<scp>MRI</scp> ‐based, <scp>3D</scp> analysis of Achilles tendon longitudinal strain under passive loading reveals limitations in conventional <scp>2D</scp> measurement

Quantifying strain in the free Achilles tendon (AT_F) is essential for understanding its mechanical function, yet existing in vivo studies report inconsistent strain values, even under comparable loading. These discrepancies are partly due to limitations of conventional two-dimensional (2D) imaging, which cannot fully capture the tendon's complex three-dimensional (3D) geometry. This study presents a novel MRI-based framework for assessing 3D centroid strain of the AT_F under passive ankle rotation. Eighteen healthy adults (10 females, 8 males, age: 24.4 ± 4.0 years) underwent static MRI scans at three joint angles (20° plantar flexion, neutral, and 20° dorsiflexion), from which 3D and 2D AT_F lengths were computed. Results showed that the 2D method failed to detect physiologically relevant strain and even indicated apparent shortening. On the other hand, the 3D method revealed significant but only a small magnitude of elongation of the AT_F (1.2% ± 0.3 from 20° plantar flexion to 20° dorsiflexion). Additionally, an offset simulation demonstrated that small sagittal-plane misalignments (2-4 mm) could introduce substantial variations of the AT_F length in the 2D measurements (2.8-6.3 mm). Such systematic errors can be due to the combined effects of imaging plane misalignment and complex features of distal soleus morphology. Collectively, these findings clearly show the limitations of the conventional 2D approaches when determining AT_F length and its changes upon loading, underscore the superior accuracy of 3D MRI in detecting subtle AT_F deformations, and provide critical insights into the anatomical and methodological sources of error in prior strain assessments.