Light micrographs showing: dissected tail before (A) and after (B) detergent skinning of a 4-dpf larvae; a part of a myotome (C) and a thin myofibrillar bundle (D) isolated after homogenization of the chemically permeabilized larval tails; a myofibrillar bundle (E) isometrically mounted in the experimental setup between a stiff needle (left) and the tip of the atomic force cantilever (right) before Ca²⁺ activation; skeletal muscle dissected from adult zebrafish before chemical permeabilization (F); part of a muscle myotome (G) and myofibrillar suspension (H) after homogenization of permeabilized skeletal muscles; myotomal myofibrils showing very thin longitudinal white bands, pointed to by arrows (I and magnified in J), which contain extended profiles of T-tubules and SR as shown in TEM (see Fig. 2, A–C); dissected adult zebrafish (K, top) to access the whole heart (K, bottom right) from which only ventricles (K, bottom left) were used for preparation of myofibrils; cardiac branched myofibrils (L and M) isolated from a permeabilized ventricle of the adult zebrafish; and relaxed cardiac myofibrils before (N) and after (O) mounting them isometrically in the experimental setup. Bars: C–E, G–J, and L–O, 10 µm; A and B, 100 µm; K, bottom, 500 µm; F and K, top, 2 mm. (P) Distribution of the slack SLs of relaxed (pCa 7.5) myofibrils isolated from skeletal muscles of the larvae (zfL-sk; filled diamonds), skeletal muscles of the adult zebrafish (zfA-sk; filled squares), ventricle of the adult zebrafish (zfA-c; filled circles), and papillary muscles of the murine heart (m-c; open circles). Horizontal lines indicate mean values; n is the number of measured myofibrils.