This short overview of the mechanical properties of smooth muscle focusses on the force-velocity relation of (mainly pig urinary bladder) smooth muscle, and its dependence on the length of the muscle and its degree of activation. Also the response of the muscle to length and force changes at a rate beyond the physiological range is discussed. The force-velocity relation of this type of muscle can be approximated by the hyperbolic Hill equation, with a normalised maximum shortening velocity in the order of 0.25 muscle lengths/s. As in striated muscle, the maximum isometric force depends on the stretched muscle length and shows a maximum at a certain length. Interestingly, smooth muscle does not normally seem to operate at this length, but far below it. Both the isometric force and the unloaded shortening velocity depend on the degree of activation of the muscle, and so does the 'curvature' of the Hill equation. The series elasticity of the muscle, which can be measured by applying length changes at a rate beyond the physiological shortening velocity, is found partly in the cross-bridges, and partly external to these. An isometric quick release of 4-10% of the muscle length is necessary to remove all tension, depending on the total force exerted by the muscle. Force recovery after such a release is biexponential in a 700 ms window. The slowest component of this recovery, with a time constant in the order of 0.45 s is mainly associated with cycling of the cross-bridges, the fastest with the external series (visco)elasticity. Isometric force development has a time constant in the order of 3 s. indicating that excitation-contraction coupling rather than cross-bridge cycling is rate limiting in this process.
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