Introduction Rotator cuff (RTC) muscle size is predictive of shoulder function and potential pathology following injuries. Ultrasound (US) can differentiate normative from pathologic RTC musculature, and inform the diagnosis of shoulder pathology and treatment selection for more efficient, patient-centered care. This study’s objective was to establish normative sonographic measurements and ratios for RTC musculature within a military population.

Methods In this cross-sectional observational study at three military treatment facilities, military beneficiaries underwent a standardized US scanning protocol to measure the upper trapezius (UT), supraspinatus (SS), infraspinatus (IS), and teres minor (TM) muscle dimensions. Ratios of these dimensions were calculated, compared to adjacent muscles.

Results 128 participants (52 female; 88.2% active duty, mean age: 32.9 years; 231 shoulders) were enrolled, excluding shoulders with a history of significant injury or surgery. Mean and standard deviation (SD) for UT and SS thickness (mm) were 8.8 ± 2.2 and 22.2 ± 4.1, respectively. Mean and SD for SS, IS, and TM cross-sectional area (CSA; cm2) were 6.48 ± 1.82, 8.65 ± 2.79, and 4.19 ± 1.45, respectively. Ratios (mean ± SD) for UT:SS thickness, SS:IS CSA, and IS:TM CSA were as follows: 0.41 ± 0.13, 0.77 ± 0.19, 2.17 ± 0.73. Subgroup analyses demonstrated significantly larger dominant-sided RTC muscle dimensions for biological males than females (p < 0.01), but corresponding ratios were not significantly different.

Conclusion This is the largest US study to date to examine RTC musculature, establishing normative sonographic measurements and ratios in a military population. Future research should determine if RTC muscle ratios are more predictive than absolute dimensions of shoulder pathology in symptomatic personnel, to guide clinical decision-making and optimize warfighter readiness.

Key Messages

US offers a cost-effective, efficient, and validated alternative to MRI for assessing RTC muscle size, which can be predictive of shoulder function and pathology.

This study established normative sonographic measurements and ratios for RTC muscle dimensions in an active, healthy, military population, providing a robust reference that accounts for demographic differences, such as biological sex, which can affect absolute muscle size.

By introducing muscle dimension ratios as a diagnostic tool, this study enhances the potential for personalized assessment of shoulder health, which can enable more timely, accurate diagnosis and patient-centered treatment plans, ultimately optimizing warfighter readiness.

Further research is needed to determine the predictive value of RTC muscle ratios for specific shoulder pathologies (e.g., RTC tendon tear, denervation), compared to absolute RTC muscle dimensions alone.

The authors have declared no competing interest.

This work was funded in part by the Musculoskeletal Injury Rehabilitation Research for Operational Readiness (MIRROR), Department of Physical Medicine & Rehabilitation, Uniformed Services University, Bethesda, MD (award HU00011920011).

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This study was conducted at three total sites. Primary research approval was obtained through the Institutional Review Board (IRB) at Walter Reed National Military Medical Center (WRNMMC-2020-0288), with additional approval from the IRBs at both Alexander T. Augusta Military Medical Center and Brooke Army Medical Center.

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