Usually when you talk about muscle strength you need to consider force per mass. With this in mind, this makes the myometrial layer of the uterus the strongest muscle in the human body relative to its weight. This achievement is based on the ability of the uterus (aproximate weight is around 35 – 40 ounces after birth) to deliver an average 7lbs infant. This myometrial layer of the uterus can exert a downward force of 100 to 400 N (25 to 100 lbs) with each contraction. Source
