The purpose of this dissertation was to investigate, in vivo, the effect of low back pain status on lumbo-sacral (L5S1) joint reaction force and moment profiles, and also on pelvis-trunk coordination during locomotion. We studied three groups of runners: runners with low to moderate LBP (LBP), runners who had recovered from a single bout of acute LBP (RES), and runners who never had LBP (CTRL). We developed an inverse dynamics model to investigate L5S1 mechanics. Results indicated that L5S1 reaction forces increased as stride length increased (p<0.024) in healthy runners, as did peak sagittal L5S1 moments (p=0.078). Subsequently, we used this model to examine L5S1 differences between our three experimental groups at their preferred running speed and at 3.8 m/s. While differences were observed between speeds for many L5S1 variables, and between groups for peak vertical GRF, no differences were found between groups for lumbar angle ROM, peak L5S1 joint reaction force, or L5S1 joint moment. In a further study, pelvis and trunk range of motion (ROM), relative phase coordination (CRP) and coordination variability (CRPvar) were compared between our groups over a range of locomotor speeds. We found increased pelvis-trunk CRP in the frontal plane for the LBP group as compared to the CTRL group during walking (p=0.029). During running, we observed increased pelvis axial ROM for the LBP group as compared to CTRL (p=0.010), and increased CRP for CTRL as compared to both RES (p=0.021) and LBP group (p=0.025), and increased CRPvar for CTRL as compared to LBP group (p=0.019). Findings from the second and third studies demonstrated differences between clinical groups despite the relative low levels of disability (mean Modified Oswestry Disability Questionnaire score = 7.9±6.2%), and suggest that mechanical and coordinative differences exist between those with and without LBP. Further, our RES group demonstrated characteristics common to both extreme groups (CTRL and LBP), supporting previous literature regarding residual performance effects. Our findings further suggest that the RES group represented a transition stage between the CTRL and LBP groups.