Representative force transients obtained at 10°C from isometrically mounted (as shown in Fig. 1, E and O) skeletal (A and B) and cardiac (C and D) myofibrils. (A) A full contraction–relaxation cycle of a skeletal myofibril isolated from skinned myotomes of zebrafish larvae (light gray trace; k_ACT = 20 s⁻¹, F_max = 38 nN/µm², and F_pass = 3.4 nN/µm²; see Video 1) and adult zebrafish (dark gray trace; k_ACT = 13 s⁻¹, F_max = 20 nN/µm², and F_pass = 1.2 nN/µm²; see Video 2). Ca²⁺ concentration is rapidly (10 ms) changed from pCa 7.5 to 4.5 and back to 7.5 to induce contraction and the relaxation, respectively. Before Ca²⁺ activation, the passive force (F_pass) and, during Ca²⁺ activation, the maximum generated force (F_max) are determined, respectively, by transiently slackening–restretching the initially pre-stretched myofibrils at SL = 1.15 × SL₀. (B) Recordings on an expanded time scale of the biphasic normalized force decay shown in A for larvae (light gray trace; k_LIN = 10.3 s⁻¹, t_LIN = 12 ms, and k_REL = 98 s⁻¹) and for adult zebrafish (dark gray trace; k_LIN = 7.6 s⁻¹, t_LIN = 14 ms, and k_REL = 79 s⁻¹). (C) A full contraction–relaxation cycle of a cardiac myofibril isolated from the myocardium of adult zebrafish (dark gray trace; k_ACT = 5.2 s⁻¹, F_max = 47 nN/µm² and F_pass = 3.3 nN/µm²; see Video 3) and from murine papillary muscles (light gray trace; k_ACT = 5.8 s⁻¹, F_max = 52 nN/µm², and F_pass = 1.8 nN/µm²; see Video 4) subjected to a protocol similar to that described for A. (D) Normalized force transients of the biphasic relaxation shown on an expanded time scale for a cardiac myofibril in C: zebrafish (dark gray trace; k_LIN = 2.3 s⁻¹, t_LIN = 46 ms, k_REL = 25 s⁻¹) and mouse (light gray trace; k_LIN = 2.1 s⁻¹, t_LIN = 32 ms, and k_REL = 42 s⁻¹). Dashed curve (shown only for the zebrafish cardiac myofibril) is the simulated force decay with the rate constant 1/τ_REL calculated from kinetic parameters of the force relaxation using Eq. 1 (τ_REL⁻¹ = 12.3 s⁻¹ for a zebrafish cardiac myofibril; for comparison, τ_REL⁻¹ = 18.5 s⁻¹ for a murine cardiac myofibril). Mono-exponential increase (myofibril contraction) and biphasic decay comprised of a linear followed by a mono-exponential phase of decay functions were used to fit (thin black lines) force transients, yielding the kinetic parameters. Steady-state and kinetic force parameters (F_pass, F_max, k_ACT, k_LIN, t_LIN, k_REL, and τ_REL⁻¹) are shown only for skeletal and cardiac myofibrils of the adult zebrafish.