The external contractile measure was the mean wave amplitude in mm at the lumenal surface. This metric was measured at the lumenal peaks and troughs only and was a measurement designed to study the relationship between endometrial wave activity, manifest as bulk motion of the uterus, versus internal contractile strength. The external contractile measure was reported in mm/contraction. The external contractile measure quantified the movement of the uterus as a whole as measured at the lumen, i.e. the motion of the uterus relative to the body.