The Global Arc of Motion (GAM): A Novel and Intuitive Way of Studying Clinical Shoulder Kinematics

Denny T. T. Lie, MBBS, FRCS, FAMS, SINGAPORE Josephine Lan Pei Wen, MSc, SINGAPORE Chou Siaw Meng, PhD, SINGAPORE Lau Jun Liang, PhD, SINGAPORE

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Introduction

The shoulder joint is one of the most complex and mobile joints of the body. It enables a broad range of movements that are essential for activities of daily living (ADLs), such as reaching, lifting, and overhead actions often seen in sports. However, the downside of such exceptional mobility is an increased susceptibility to injuries and dysfunctions.

Accurate assessment of shoulder kinematics is essential for diagnosing conditions such as rotator cuff tears and frozen shoulder. Clinicians commonly use range of motion (ROM) measurements with tools such as goniometers and inclinometers in addition to functional scoring systems to evaluate shoulder function. However, these methods do not fully capture the shoulder’s complex 3-dimensional (3D) movement or the interactions between the humerus, scapula, clavicle, and thorax.

In traditional biomechanics, the term joint kinematics refers to the study of the motion of joints in the body, including the movement of bones relative to one another within a joint, without considering the forces that drive these movements. It involves analyzing the types of motion (e.g., flexion, extension, rotation, and translation), velocities, and accelerations of different parts of the body, providing insight into how joints function during various physical activities. Kinematics provides a comprehensive understanding of how a joint moves in space, which is crucial for biomechanics, physical therapy, and sports science.

The study of joint movements typically involves technologies such as motion capture and inertial measurement units (IMUs). While motion capture is widely used in clinical gait analysis and for analyzing lower-limb kinematics, its use in studies of the upper limb, particularly the complex 3D movements of the shoulder joint, remains much less common. Despite the well-established accuracy of motion capture for tracking anatomical landmarks, its reliability may be diminished when applied to the upper limb.

Shoulder Kinematics: Challenges in Clinical Practice

The shoulder, as a ball-and-socket joint, is complex and highly mobile, which allows for a wide range of motion. This extensive mobility illustrates the sophisticated dynamic relationship involving not just the humeral head and the scapula but also the clavicle, acromioclavicular joint, and surrounding soft tissues such as muscles, tendons and ligaments. However, this complexity makes it particularly challenging to assess and accurately measure shoulder movement using traditional clinical tools.

While standard clinical assessments, such as functional shoulder scores, provide useful information on pain, range of motion, and strength, they fail to fully capture the intricate 3D movements of the shoulder. The ability of the shoulder joint to move in multiple planes requires a more nuanced approach to understanding its kinematics.

The International Society of Biomechanics (ISB) Joint Coordinate System (JCS) has long been a widely used framework for studying joint kinematics. It integrates the local coordinate systems (LCS) of the thorax and humerus to describe thoracohumeral movement, or humeral motion relative to the thorax 1. In the shoulder, this system uses 3 distinct angles: plane of elevation (PoE), angle of elevation (AoE), and humeral axial rotation (AR)2. While the JCS has been instrumental in studying joint movement, it has substantial limitations when applied to the shoulder because of the complexity and multidirectional nature of shoulder motion.

The JCS is better suited for simpler joints, such as the knee or elbow, in which movement is often confined to 1 or 2 planes. However, the ball-and-socket structure of the shoulder allows for a wide ROM in all 3 dimensions, making shoulder motion far more complex. Moreover, ROM measurements are currently predefined by anatomical planes such as flexion-extension in the sagittal plane, abduction-adduction in the coronal plane, and internal-external rotation in the rotation axis of the humerus.

This is where the ISB’s “Globe model” attempts to offer a more holistic approach to shoulder kinematics. The globe model uses a spherical coordinate system centered on the center of rotation of the glenohumeral (GH) joint, which aims to better represent the complex and multi-directional movements of the shoulder (ref to Fig 2). Although the Globe model conceptually and mathematically overlaps with the JCS, it has been largely broken down into its 3 constituent angles (PoE, AoE and AR), reducing its intuitive nature and conceptual integrity. When decomposed in this way, the Globe model simply becomes an easier way to derive the angles of the JCS, losing its original advantage of providing a more comprehensive understanding of shoulder movement.

