Fig. 1

Instrumentation and experimental protocols. (A) The unipennate rat superficial masseter (grey is tendon, red is fascicles), instrumented with EMG to measure muscle activation (â), sonomicrometry to measure length-change of muscle fascicles (ε), and a custom miniature strain-gauge buckle to measure muscle force ( f ). (B) Representative in vivo measurements of muscle activation, length, and force for incisor biting on food with varying hardness (color). Grey bars delineate biting power strokes. Note how starting length (L_MAX) can exceed activation length (L @_âON) due to earlier activation of other adductor muscles. (C) The muscle preparations tested in vivo were also tested in situ using an ergometry procedure to measure muscle FL with needle stimulation (, 250ms for submax tetanic, and 2ms for twitch). (D) Representative contractions used to construct the muscle’s FL relationship. Grey bars indicate stimulation intervals.