Fig. 4. Ca distribution in the five regions at various stages in the contraction–relaxation cycle. Inset (top) is a diagram illustrating the six different stages of force change used for quick-freezing, i.e. at rest (A), during contraction (B), and at 0.1 s (C), 1 s (D), 3 s (E) and 5 s (F) after onset of relaxation. The bar shows period of stimulation (S). (A–F) [Ca] in the five regions along the half sarcomere. Values are means ± S.E.M. (N=36). Significant differences in [Ca] in the five regions are also shown; *P<0.05; ***P<0.001.