Force x is the force that gives the runner power to move forward quickly, and force g acts through the center of mass of the runner, as represented by the purple dot.
Sprinting around a turn, such as the 200 m and 400 m races, is proved to result in a longer race time than if the race were run on a straight track. This is due to the centrifugal force experienced by the runner as he goes around the curve of the track. This has the effect of decreasing the force available for the runner to propel himself around the track. This effect is more clear the smaller the turn radius is.
The figure below shows the contact forces acting between the runner’s foot and the curved track.