Ultimately, while both the JCS and the Globe model have contributed to understanding shoulder kinematics, they fall short of providing clinicians with a simple, intuitive, and clinically useful tool. As a result, there is a need for a more intuitive approach that accurately captures the complex, 3D movement of the shoulder while being accessible and easy to interpret in real time during clinical assessments and treatments. Such an approach would provide clinicians with a clearer view of shoulder function.

Concept of Global Arc of Motion (GAM)

In recent years, the concept of the “Globe model” has been gaining popularity. Researchers have proposed that the use of this model could potentially portray 3D shoulder joint kinematics more accurately.

As mentioned, the ISB Globe model illustrates the position of the humerus in relation to the thorax (referred to as thoracohumeral motion) by utilizing the plane of elevation (PoE) and the angle of elevation (AoE) rather than flexion-extension and abduction-adduction. The PoE indicates the plane in which the humerus is raised, whereas the AoE signifies the degree of elevation of the humerus. Essentially, the concept of the Global Arc of Motion (GAM) is based on the Globe model and is used to describe and quantify the global 3D ROM of the shoulder joint.

The GAM model should provide a more comprehensive view of shoulder kinematics than traditional angular measures. It represents the ability of the shoulder to move through space, incorporating the dynamic adjustments that occur during functional tasks. For example, during overhead motions such as throwing and lifting, the scapula must rotate and tilt to allow the humerus to move freely, and the clavicle must elevate to maintain the integrity of the shoulder complex. The GAM model accounts for these movements and provides a better understanding of shoulder function.

The main advantage of this newly proposed model (GAM) is its ability to consider the elevation of the humerus in all planes, unlike the orthogonal system, which only assesses elevation in the sagittal and coronal planes.

The current method of reporting shoulder joint motion with use of the JCS typically isolates the motion of the humerus with respect to the thorax, failing to capture the complex coordination between the humerus, scapula, clavicle, and surrounding muscles. The lack of integration between the various joints in the shoulder complex could lead to an incomplete picture of shoulder function, especially when considering functional tasks that require the coordination of multiple components. The JCS mainly focuses on specific planes of motion (e.g., flexion, extension, abduction), which are useful for some assessments but do not adequately capture the 3D movement of the shoulder.

Figure 1 portrays the activity of daily living (ADL) action for hair-combing and its current standard for reporting shoulder kinematics with the JCS method. While the solid red lines in Figures 2 (Globe model) and 3 (GAM model)show the shoulder trajectory during the hair-combing motion, Figure 2 illustrates the both superior and lateral views of Globe model. Figure 3 (GAM model) clearly shows that shoulder ROM had a longitude ranging from 135° to -125° and a latitude of 150°. This model also can be used for more than just describing angles. The shaded region of the globe shows the total area covered during circumduction. The benefit of using GAM model lies in its visual simplicity and intuitive nature.

Fig. 1 JCS model of shoulder kinematics for hair-combing (solid red line).

Fig. 2 The Globe model, with the top (superior) view shown in blue and the side (lateral) view shown in red.

Fig. 3 GAM model for shoulder kinematics, with the solid red line defining the hair-combing action.

Using ISB recommendations, the local coordinate system (LCS) of the humerus, thorax, and scapula was constructed to analyze shoulder kinematics within the GAM model. The scapular LCS was used to determine the center of rotation of the GH joint by using a sphere-fitting method. The humeral LCS was then adjusted to account for thoracic rotation. The long axis of the corrected humeral LCS represented thoracohumeral motion, which was converted from a Cartesian coordinate system to a spherical coordinate system centered at the center of rotation of the GH joint.

