The mechanical properties of the actomyosin bond were investigated by applying to activated frog muscle fibres, fast stretches (~25 nm hs^-1 amplitude and ~350 µs duration) which forcibly detached the crossbridge ensemble. Stretches were applied before and up to 20 ms after a conditioning step release (~4 nm hs^-1 amplitude, 120 µs duration) during the time course of the quick force recovery. Experiments were made at 5°C. Frogs (Rana esculenta) were killed by decapitation followed by destruction of the spinal cord, according to the official regulation of the European Community Council (Directive 86/609/EEC). Force and sarcomere length were measured with a fast force transducer (~50 kHz natural frequency) and a striation follower device. To reduce fibre damaging by the stretches, experiments were made on the tetanus rise at tension (P_t0) of about 0.5 the plateau tension. All the force values reported here are normalized for P_t0. The rupture force of the crossbridge ensemble, the critical force P_c and the sarcomere elongation at P_c (critical length, L_c), were measured. Before the release P_c was 3.60 ± 0.15 (n=8) times P_t0. Critical length L_c was 12.02 ± 0.45 nm hs^-1. In contrast with previous data on the tetanus rise (Bagni et al. 2005) during the quick force recovery P_c was almost constant independently of the tension P_t developed by the fibre at the time of the stretch. L_c increased immediately after the step release (0.2 ms delay) to 16.24 ± 0.75 nm hs^-1 and remained elevated up to at the end of the phase 2 of the recovery (2 ms delay) when it started to decrease returning to the isometric value in about 20 ms. The ratio (P_c-P_t)/L_c, which represents the chord stiffness of the half-sarcomere, started to decrease progressively after the release and reached a maximum reduction of 27 ± 5 % at 2 ms delay. The return to the pre-release value occurred in about 20 ms. Data analysis suggested that: 1) crossbridge population remained almost constant during the quick force recovery; 2) the forced rupture of the actomyosin bond produced by the stretches is preceded by the reversal of the power stroke; 3) the average myosin lever arm movement during the power stroke can be measured by the changes of L_c; 4) after the execution of the power stroke, myosin heads are progressively detached and substituted by new heads attached with the pre-power stroke configuration. The complete substitution occurs in a relatively short time of 20 ms.