Rotator cuff (RC) tears affecting ∼40 % of the U.S. population over 60, cause pain and reduced shoulder function (Collin et al., 2019). While previous research has focused on RC muscle pathologies like muscle atrophy and fatty infiltration (Goutallier et al., 1994) and their effects on strength, function, and repair outcome (Gladstone et al., 2007, Goutallier et al., 2003, Shen et al., 2008), little attention has been given to the muscle’s architectural changes. Notably, chronic changes in muscle length could significantly affect function (Sachdeva et al., 2022, Ward et al., 2010, Ward et al., 2006), given the critical force–length relationship of muscles, where optimal muscle length is key for maximum force generation. Understanding how RC pathology, clinical interventions, and post-operative recovery influence muscle length changes could provide insights into persistent strength deficits associated with RC tears, even post-repair.

Clinical imaging modalities have been used to assess muscle fiber length changes associated with RC tears and surgical repair. Kim et al., (2013) and Sachdeva et al., (2022) used conventional ultrasound imaging to demonstrate supraspinatus fiber bundle length variations pre- and post-surgery. However, the accuracy of ultrasound imaging in quantifying fiber bundle length is compromised by its heavy reliance on probe orientation. Notably, probe deviations exceeding 5° have been associated with ∼25 % error in fiber length measurement (Bénard et al., 2009).

Magnetic resonance imaging (MRI) has also been used to estimate muscle length by analyzing the muscle–tendon junction (MTJ) location. Ma et al., 2021, Meyer et al., 2012b, and 2012a reported muscle fiber shortening in RC tears patients and lengthening at 24-months post-surgery. However, the reliability in identifying MTJ location was only fair to moderate and assessing MTJ location through 2D images posed additional challenges. These limitations can be overcome by leveraging the advantages of 3D MRI, which provides more precise visualization and analysis of complex structures (Gyftopoulos et al., 2016) like the MTJ. Therefore, the objective of this study was to develop a reliable 3D approach for assessing supraspinatus muscle length.