Tension Transients after Quick Release in Rat and Frog Skeletal Muscles

THE elastic energy of isometrically tetanized muscle can be measured by applying stepwise shortenings of different magnitudes to one end of the muscle while recording the resulting drop in tension. According to A. F. Huxley's 1 sliding filaments model this energy should reflect a property of the so-called cross-bridges between the actin and myosin filaments. We have measured this with a technique which enabled us to obtain the shortening of a tetanized muscle over a distance of 0.5 mm within 0.3 ms; smaller shortenings were achieved even faster. Resonance of the force transducer occurred at frequencies higher than 7 kHz, and stiffness of the whole apparatus was 10 g µm −1 . The sudden length-change induced an oscillation of approximately 4 kHz in the moving part of the apparatus. This oscillation can easily be discriminated as an artefact in the force recordings.