motion of the scapula. Instead of rotation, the olecranon marker, which is a strategic point used to monitor the movement of the arm, moved downward. This can lead to shoulder dysfunctions such as subacromial impingement syndrome due to changes in glenohumeral joint kinematics, such as decreased SIR caused by posterior shoulder tightness. Tightness of posterior shoulder structures may be associated with the possibility of superior translation [22]. Excessive superior and anterior humeral head translation can reduce the size of the subacromial space and result in increased mechanical compression on the structures within the subacromial space [23-25]. Harryman et al. [15] also suggested that the selective tightening of the posterior portion of the shoulder capsule may lead to anterior and superior translation. Therefore, it may be inferred that as the superior translation of the humeral head occurs due to restricted SIR ROM, the marker on the olecranon exhibited significant vertical displacement. In contrast, the participants in the control group rotated the humeral head effectively and maintained shoulder abduction, thereby resulting in minimal VD. Therefore, individuals with restricted SIR demonstrated a significant difference in VD compared with that in the control group.

In the current study, only the total amount of marker movement during rotational control was calculated. Although both groups exhibited a tendency for VD, with a significant downward drop, the patterns of horizontal movement differed. Therefore, although the average of HD at 3 cm was similar between the two groups, determining the predominant direction of movement was not possible. Previous studies have suggested that people with restricted SIR display anterior translation of the humeral head during SIR. Harryman et al. [15] observed a significant anterior displacement of the humeral head during internal rotation in operative tightened posterior capsules. In contrast, minimal displacement was observed in the intact capsules. They demonstrated that this finding was due to the capsular constraint mechanism, which involves rotational tension exerted by the humeral head on the tightened posterior capsule, causing displacement in the opposite direction of the constrained tight tissue [15]. Similarly, Branch et al. [26] indicated that the posterior capsule has a significant impact on humeral head translation during SIR at shoulder 90° abduction and 0° flexion or extension. Additionally, Itoi et al. [27] discovered in a cadaveric study that anterior translation of the humeral head occurred during SIR and posterior translation occurred during external rotation without loaded condition. Based on these findings, investigating the direction of anterior-posterior movement using olecranon marker in future studies may be useful. Furthermore, as the test involved performing SIR with the scapula fixed, anterior-posterior movement of the marker was believed to be not well represented.

Our study has several limitations. First, we utilized 2-dimensional video analysis to observe rotation control during SIR. Although this method is convenient and user-friendly, accurately investigating complex humeral head movements is difficult. Second, the assumption of treating the entire arm as a rigid body in this study to investigate ICR of the GH joint has not been verified for accuracy. Therefore, validation of the approach would be necessary. Third, as the humerus of each participant is not a perfectly straight rod, the variations in the geometry of the humerus could influence the results. Thus, it is essential to examine the variations in the shape of the humerus in participants using techniques such as computed tomography (CT) or X-ray imaging. Fourth, we included only individuals without shoulder pain. In future studies, it would be clinically valuable to include individuals with shoulder pathologies such as shoulder impingement syndrome, to assess the rotation control of the shoulder joint. Therefore, investigating whether improving the limited SIR ROM results in reduced VD, that is, improved rotational control is clinically important.

The present study compared the displacement of the olecranon to measure the rotational control of the humeral head during SIR among individuals with and without restricted SIR ROM. The results of this study indicated that the restricted SIR group had a greater VD of the olecranon than that displayed by the control group, whereas no significant difference was observed in HD of the olecranon between the two groups. These results suggest that the limitation of SIR ROM may lead to difficulty in rotation control of the humeral head.

None to declare.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: MC, OK. Data curation: MC. Formal analysis: MC, JK, UH. Investigation: MC, IA. Methodology: MC, OK. Project administration: MC. Resources: MC, IA. Supervision: JK, UH. Visualization: MC. Writing - original draft: MC. Writing - review & editing: JK, UH, OK.