The 3D in vivo Achilles' tendon moment arm, quantified during active muscle control and compared across sexes

The Achilles' tendon moment arm (ATma) is a critical quantity in that it defines the triceps surae's ability to generate a moment on the calcaneus, which is then transferred to the foot. This measure has been primarily acquired two-dimensionally in small male populations. Thus, the primary purpose of this study was to establish the first in vivo three-dimensional measures of the ATma, measured non-invasively during dynamic activity in a large normative population, inclusive of both males and female subjects (n=20). Subjects were each placed supine in a 1.5 T MRI and asked to repeat a simulated toe-raise while a full sagittal-cine-phase contrast (dynamic) MRI dataset was acquired. From these data, the 3D and 2D ATma was calculated. The ATma was scaled by the distal tibial width, based on a correlation analysis. The 2D ATma overestimated its 3D counterpart by 3.1 mm, on average. The scaled ATma was no different between the male and female cohorts, but the scaled Achilles' tendon area was smaller in the male cohort. The magnitudes of the ATma were most similar to previously reported values when variations in ankle angle were taken into account. The results of this study have important implications for the applicability of ATma data to both clinical questions and modeling. Any future studies should adapt the ATma based on subject size and/or sex, ensure compatibility between the manner in which the ankle angle is defined and the data being used, and account for the influence that muscle force has on the 3D ATma.