![]() The force needed to break the single crossbridge was insensitive to solution tonicity suggesting a non-ionic nature of the actomyosin bond. In agreement with the lever arm theory, both X-ray diffraction and mechanical data show that solution tonicity affects S1 mean position and consequently crossbridge individual extension and force, with no effect on crossbridge number. The presence of the distortion, which decreased with tonicity, allowed calculation of the mean position of the myosin head (S1) during the oscillation cycle. I M3 changed during the length oscillations in a sinusoidal manner in phase opposition to length changes, but a double peak distortion occurred at the peak of the release phase. M3 meridional layer line intensity ( I M3) was measured during the application of sinusoidal length oscillations (1 kHz frequency, and about 2% fibre length amplitude) at tetanus plateau. For a given isometric tension, P c increased with solution tonicity and occurred at a precise sarcomere elongation (critical length, L c) which also increased with tonicity. In all solutions, P c was proportional to the initial isometric tension developed. ![]() Force increased during the stretch up to a peak (critical tension, P c) at which it started to fall, in spite of continued stretching. Fast stretches (0.4–0.6 ms duration and 16–25 nm per half-sarcomere (nm hs −1) amplitude) were applied at various tensions during the force development in isometric tetani. Experiments were made in single muscle fibres and in fibre bundles from the frog, using both fast stretches and time-resolved X-ray diffraction, in isotonic Ringer solution (1T), hypertonic (1.4T) and hypotonic (0.8T) solutions. The aims of this study were to investigate the effects of solution tonicity on muscle properties, and to verify their consistence with the lever arm theory of force generation.
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