The purpose of this study was to calculate, as a function of time, segmental contributions to the vertical ground reaction force Fz from positional data for the landing phase in running. In order to evaluate the accuracy of the method, time histories of the sum of the segmental contributions were compared to Fz(t) measured directly by a force plate. The human body was modeled as a system of seven rigid segments. During running the positions of markers defining these segments were monitored using a video analysis system operating at 200 Hz. Special care was taken to minimize marker movement relative to the mass centers of segments, and low-pass cutoff frequencies of 50 Hz (markers defining leg segments) and 15-20 Hz (markers defining upper body) were used in filtering the position time histories so as to ensure that high signal frequencies were preserved. The magnitude of the high-frequency peak in Fz, also known as 'impact force peak', was estimated with errors less than 10%, while the time of occurrence of the peak was estimated with errors less than 5 ms. It would appear that the positional data were sufficiently accurate to be used for calculation of intersegmental forces and moments during the landing phase in running. Analysis of the segmental contributions to Fz(t) revealed that the first peak in Fz has its origin in the contribution of support leg segments, while its magnitude is determined primarily by the contribution of the rest of the body. These contributions could be varied independently by changing running style. It follows that if the possible relationship between 'impact force peaks' and injuries is to be investigated, or if the effects of running shoe and surface construction on these force peaks are to be evaluated, the calculation of segmental contributions to Fz(t) is a more suitable approach than measuring only Fz(t).