There is a controversy whether cross-bridge (XB)
dynamics is determined by XB displacement, filaments velocity or the load
experienced by each XB. The study tests these three hypotheses at the sarcomere
level. Methods: Trabeculae were isolated from rat right ventricles (n=9).
Sarcomere length was measured by laser diffraction. Changes in the number of
strong XBs (N_XB) were evaluated by measuring the dynamic stiffness.
Ramp stretches and releases at different velocities and onset times were
imposed over isometric sarcomere contractions. Results: Stretches yielded parallel increase in the force and
stiffness, at all stretch velocities. The force per XB during stretch was
constant, independent of the velocity, and equal to the isometric force per XB.
This observation is incongruent with a load dependent kinetics. The instantaneous stiffness during the ramp
stretches and releases was normalized by the isometric stiffness. Identical changes in the normalized stiffness were
observed when identical ramp perturbations (stretch or release) were imposed at
different onset times during the twitches. Thus changes in the normalized N_XB
are not dominated by XB recruitment processes, although the number of available
XBs varies with time during the twitch. The normalized stiffness development
rate linearly depended on the lengthening velocity with a slope of 6.73±0.98
[1/µm].  During shortening the normalize stiffness
decline rate depended linearly on the shortening velocity with identical slope
of 6.70±1.43 [1/µm]. Conclusions: The symmetrical dependence of the
normalized stiffness development rate on the velocity and the independence on
the perturbation onset time are conveniently explained by the velocity
dependent hypothesis in the framework of an integrated sarcomere, where there
is a cooperative interaction between the XBs. This mechanism explains the force-velocity
relationship and the muscle high contractile efficiency.