Fig. 1. Activation of skinned fibre bundles of anterior byssus retractor muscle (ABRM) with stepwise increase in Ca²⁺ concentrations. (A) Original recording of force and stiffness. The force spikes are induced by repetitive rectangular stretches of 20 ms duration and 0.1% L₀ in amplitude. The free Ca²⁺ concentration is expressed as pCa (= -log[Ca²⁺]). The force recordings (insets) were obtained on another preparation; they show the force transients induced by rectangular stretches of about 5 s in duration and 0.2% L₀ in amplitude applied at different stages of the experiment. (B) Relationship between stiffness and force at different Ca²⁺ activations in the experiment shown in A. The data points were obtained at the end of each individual Ca²⁺ activation.