Theories that could fully explain the biomechanism of labor are many, but none of them can fully explain their causal nature. According to A. Ya. Krasovskiy, the flexion of the head is explained by the pressure on her spine, transmits her action by driving away the force. In this case, the head is considered as an unequal lever, as a result of locating the point connecting the ridge with the base of the skull not in the center, but closer to the occiput than to the chin. Therefore, most of the driving forces are concentrated on the back of the head. A long lever arm (face, forehead) meets resistance from the unnamed pelvic line.
In this case, two opposing forces arise. : one of them presses on the back of the head and causes it to move forward, the second - contains the face of the fetus. As a result, the nape falls below the forehead - there is a bending of the head.
The internal turn of the head is explained by the interaction of the fetal head and the pelvic wall and the pelvic floor muscles. Gliding along the walls of the pelvis and being under the action of the muscles of the pelvic floor, the head makes its internal turn. After the internal rotation, the head is in a state of considerable flexion. The occiput is located between the lower branches of the bones. Here the nape is subjected to pressure from the side of driving forces and is shown from under the lumbar arch first. Further, as a result of the interaction of the two cross forces (force, develops under the action of the abdominal wall and diaphragm, as well as the muscles of the uterus, and the strength of the pelvic floor muscles), the head unbends.
The inner turn of the trunk and the outer turn of the head are mainly due to the rotation of the shoulder girdle.