Research concerning forefoot strike pattern (FFS) versus rearfoot strike pattern (RFS) during running has focused on the ground reaction force. The main purpose of this study was to determine the internal loading of the joints for each strike pattern. A secondary purpose was to determine if a converted runner can adequately represent the habitual population for both FFS and RFS. Using inverse dynamics to calculate the net joint moments and reaction forces, and optimization techniques to estimate muscle forces we determined the axial compressive load and rate of loading at the ankle, knee, and hip. Effect sizes (ES) were used to categorize comparisons into small (ES>0.2), medium (ES>0.5) and large (ES>0.8) effects. Subjects consisted of 15 habitual FFS and 15 habitual RFS competitive runners. Each subject ran with their habitual strike pattern and then converted to the opposite strike pattern. Converting to a FFS pattern had the opposite effect of converting to a RFS pattern at the knee and hip. Converted FFS runners had decreased knee (ES=0.98) and hip (ES=1.23) contact forces while converted RFS runners had increased knee (ES=0.77) and hip (ES=0.71) contact forces. Habitual FFS runners had higher contact forces at the ankle (ES=0.51) and hip (ES=0.81) compared to habitual RFS runners. The habitual FFS runners also demonstrated a substantially larger compressive load at the ankle and knee during the first 40% of stance. This could be important when considering the high number of cyclical loads experienced by these runners.