In vivo ultrasound–MRI 3D mapping of neuromuscular compartments reveals the neuromechanical architecture of finger-specific control

Neuromuscular compartments provide the functional substrate for finger-specific motor control, yet their in vivo three-dimensional (3D) architecture remains unresolved because of the absence of clear fascial landmarks for imaging. Here, we present a multimodal imaging framework combining ultrasound and magnetic resonance imaging (MRI) to provide volumetric compartment segmentation and in vivo architectural reconstruction of the flexor digitorum superficialis. During isolated finger flexion, tissue movement directions estimated from dynamic ultrasound delineate compartment contours, which are mapped into a 3D MRI reference to generate volumetric compartment masks. Using these masks as constraints, tractography reconstructs 3D fiber trajectories and yields compartment-specific architectural measurements. In ten healthy individuals, compartment topology and fiber orientations were consistent with classic cadaveric anatomy, and estimated compartments accurately predicted surface electromyography amplitude distributions. This framework provides 3D fiber trajectories and quantitative parameters for finger-specific computational models, establishing a foundation for subject-specific neuromuscular digital twins and next-generation neuromuscular interfaces.