Fig. 4.

Direct fascicle sensing through sonomicrometry. a Schematic depicting closed-loop experimental paradigm using sonomicrometry crystals for fascicle state measurement. b Plot of points derived from a closed-loop run defining the fascicle length across the range of joint angles. The relationship between relative fascicle length and joint angle demonstrates passive and active contracting segments throughout range of dorsiflexion (90°–70°). These representative data were obtained from trials in virally transduced rats. c Closed-loop control of joint angle using sonomicrometry feedback regarding fascicle length demonstrating smooth sinusoidal movement (top: red—desired, black—actual) and dynamic range in end angle (bottom: red—desired, black—actual). This representative trial was performed in a transgenic mouse. d Schematic of the implementation of closed-loop optogenetic/sonomicrometry-driven systems; transdermal (left) and implantable (right)