DTI‐based fiber tracking reveals a heterogeneous alteration of pennation angle upon muscle lengthening

Abstract

Introduction Diffusion tensor MRI (DTI)-based fiber tracking can determine the 3D structure of muscle, including pennation angle (𝛉). The goal of this study was to determine how 𝛉 changes upon muscle lengthening. Methods DTI and anatomical MR images were acquired of the tibialis anterior (TA) with the foot in 0° of plantarflexion (anatomical position), +15° of plantarflexion, and at 0° on a different day (n=4). Fiber tracking was performed by following the direction of greatest diffusion from 28,000 starting points along each of the deep and superficial aspects of the TA’s aponeurosis. 𝛉 was measured as the orientation of the fiber tract with respect to the plane tangent to the starting point. Results Reproducibility: 𝛉 did not differ when the 0° measurement was repeated. Deep compartment: at the 0° foot position, 𝛉 increased from 19±4° (proximal portion) to 37±6° (distal portion). When the foot was rotated to +15°, 𝛉 decreased to 18±4° (proximal portion) and increased to 40±6° (distal portion). Superficial compartment: at 0°, 𝛉 increased from 19±4° (proximal portion) to 30±3° (distal portion). When the foot was rotated to +15°, 𝛉 decreased to 18±4° (proximal portion) and did not change in the distal portion. Conclusion Muscle lengthening causes heterogeneous changes in 𝛉 along the aponeurosis. This may relate to the non-uniform material properties of the aponeurosis. Support NIH R01 AR050101, R01 NS034834, M01 RR00095