A New Method for Accurate Shoulder Strength Measurements

Muscle strength assessment is a key outcome measure for evaluating shoulder function in patients with orthopedic dysfunctions. Hand-held dynamometers (HHDs) are widely used in clinical practice due to their ease of use, costeffectiveness, and portability. However, factors such as examiner strength, subject positioning, and HHD placement often compromise their reliability. Recent studies have explored advanced measurement devices, such as load cells, which offer higher accuracy and improved data reliability. Despite these advantages, their clinical use remains limited due to complex setup requirements and the need for specialized software for data extraction. This study proposes a novel system integrating an HHD with a load cell mounted on a rigid, 3D-printed modular support to enhance measurement reproducibility. The system was validated through static weight tests and a feasibility assessment involving healthy volunteers. Validation tests using static weights demonstrated good consistency between the two devices, though discrepancies became more pronounced as the weight increased. Feasibility testing in healthy volunteers further confirmed the system's ability to measure shoulder muscle strength and compare data across different systems. Specifically, Bland-Altman analysis revealed a small systematic bias (MOD: 1.6 N) and narrow Limits of Agreement (LOA:-3.8 N to 7.0 N). The Mean Absolute Error (MAE: 2.9 N) and Mean Absolute Percentage Error (MAPE: 9.4%) demonstrated acceptable error levels in both dominant and non-dominant arms. These results suggest the system provides a standardized and reproducible approach to muscle strength assessment by reducing human error. The proposed system shows strong potential for clinical applications, particularly in the monitoring and diagnosis of musculoskeletal shoulder disorders, where measurement precision is critical.