P1.154 Wednesday, Jan. 4 Myofilament lattice spacing increases as muscles shorten GEORGE, NT; SALCEDO, MK*; WILLIAMS, CD; IRVING, TC; DANIEL, TL; Univ. of Washington, Seattle; Univ. of Washington, Seattle; Univ. of Washington, Seattle; Ill. Inst. of Technology, Chicago; Univ. of Washington, Seattle firstname.lastname@example.org
Under constant volume muscle contraction myofilament overlap increases and muscle expands radially. While myofilament overlap is considered a major factor in force generation, considerably less is known about the role of myofilament expansion accompanying shortening during contraction. Recent theoretical evidence suggests that the spacing between filaments will indeed be an important determinant of force generation at any level of filament overlap, however, there are scant data that show the extent to which filament overlap changes in intact muscles. To address this issue we performed work loops on intact Manduca sexta flight muscles while simultaneously measuring myofilament spacing, force, and length using time resolved small angle x-ray diffraction imaging. Muscles were stimulated at 25 Hz and held at a temperature of 35C while simultaneously measuring length and force. Five x-ray images were taken during each wingbeat cycle for 100 contractions. These images were averaged over all cycles, and lattice spacing was measured for each phase. During work loops with a 6% strain cycle, lattice expansion changes by 2.77 ± 0.839%. Thus we do not reject the constant volume assumption, which would predict a 2.4% change in radial dimension for a 6% longitudinal strain. Taken together these data suggest that lattice expansion during muscle shortening is a critical component of muscle force generation.