Various models have been used to describe distance running technique. Among these, the mass-spring model is fairly simple to use and apply, but when employed as a model, does not predict vertical force accurately especially when a heel strike is exhibited. The purpose of this article is to demonstrate how the mass-spring model can be modified to provide a simple, yet accurate prediction of ground reaction forces in distance running. Sixteen subjects ran on a force instrumented treadmill. Vertical forces during running at a self-selected pace were collected at 500 Hz. Vertical stiffness was calculated using the conventional mass-spring model with a constant stiffness and then a high-low method where stiffness was varied from a high to low value during the heel strike. Fishers z-test was used to compare correlations between predicted and measured ground reaction forces for each method of calculating stiffness. The high-low method of calculating stiffness provided a better fit of predicted to measured ground reaction forces than the constant stiffness method (p < 0.01). The high-low method of calculating stiffness avoids the difficulties of applying multiple masses, springs, or dampers while simply, yet accurately matching predicted to measured ground reaction forces.