The primary objective of this research is to associate lumbar segmental mechanical response with intervertebral disc degeneration under physiologic testing conditions. Because no mathematical model exists for lumbar spine segmental rotations, a portion of this thesis evaluates potential methods for curve fitting the torque-rotation curves. The Dual Inflection Point (DIP) Boltzmann equation was developed during the course of this research and is presented here as a method for fitting spinal motion data wherein a physical meaning can be assigned to each of the model coefficients. This model can tell us more about the effects of degeneration, testing conditions, and other factors that are expressed in the change in spinal motion. Previous studies have investigated the relationship between the degeneration grade and flexibility of the intervertebral disc, but were completed without the presence of a compressive follower load. This study builds on past work by performing the testing under a compressive follower load. Segmental stiffness, range of motion (ROM), hysteresis area, and normalized hysteresis (hysteresis area/ROM) were evaluated and the effect of degeneration, segment level, temperature, and follower load were analyzed. Twenty-one functional spinal units (FSUs) were tested in the three primary modes of loading at both body temperature and room temperature in a near 100% humidity environment. A compressive follower load of 440 N was applied to simulate physiologic conditions. Fifteen of the twenty-one segments were also tested without the follower load to determine the effects of the load on segmental biomechanics. The grade of degeneration for each segment was determined using the Thompson scale and the torque-rotation curves were fit with the DIP-Boltzmann sigmoid curve.The effect of degeneration was statistically significant (α = 0.05) for stiffness, ROM, and hysteresis area in axial rotation (AR) and lateral bending (LB); it was also statistically significant for ROM and normalized hystersis in flexion-extension (FE). The lumbosacral joint (L5-S1) was significantly stiffer in AR and LB; the decrease in ROM and hysteresis area in AR and LB were also statistically significant for the lumbosacral joint compared to L1-L2 and L3-L4. Temperature had a significant effect on stiffness and hysteresis area in AR and on hysteresis area in LB. The follower load increased stiffness in all three modes of loading, but was significant only in AR and LB; it also reduced ROM and increased normalized hysteresis in all three modes of loading.

Abrupt decreases and increases in systolic arterial blood pressure produce baroreflex mediated shortening and lengthening, respectively, of the R-R interval. This phenomenon, otherwise known as the cardivagal baroreflex, is best described by the sigmoid relationship between R-R interval length and systolic blood pressure. The linear portion of this relationship is used to derive the slope or gain of the cardiovagal baroreflex. Importantly, lower levels of cardiovagal baroreflex have been associated with poor orthostatic tolerance and an increased cardiovascular disease-related mortality. The most commonly used and accepted technique to assess cardiovagal barorelex gain is the modified Oxford techinique. Bolus injections of sodium nitroprusside followed by phenylephrine HCL are used to decrease and raise blood pressure ~15 mmHg, respectively. The baroreflex control of the cardiac vagal outflow can then be assessed by the relation of the R-R interval to systolic blood pressure. However, the modified Oxford technique does not always reveal the nonlinear nature of baroreflex relations. The reasons for this has been unclear. Thus, analysis of baroreflex gain when nonlinearities are not revealed is problematic. Five classifications of baroreflex trials have been identified: acceptable, threshold-heavy, saturation-heavy, linear-heavy, and random trials. A new method of gain estimation was developed that combines the strengths of the current methods of gain estimation with the knowledge of the classifications of baroreflex trials. Using this method, cardiovagal baroreflex gain assessment can be maximized if threshold-heavy, saturation-heavy, and random trials are filtered out of the analysis and the manual method is used to estimate gain on the remaining trials. In addition, a link seems to exist between the variability of delta and the variability in baroreflex gain between different subjects. / Master of Science