Form follows force: Activation-dependent, 3D ultrasound-imaging-based analysis of human tibialis anterior's muscle architecture.

Abstract

Muscle architecture, particularly the pennation angle, plays a crucial role in force transmission and muscle function. The tibialis anterior muscle exhibits compartment-specific architectural patterns that may contribute differentially to force production. Three-dimensional ultrasound (3DUS) enables comprehensive spatial assessment of muscle architecture, overcoming limitations of traditional 2D ultrasound imaging. Twelve participants performed graded isometric dorsiflexion contractions at passive, 10%, 20%, 40%, and 80% of maximum voluntary contraction (MVC) at -10° ankle dorsiflexion. 3DUS data were acquired using an automated linear scanning system. Muscle architecture was analyzed using two complementary pipelines: MATLAB-based processing for pennation angles and central aponeurosis metrics with compartment-specific assessment, and Python-based streamline fiber tracking for fiber lengths and curvatures. Pennation angles increased progressively with contraction intensity (passive: 10.6° ± 0.4°; 80% MVC: 12.6° ± 0.4°, p<0.05), with the deep compartment showing significantly higher angles than the superficial one (p<0.05). Fiber length decreased significantly with contraction intensity (passive: 67.8 mm ± 2.8 mm; 80% MVC: 58.7 mm ± 1.8 mm, p<0.05), while fiber curvature decreased significantly (passive: 0.48 1/100 mm ± 0.02 1/100 mm; 80% MVC: 0.30 1/100 mm ± 0.02 1/100 mm, p<0.05). Central aponeurosis length increased with contraction intensity (passive: 164.0 mm ± 5.4 mm; 80% MVC: 169.7 mm ± 5.8 mm), and aponeurosis width increased significantly (passive: 29.0 mm ± 1.2 mm; 80% MVC: 32.2 mm ± 1.3 mm, p<0.05). The integration of streamline-based fiber tracking with compartment-specific analysis reveals progressive architectural changes in the tibialis anterior during graded contractions. Compartment-specific differences in pennation angles and fiber shortening may suggest distinct functional roles, with implications for understanding muscle force production and clinical assessment.