Muscle is highly organized across multiple length scales. Consequently, small changes in the arrangement of myofilaments can influence macroscopic mechanical function. Two leg muscles of a cockroach, have identical innervation, mass, twitch responses, length-tension curves, and force-velocity relationships. However, during running, one muscle is dissipative (a "brake"), while the other dissipates and produces significant positive mechanical work (bifunctional). Using time resolved x-ray diffraction in intact, contracting muscle, we simultaneously measured the myofilament lattice spacing, packing structure, and macroscopic force production of these muscle to test if structural differences in the myofilament lattice might correspond to the muscles' different mechanical functions. While the packing patterns are the same, one muscle has 1 nm smaller lattice spacing at rest. Under isometric activation, the difference in lattice spacing disappeared consistent with the two muscles' identical steady state behavior. During periodic contractions, one muscle undergoes a 1 nm greater change in lattice spacing, which correlates with force. This is the first identified structural feature in the myofilament lattice of these two muscles that shares their whole muscle dynamic differences and steady state similarities.

X-ray diffraction data during work loops and isometric twitches of Blaberus discoidalis

X-ray diffraction can be used to measure the myofilament lattice spacing of muscle. We measured the lattice spacing of two leg muscles from Blaberus discoidalis (cockroach) under sinusoidally driven, periodically activated strain trajectories (work loop) and isometric twitches. X-ray diffraction images have been analysed and compiled into two Matlab structures, along with force and length.

d10_blaberus_discoidalis.mat