Passive stretch relationships in human uterine muscle

Abstract

The passive length-tension relationship was studied using human myometrium. The strips of muscle were extended by step pulls of 3 mm. per extension and the tension recorded after the first or rapid stage of stress relaxation had occurred. The cross-sectional area was measured at the narrowest point of the muscle and tension was computed as grams per square millimeter.The material from pregnant patients gave curves which were flatter and rose less steeply than those from the nonpregnant. The alteration in the passive length-tension properties due to the pregnant state was also found to be present when 20 mm. strips were compared to 30 mm. strips from nonpregnant patients. This comparison was necessary due to what appeared to be elastic recoil in the excised muscle from pregnant patients.The passive length-tension relationship in the nonpregnant patient material was not altered by age, previous parity, or phase of the menstrual cycle. There were no differences noted in strips taken from various sites in the pregnant uterus.Histologic counts of the relative amounts of muscular and nonmuscular tissue between the pregnant and nonpregnant material could not account for the differences in the passive length-tension curves. The relative amounts of muscle and connective tissue were the same in all categories.