Coolbaugh, C. | bioRxiv Science

bioRxiv2025

MRI-based 3D Estimation of Skeletal Muscle Architecture and Strain during Contraction

Skeletal muscle generates forces that drive the motion of the human body. Three-dimensional (3D) quantification of whole-muscle architecture and strain, and their relationship during contraction is critical to understanding the mechanical function of skeletal muscle in health and disease. This has proven to be challenging, as brightness mode ultrasound is capable of measuring muscle architecture during contraction but cannot capture 3D changes in whole-muscle architecture, while Diffusion Tensor Imaging (DTI)-based tractography can measure 3D whole-muscle architecture but its use during contraction is precluded by long scan times (>5 minutes). In this study, we implement DTI-based tractography with an image registration-based approach, previously validated under passive deformation, to estimate 3D whole-muscle architecture of the tibialis anterior (TA) muscle during moderate intensity contractions (20-40% MVC). Moreover, this approach allows the measurement of whole-muscle strain during contraction, facilitating the evaluation of intramuscular relationships between architecture and strain. Our results show a decrease in the fiber-tract length, an increase in the pennation angle, and an increase in the fiber curvature of the TA during contraction. Intramuscular strain heterogeneity was observed between and within different regions of the muscle, with exploratory analyses suggesting that regional strain heterogeneity could be influenced by muscle architecture. Our results showcase the potential of MRI-based methods to obtain 3D estimates of whole-muscle architecture and strain during contraction, providing a breadth of new data that allows for new avenues of skeletal muscle biomechanical research.

bioengineering↗

bioRxiv2024

A registration strategy to characterize DTI-observed changes in skeletal muscle architecture due to passive shortening

Skeletal muscle architecture is a key determinant of muscle function. Architectural properties such as fascicle length, pennation angle, and curvature can be characterized using Diffusion Tensor Imaging (DTI), but acquiring these data during a contraction is not currently feasible. However, an image registration-based strategy may be able to convert muscle architectural properties observed at rest to their contracted state. As an initial step toward this long-term objective, the aim of this study was to determine if an image registration strategy could be used to convert the whole-muscle average architectural properties observed in the extended joint position to those of a flexed position, following passive rotation. DTI and high-resolution fat/water scans were acquired in the lower leg of seven healthy participants on a 3T MR system in +20{degrees} (plantarflexion) and -10{degrees} (dorsiflexion) foot positions. The diffusion and anatomical images from the two positions were used to propagate DTI fiber-tracts from seed points along a mesh representation of the aponeurosis of fiber insertion. The -10{degrees} and +20{degrees} anatomical images were registered and the displacement fields were used to transform the mesh and fiber-tracts from the +20{degrees} to the -10{degrees} position. Students paired t-tests were used to compare the mean architectural parameters between the original and transformed fiber-tracts. The whole-muscle average fiber-tract length, pennation angle, curvature, and physiological cross-sectional areas estimates did not differ significantly. DTI fiber-tracts in plantarflexion can be transformed to dorsiflexion position without significantly affecting the average architectural characteristics of the fiber-tracts. In the future, a similar approach could be used to evaluate muscle architecture in a contracted state.

physiology↗