Experimental and theoretical biomechanical analyses of the second stage of labor

Injuries during vaginal birth affect 10 to 15% of first-time mothers. The goal of this thesis was to use biomechanics to help elucidate the underlying mechanisms.;Biaxial ramp-hold test results from the rodent vagina showed that term pregnancy decreased both tissue tensile stiffness and the long-term residual stress, and increased tensile strain at failure, by nearly two-fold (Chapter 2). Similar trends were observed the levator ani muscle of squirrel monkeys (Chapter 3). An anisotropic visco-hyperelastic constitutive equation was used to fit tensile tissue test data from the non-pregnant human levator ani muscle and perineal body (Chapter 4) for the purposes of finite element (FE) modeling.;A 3-D axisymmetric FE model of the energetics of the second stage of labor, using a clinically recorded intrauterine pressure profile, showed that the duration of the second stage of labor is sensitive to the magnitude, number of uterine contractions, timing with respect to the peak uterine contraction, and intra-push interval. The 'Peak' pushing style was the most efficient pushing strategy, the 'Triple' style the least efficient (Chapter 5). The duration of labor was most sensitive to pelvic floor tissue stiffness, followed by long-term relaxation behavior (Chapter 6).;The same simplified 3-D axisymmetric FE model was used to explore the effect of operator behavior on the pelvic floor mechanics during vacuum extraction delivery. Increasing the continuous vacuum extraction force by 25% increased muscle injury risk by 112%. Intermittent extraction forces kept the risk of maternal injury lower (Chapter 7).;A motor unit-based striated muscle fatigue model was integrated into the Chapter 6 energetics model. Increasing volitional pushing magnitude did not necessarily reduce the duration of labor, due earlier fatigue onset (Chapter 8).;An anatomically realistic 3-D FE model labor predicted a peak stretch of 5.04 near the origin of the pubovisceral muscle, and a value of 4.15 in the perineal body. Decreasing perineal body stiffness lowered the peak stretch and stress near pubovisceral muscle origin (Chapter 9).