This allowed us to obtain the PoE and AoE, with the POE being defined as +90˚ anteriorly, -90˚ posteriorly, and 0˚ laterally, and the AoE ranging from 0˚ inferiorly to 180˚ superiorly. The PoE and AoE were then plotted as longitude and latitude, respectively, on a standard map axis (Robinson’s projection, described below). The enclosed arc of the circumduction motion formed the GAM for the shoulder joint, and the area within this arc was used to quantify the shoulder's global range of motion (ROM).

Robinson’s projection, mentioned above, refers to a type of map projection that was used to represent the Earth’s surface, like one would see in world maps. The projection of the GAM model aims to be clearer and more evenly distorted allowing it to be more intuitive(Fig. 4).

Fig. 4 Robinson's projection of extreme shoulder ROM.

Validation and Outcomes of GAM

Thirteen young and healthy male patients and a 65-year-old female patient with a 0.9 × 1.0-cm supraspinatus tear and functional reduction in ROM were recruited for a study conducted at Singapore General Hospital. The participants were instructed to complete a standard series of ROM exercises, including a full humeral circumduction movement, spanning the entire global ROM of the shoulder joint in the anterior, posterior, and lateral directions.

Motion capture was used to track upper-limb motion with use of reflective markers on osseous anatomical landmarks. The trajectory data of the reflective markers was processed and analyzed to generate the GAM.

Figure 5 reflects the comparison of means (and standard deviation) between 13 participants and patient’s ROM in both shoulders. The raw GAM data for the 5 ROM tasks (flexion, extension, abduction, internal rotation, and external rotation) is presented, with each colored dotted line representing the raw GAM of an individual participant.

Fig. 5 Mean GAM profiles (black solid line) with standard deviation (black dotted lines).

The blue and red lines in Figure 5 show an example of the use of the GAM for a patient with a shoulder abnormality (a supraspinatus tear in the left shoulder). This figure clearly shows that both shoulders in this patient had a reduction in shoulder ROM compared with the findings in young healthy individuals, with the reduction on the affected (left) side (red line) being more severe than that on the unaffected (right) side (blue line).

A few limitations should be noted. The normative dataset has to be built over time across all age-groups and genders, as the dataset described here was only representative of the 13 young male participants. Also, the angular description of the humeral rotation was left out, and is known to also be influenced by the position of the humerus (range of axial rotation typically decreases with increasing humeral elevation). However, with the GAM representing the extreme ROM of the shoulder joint, measuring the axial rotation of the humerus at these extremes may not be entirely accurate.

Future Directions

The GAM has a wide range of potential applications, including assessing the severity of ROM limitations in patients with shoulder abnormalities and the success of not only rehabilitation but surgery as well.

Although current technologies and the ISB JCS have enhanced our understanding of shoulder kinematics, they still have limitations that affect their practical use in clinical settings. Developing a more comprehensive and intuitive method such as the GAM model for measuring shoulder function could potentially improve the diagnosis, treatment, and rehabilitation of patients with shoulder conditions. This approach will enable clinicians to deliver better care, and has the potential to optimize recovery times and improved patient outcomes.

eNewsletter Current Perspective Author Interview & Discussion

Click here to listen as Deputy ISAKOS eNewsletter Deputy Editor, Dr. Nik Paschos, interviews Dr. Denny Lie on this article: The Global Arc of Motion (GAM): A novel and Intuitive Way of Studying Clinical Shoulder Kinematics.

References

1. Wu, G., Van der Helm, F. C., Veeger, H. D., Makhsous, M., Van Roy, P., Anglin, C., ... & Buchholz, B. (2005). ISB recommendation on definitions of joint coordinate systems of various joints for the reporting of human joint motion—Part II: shoulder, elbow, wrist and hand. Journal of biomechanics, 38(5), 981-992.
2. Doorenbosch, C. A., Veeger, H. E. J., & Harlaar, J. (2003). The globe system: an unambiguous description of shoulder positions in daily life movements. Journal of rehabilitation research and development, 40(2), 147-155.

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