In vivo determination of the stress-strain relation of the human myometrium.

Smooth muscle is capable of responding by contraction to mechanical distension. Neither the structural basis for this phenomenon nor the theories of smooth muscle behavior are completely clear. A method is now described by which the contractile response to mechanical distension of the human uterine muscle in vivo is determined. The distension is enforced sinusoidally within the frequency range 0.005-2 Hz, by filling and emptying a rubber balloon introduced into the uterine cavity. A tonometric index of the myometrium is defined using Hook's law after the approximate evaluation of a contractile modulus and the tangential stress and strain in the inner layer of the uterine wall. The regression line of indices upon frequency is calculated and intercept and regression coefficient of the equation for basic conditions are compared with those obtained during administration of pharmacologically active agents. It is suggested that the differences in intercepts and regression coefficients be used as a measure of drug effect.