Figure 5. Mechanical and structural signals at the onset of contraction.

In all the frames the vertical continuous line marks t_s, the dashed lines mark the boundary of the period of unloaded shortening. A–D, thick line, isometric condition; thin line, unloaded shortening imposed 5 ms after the start of stimulation. A, fibre length change imposed by the motor; shortening in %l₀, where l₀ is the fibre length. B, length change in nm per half-sarcomere (Δl) measured in the selected fibre segment. Note that during the first 5 ms of imposed shortening (denoted by *), the half-sarcomere shortens at a velocity lower than V₀; V₀ shortening starts 10 ms after the first stimulus, at the end of the latent period, when in the isometric contraction the half-sarcomere starts to shorten against the end compliance. C, difference between fibre length change in A and half-sarcomere length change in B, both expressed in %l₀ (where l₀ for B is the half-sarcomere length). D, force. E and F, S_M3 and S_M6 from RAPID experiments in isometric conditions (•) and during the imposed V₀ shortening (○). G and H, S_M3 and S_M6 from FReLoN experiments. Same symbols as in E and F.