Filament lengths in frog semitendinosus and tibialis anterior muscle fibres.

In frog semitendinosus muscle the descending limb of the length-tension curve is shifted rightward relative to that of tibialis anterior. Both the plateau right corner and the zero-force intercept are equally shifted. To investigate the reason for this shift, we compared filament lengths in the two muscles. Single fibres were mechanically skinned, stretched to reveal filaments clearly, incubated in a solution containing one of several antibodies to enhance filament visualization, and examined by electron microscopy. We found no differences of filament length. Thick filament lengths were 1.62 and 1.61 microns, respectively. I-segment lengths were measured by two methods. With the first, filament length was the same for both muscles, 1.95 or 1.98 microns, depending on the value taken for the troponin repeat; with the second it was 1.92 and 1.94 microns, respectively, for the two muscles. These differences are insignificant. Thus, the reported differences of shape of the length-tension curve are not explainable in terms of differences of filament length.