Effect of thin filament length on the force-sarcomere length relation of skeletal muscle

We studied a slow- and a fast-twitch muscle fiber type of the perch that have different thin filament lengths. The force-sarcomere length relations were measured, and it was tested whether their descending limbs were predicted by the cross-bridge theory. To determine the predicted relations, filament lengths were measured by electron microscopy. Measurements were corrected for shrinkage with the use of I-band and H-zone periodicities. Thick filament lengths of the two fiber types were found to be similar (1.63 ± 0.06 and 1.64 ± 0.10 μm for slow- and fast-twitch fibers, respectively), whereas the thin filament lengths were clearly different: 1.24 ± 0.10 μm (n = 86) for the slow-twitch type and 0.94 ± 0.04 μm (n = 94) for the fast type. The descending limbs of the two fiber types are therefore predicted to be shifted along the sarcomere length axis by ~0.6 μm. Sarcomere length was measured on-line by laser diffraction in a single region in the center of the fibers. The passive force-sarcomere strain relation increased much more steeply in the slow-twitch fibers. The descending limb of the active force-sarcomere length relation of fast twitch fibers was linear (r = 0.92), but was found at sarcomere lengths ~0.1 μm greater than predicted. The descending limb of the slow-twitch fibers was also linear (r = 0.87) but was now found at sarcomere lengths ~0.05 μm less than predicted. The difference in position of the descending limbs of the two fiber types amounted to 0.5 μm, ~0.1 μm less than predicted. The difference between measured and predicted descending limbs was statistically insignificant.