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The occurrence of low back pain in society is a widespread and costly problem, while running is an accessible and common form of physical exercise. The intervertebral disc is a commonly studied location of interest within the low back, however limited research has been performed attempting to assess the risks or benefits associated with running on the health of the intervertebral disc, with zero research which estimates in-vivo loading on the intervertebral disc during running. Meanwhile, the available literature is not in agreement on whether running poses more of a risk or a benefit to intervertebral disc health, with some research suggesting that running has the potential to positively affect the intervertebral disc, while additional research suggests that long distance running will increase the likelihood of injury among runners. Thus, it is of interest to determine in-vivo estimates of forces on the L5/S1 intervertebral disc in order to assess changes in pressure within the disc nucleus pulposus during running. As such, for this dissertation, three studies were completed. The first involved model development in order to estimate L5/S1 joint angles, forces, and moments, followed by further model development to estimate muscular forces crossing the L5/S1 joint and joint compressive forces, and concluding by utilizing a finite element model of the intervertebral disc to estimate joint pressure. Following model validation, two studies were completed using repeated measures study designs in order to compare changes in intervertebral disc pressure due to running at different velocities, using different footstrike patterns, and following fatigue. Validation of the models used when estimating in-vivo intervertebral disc pressures resulted in estimates for joint moments that were similar to those estimated during a previous research study. The shape of the curves estimating muscular forces were similar to muscular stimulation curves derived via electromyography (EMG), however due to limitations of the data collection process for both muscle modeling and EMG it was impossible to reach a strong agreement when comparing these data sources. Validation of the finite element model resulted in estimated disc compression leading to percent stature loss that was similar in error magnitude to some previously published research comparing in-vivo and simulated data. Comparisons of previously recorded in-vivo disc pressure for a single subject to estimations for the present study resulted in minimum pressures that were similar across the studies, with greater maximum pressure estimated using the current modeling approach. An increase in velocity resulted in an increase in the average and peak pressures on the intervertebral disc during running, while faster velocities resulted in reaching peak pressure later on in stance than slower velocities. Changing footstrike patterns did not cause any differences in average or peak intervertebral disc pressure, however running with a forefoot strike did cause runners to achieve peak intervertebral disc pressures earlier on during the stance phase than when running with a rearfoot strike pattern. During fatigued running, a moderate-large effect size was observed with higher pressures on the disc during the fatigued state which were not statistically significant (p>0.05), and with no change in the percent stance needed to reach peak pressure. The models appear to perform adequately when utilized in a repeated measures study design, but are not able to accurately detect specific pressures within the intervertebral disc. The true meaning behind these results is unknown as higher pressures or loading rates during certain conditions may lead to increased risk of injury, or alternatively there might be little effect on injury risk as the higher pressure/loading may increase fluid flow into and out of the disc thus enhancing nutrient absorption. Further research needs to be performed in order to determine the risks associated with increases in pressure due to running at faster velocities, potential increases in the loading rate during forefoot strike running due to reaching peak pressure sooner than rearfoot strike running, and potential risks associated with increased intervertebral disc pressure during fatigued running.

Little is known regarding the effect that footwear cushioning can have on the mechanics of the low back. The purpose of this study was to 1) determine the material characteristics of a minimalist running shoe tested with and without a commercially available shoe insole, 2) determine if there are differences in lower back or knee kinematics when minimalist shoes are worn with and without a shoe insole during treadmill running, and 3) determine if there are differences in levels of muscle activation when minimalist shoes are worn with and without a shoe insole during treadmill running. Following the receipt of informed consent 10 subjects (age 33.3±13.0 years, height 168.5±9.8 cm, mass 64.5±13.5 kg) ran on the treadmill while wearing a minimalist running shoe with (IN) and without (OUT) added cushioning. Following determination of each subjects preferred running speed, a running warm-up was performed while wearing the test shoes during IN and OUT. Study subjects were then instrumented with surface electrodes on the left erector spinae, rectus abdominis, and biceps femoris, while also being instrumented with electrogoniometers placed over the lumbar spine, and the lateral side of the left knee. Subjects ran on the treadmill for two minutes at their preferred speed after which data collection took place for an additional 45 seconds with shoe condition order being counterbalanced. The first ten running strides were extracted for analysis. Muscle activity and kinematics were extracted using a telemetry system for electromyography (TeleMyo 2400T, G2; Noraxon USA Inc. Scottsdale, AZ; 1500Hz), with the fully rectified, normalized signal from the surface electrodes being used to calculated average muscle activity for the erector spinae (ES), the rectus abdominis (RA), and the biceps femoris (BF) during the stance phase of running, using peak extension from the knee electrogoniometer to determine stance. Kinematic analysis was performed using the knee and back electrogoniometers which included calculating knee range of motion (KnROM), knee angle at the moments of peak extension (KnExt) and peak flexion (KnFlx), low back range of motion (BaROM), and average flexion/extension of the low back (BaPos). Following subject testing, Paired T-tests (α=0.05) were performed to compare the test conditions. Impact testing of the test shoes was also performed at the heel (HL) and forefoot (FF) of all shoes during IN and OUT using a mechanical impact tester (Exeter Research Inc. Brentwood, NH; 3000Hz). Testing followed a modified American Standard for Testing Materials (ASTM) test procedure (ASTM F-1614). A missle head (mass 8.5kg; diameter 45mm) was dropped from a height of 50 mm with twenty pre-impacts being performed, followed by data being collected during ten test impacts. Peak acceleration (PA) and peak pressure (PP) were extracted from test results, and Independent T-tests (α=0.01) were used to separately compare HL and FF during IN and OUT while also comparing IN and OUT during HL and FF. Results for impact testing showed differences between HL and FF during IN for all variables, with differences between IN and OUT being observed during HL and FF for all variables. Results for KnFlx showed increases in maximum knee flexion when cushioned inserts were placed in the shoes (32.2±4.7° with inserts vs. 30.3±5.5° without inserts). These results suggest that differences in shoe cushioning material do not significantly affect mechanics of the low back during running, although implications for knee stiffness do exist.