<p>Since a secure stable measurement of ground reaction force exerted by support leg during running motion is difficult to achieve in sprint running, investigating the motion generating mechanism of running motion with use of a treadmill installed with a force platform could have a great advantage in the study of running motion from a view point of the stable measurement of 3-dimensional coordinate data as well as of ground reaction force exerted by support leg on the belt during the support phase of running. The purpose of this study were to obtain not only kinematic data of running motion but also kinetic data (i.e. belt reaction force) exerted by support leg in running, and then to quantify dynamic contributions of support leg joint torques to the generation of horizontal and vertical components of the belt reaction force. The belt tensile force was set under two conditions (i.e. low:20N, high:50N of tensile belt force) The support leg was modeled as a 3-segment rigid link model consisting of foot, shank and thigh segments. The equation of motion for the support leg model was derived from a combination of three equations, such as the equation of motion for the individual segments, the equations for constraint conditions arising from the connection of adjacent segments at joints, and the equations for anatomical constraint axes at certain joints. The result obtained in this study shows that the ankle plantar-dorsal and ankle eversion-inversion torque are main contributors to the generation of the individual components of the ground reaction force under both the belt force loading conditions.</p>