Changes in muscle stiffness during contraction recorded using ultrasonic waves

Abstract

Although it is generally accepted that muscles contract by means of cross-links between the thick and thin filaments^1,2, the molecular mechanism of contraction is still a matter for debate and speculation. To investigate the number and state of the cross-links at various stages of muscle contraction, muscle stiffness changes have been studied by applying step or sinusoidal length changes and measuring the resulting force changes^3–5, or the propagation of longitudinal mechanical waves^6,7. These methods, however, involve relatively large perturbations to the contractile system, and may not be free from the possibility that the state of the contractile system is altered by the measurement procedure. We have developed a technique in which muscle stiffness can be continuously recorded during a single mechanical response in frog skeletal muscle by measuring the propagation velocity of ultrasonic waves with negligibly small perturbations to the contractile system and a high time resolution. Using this technique we have obtained the novel and unexpected result that during contraction muscle stiffness decreased in the transverse direction, though it increased in the longitudinal direction.