Stress relaxation in human uterine muscle

Abstract

The passive stress relaxation behavior of human uterine muscle was investigated. Specimens were stretched at four elongation rates to a nominal peak tension of 5 Gm. per square millimeter and allowed to relax. The stress relaxation behavior was described by the parameters,Sd/Soand time, where Sd was the decay in stress after a certain period of relaxation, t, and So was the initial peak stress which occurred at the time elongation was stopped or at t = 0. The stress relaxation behavior, described by curves ofSd/Soversus time (time plotted on a log scale), exhibited four characteristic stages. The position of theSd/Soversus time curve was proportional to approximately13/R, where R was the elongation rate in millimeters per minute.Also investigated were the effects of glycerination, temperature changes, site of the specimen in the uterus, aging, pregnancy, trypsin, and pronase on the stress relaxation behavior. From these experiments it was concluded that stress relaxation is the result of a physical process in the tissue and the mechanism responsible was located primarily in the connective tissue framework. Aging did not cause a significant change in the normal stress relaxation behavior. Pregnancy slowed down the rate of stress relaxation about 5 per cent.This type of mechanical, passive characteristic of uterine muscle was discussed in relationship to its functional importance in pregnancy.