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.