In vivo x-ray diffraction and simultaneous EMG reveal the timecourse of myofilament lattice dilation and filament stretch

Muscle function within an organism depends on the feedback between
molecular and meter-scale processes. Although the
motions of muscle’s contractile machinery are well
described in isolated preparations, only a handful of experiments have
documented the kinematics of the lattice occurring when
multi-scale interactions are fully intact. We used time-resolved
X-ray diffraction to record the kinematics of the myofilament lattice
within a normal operating context: the tethered flight
of Manduca sexta. As the primary flight muscles
of M. sexta are synchronous, we used these
results to reveal the timing of in
vivo cross-bridge recruitment, which occurred 24 ms
(s.d. 26) following activation. In addition, the thick filaments stretched
an average of 0.75% (s.d. 0.32) and thin filaments stretched
1.11% (s.d. 0.65). In contrast to other in
vivo preparations, lattice spacing changed an average of 2.72%
(s.d. 1.47). Lattice dilation of this magnitude significantly
affects shortening velocity and force generation, and filament stretching
tunes force generation. While the kinematics were consistent
within individual trials, there was extensive variation between
trials. Using a mechanism-free machine learning model we searched for
patterns within and across trials. Although lattice kinematics
were predictable within trials, the model could not create predictions
across trials. This indicates that the variability we see across trials
may be explained by latent variables occurring in this naturally
functioning system. The diverse kinematic combinations we documented
mirror muscle’s adaptability and may facilitate its
robust function in unpredictable conditions.