3D muscle architecture in the triceps surae muscle: 3D ultrasound methods and maps of fascicle orientation and curvature

Muscle fascicle architecture is an important parameter affecting the mechanical function of skeletal muscle. Most previous studies on fascicle architecture have been in 2D and the importance of 3rd dimension has not been much explored. The 3D orientation of the whole muscle may be regionalized in the muscle and can change with the contraction state of the muscle. Fascicles are arranged as sheets in muscles and the sheets’ arrangement may change when the muscle bulges during contraction. With the muscle bulging, fascicle sheets may deform and affect the 3D fascicle orientations which will further influence the force generated by the muscle. In this thesis methods were developed and validated to study the in-vivo muscle fascicle architecture in 3D using B-mode ultrasound and optical tracking systems. Images were obtained from multiple scans of the muscles with scan times less than two minutes and analyzed for fascicle orientations, fascicle curvatures, fascicle sheet orientations and fascicle sheet curvatures. The 3D architecture information further was used to study the effect of ultrasound probe orientation and position on the measured 2D fascicle orientations. The orientation and curvature values of the fascicles and the fascicle sheets were quantified in the soleus and the gastrocnemii muscles in six male subjects for three torque levels 0%, 30% and 60% of MVC and four ankle angles -15°, 0°, 15° and 30° of planter flexion. The probe orientation and position was more critical in soleus than the gastrocnemii muscle due to more complex fascicle arrangement. Fascicle orientations and curvature values were regionalized across the muscles and changed with the change in ankle angle and relative torque level, p<0.01. The change in fascicle arrangement may be in response to the intramuscular pressure, and these changes can alter the mechanical output. The 3D information obtained in this thesis will be useful to understand the force generation of muscle and also to understand the change in muscle function with diseases affecting the muscle architecture