Impact forces analysis in heel-toe running is often used to examine the reduction of impact forces for different running shoes and/or running techniques. Body mass is reported to be a dominant predictor of vertical impact force peaks. However, it is not evident whether this finding is only true for the real body mass or whether it is also true for additional masses attached to the body (e.g., running with additional weight or heavy shoes). The purpose of this study was to determine the effect of additional mass on vertical impact force peaks and running style. Nineteen subjects (9 males, 10 females) with a mean mass of 74.2 kg/56.2 kg (SD = 10.0 kg and 6.0 kg) volunteered to participate in this study. Additional masses were attached to the shoe (.05 and .1 kg), the tibia (.2, .4, .6 kg), and the hip (5.9 and 10.7 kg). Force plate measurements and high-speed film data were analyzed. In this study the vertical impact force peaks, F_zi, were not affected by additional masses, the vertical active force peaks, F_za, were only affected by additional masses greater than 6 kg, and the movement was only different in the knee angle at touchdown, ϵ₀, for additional masses greater than .6 kg. The results of this study did not support findings reported earlier in the literature that body mass is a dominant predictor of external vertical impact force peaks.