Three-Dimensional Postural Characteristics and Gender Differences in Human Shoulder Joint Stiffness

Research Background

The shoulder joint is one of the most complex joints in the human structure. Its stability is crucial for the effective control of arm movement, including the natural control of distal joints like the elbow and wrist, as well as fine hand functions in daily activities. The stability of the shoulder joint is achieved through the complex interactions of bones, ligaments, tendons, and muscles, with stiffness being the characteristic that provides resistance to external disturbances. Recent studies have indicated that women experience higher incidences of shoulder injuries in occupations and sports, suggesting potential gender differences in shoulder joint stiffness. However, research on shoulder joint stiffness and its gender differences in multidimensional space, especially in three-dimensional (3D) space, has not been fully developed.

Paper Source

This study was jointly completed by researchers Seunghoon Hwang, Dongjune Chang, Aditya Saxena, Ellory Oleen, Soe Lin Paing, John Atkins, and Hyunglae Lee. The article was published in the 2023 issue of IEEE Transactions on Biomedical Engineering.

Research Objective

The main objective of this study is to characterize shoulder joint stiffness in three-dimensional space, particularly in a relaxed muscle state, and to explore its gender differences. This will help understand the role of shoulder joint stability and the risk of shoulder injuries that are gender-dependent.

Research Methods

Participants

The study recruited 40 healthy participants, including 20 males (age: 24.8±2.6 years, height: 171.1±11.5 cm, weight: 67.5±8.1 kg) and 20 females (age: 23.8±4.1 years, height: 163.8±8.5 cm, weight: 58.9±7.3 kg). All participants were right-handed and had no history of neuromuscular disorders or shoulder injuries. The ethics committee of Arizona State University approved the study, and written informed consent was obtained from each participant before data collection.

Experimental Setup

The study used a custom-designed shoulder exoskeleton robot (4-bar spherical parallel manipulator 4B-SPM) capable of generating rotational movements of the shoulder joint in three-dimensional space. It measured the posture and torque of the shoulder joint in specific postures using sensors. To ensure accurate measurements, a Vicon Nexus2 3D motion capture system and a 6-axis force/torque sensor were used. Additionally, the electromyographic activity of major shoulder muscles (anterior deltoid, middle deltoid, and posterior deltoid) was monitored during the experiment to ensure participants maintained a relaxed muscle state.

Experimental Procedure

Before the start of the experiment, the maximum voluntary contraction (MVC) of the major shoulder muscles was determined. The experiment included 15 arm postures involving different levels of shoulder flexion and horizontal abduction/adduction angles. In each posture, participants maintained a purely passive movement state without active muscle control for 45 seconds, and the robot applied slight postural disturbances to the shoulder joint with a root mean square error of 2° and a cutoff frequency of 3Hz.

Data Analysis

Impulse Response Function (IRF) was used to quantify the relationship between input postural disturbances and output torque responses. IRFs were estimated using short data segment system identification methods. Shoulder joint stiffness was estimated by integrating the IRFs, and the quality of the stiffness estimation was evaluated using variance accounted for (VAF).

Research Results

Effect of Posture on Shoulder Joint Stiffness

Two-way repeated measures analysis of variance (ANOVA) results showed that arm posture significantly affected shoulder joint stiffness (p < 0.001). Stiffness increased as the shoulder flexion angle decreased and also increased near the limits of shoulder joint range of motion. For example, at flexion angles of 45°, 67.5°, and 90°, stiffness was 17.4±3.3, 12.7±1.7, and 10.3±1.9 N·m/rad, respectively.

Study on Gender Differences

Mixed ANOVA results showed that gender had a significant effect on shoulder joint stiffness (p < 0.001), with males having higher shoulder joint stiffness than females. Additionally, stiffness data normalized by body mass also showed significant gender differences (p < 0.001), indicating that the differences were not solely due to body weight differences. Gender differences reached statistical significance in 12 out of 15 postures.

Analytical Discussion

The study demonstrated that even in a relaxed muscle state, shoulder joint stiffness exhibited significant variation in different arm postures, primarily due to the inherent physical properties of the shoulder joint rather than compensations by neural control or muscle reflexes. Moreover, the study systematically revealed for the first time the significant differences in shoulder joint stiffness between males and females, which is important for understanding gender-dependent mechanical properties of the shoulder joint and associated risks.

Research Conclusion

The study provides fundamental data on the changes in shoulder joint stiffness in three-dimensional space with varying arm postures and its gender differences. It lays the foundation for future studies characterizing shoulder joint stiffness under various task conditions (e.g., dynamic shoulder movements, muscle co-contraction). The results have significant implications in multiple fields, including physical therapy, rehabilitation robotics, and assistive robotics.

Research Highlights

• First systematic characterization of shoulder joint stiffness in three-dimensional space and its gender differences.
• Use of a custom shoulder exoskeleton robot provided accurate measurements of shoulder joint physical properties.
• Revealed the postural dependence of shoulder joint stiffness, especially changes at different flexion and horizontal abduction angles.
• Found and quantified significant differences in shoulder joint stiffness between males and females, which is important for understanding the mechanical properties of the shoulder joint and associated risks.

Future Research Directions

Future research should explore shoulder joint stiffness under multi-directional disturbances and study changes in shoulder joint stiffness under various conditions (e.g., different movement tasks, dynamic postures, and neuromuscular diseases). By integrating studies on the static and dynamic characteristics of the shoulder joint, a more comprehensive understanding of shoulder joint function and mechanical properties will be achieved, providing stronger theoretical support for rehabilitation therapy.