Running performance, energy requirements, and musculoskeletal stresses are directly related to the action-reaction forces between the limb and ground. For human runners, the force-time patterns from individual footfalls can vary considerably across speed, foot-strike, and footwear conditions. Here, we used four human footfalls with distinctly different vertical force-time waveform patterns to evaluate whether a basic mechanical model might explain all of them. Our model partitions the body's total mass (1.0 Mb) into two invariant mass fractions (lower-limb=0.08, remaining body mass=0.92) and allows the instantaneous collisional velocities of the former to vary. The best fits achieved (R2 range: 0.95-0.98, mean=0.97±0.01) indicate that the model is capable of accounting for nearly all of the variability observed in the four waveform types tested: barefoot jog, rear-foot strike run, fore-foot strike run, and fore-foot strike sprint. We conclude that different running ground reaction force-time patterns may have the same mechanical basis.