Tibial stress fractures are one of the most common and potentially serious overuse injuries in runners. Despite the fact that the tibial stress fractures are related to vertical ground reaction force at early contact phase, a few study has been done for quantification of the variability of the risk parameters. The aim of this study was to quantify the variability of risk parameters for the stress fractures in able-bodied runners. Eight male subjects were asked to run at three different velocities (2.5, 3.0, 3.5 m/s) on a treadmill mounted on the force plates. From the vertical ground reaction force, we calculated coefficient of variation (CV) following lower extremity loading variables; vertical impact peak, vertical instantaneous loading rate and vertical average loading rate. We found that there were no significant differences in the CV of the loading variables in each speed, indicating that variability of loading variables during running is independent from running speeds.