Reliability and validity of shoulder muscle strength measurements using portable dynamometers: A systematic review

Dynamometric shoulder strength testing is crucial in clinical, research, and sports settings. Reliable and valid protocols are required to track changes in muscle strength over time. Therefore, this study aims to compare the reliability and validity of portable dynamometer protocols, provide an overview of available protocols and their methodological quality, and develop a checklist to guide the planning and reporting of reliability studies. A systematic search of PubMed, Cochrane Library, Web of Science, and CINAHL databases (up to February 2024) identified 53 studies meeting the inclusion criteria. Methodological quality was assessed using the COSMIN Risk of Bias tool and synthesis of evidence was based on criteria for good measurement properties. In case of consistent results meta-analysis was performed. Relative reliability showed sufficient results, while absolute reliability and validity results were less consistent. Although included studies showed limitations in execution and reporting, portable dynamometers can be used to distinguish between individuals. However, interpreting changes in individual shoulder strength requires caution. Examiners should be aware of the measurement properties within their applied protocols. In protocols, where reliability is unknown, it should be investigated to allow appropriate interpretation of results. To address the persistent methodological limitations observed in literature, we propose the "Muscle Strength Reliability Checklist", a structured instrument intended to enhance methodological transparency and consistency. Based on the findings of this systematic review, future protocols should use devices with external fixation, place participants in standardized, stable body positions, and adhere to a standardized timeline, instructions, and encouragement procedures.