The purposes of this study were to define the start and end of the transition from curved to straight running, and to investigate the kinematic factors that influence the transition distance. We captured sprinting motion from 40-m before to 15-m after the connection between the curved and straight paths meet in 200-m race. This study detected the transition phase based on the trajectory of the participant that an orthogonal distance between the midpoint of hip joints and a course line. The start of the transition phase was determined as the position of the midpoint of hip joints with the maximum log-likelihood of the standard deviation. Then, the sum of the residual squares of the body trajectory evaluation quantities to the approximate straight line was calculated, and the end was determined as the frame with the maximum log-likelihood of the sum of the residual squares. Our findings revealed a difference of 25.60-m in the transition distance between participants. The transition distance had significant correlations with sprinting motion during the pre-transition phase: a positive correlation with the inward inclination angle of the trunk at right touchdown, a negative correlation with the inward inclination angle of the support leg at right touchdown, and a negative correlation with the inward inclination angle of the trunk in relation to the support leg at left touchdown. In addition, the transition distance exhibited a positive correlation with the step width at left touchdown during the transition phase. These findings suggest that the transition distance would be greater if the moment is induced by altering the direction of the force vector, and conversely, it would be shorter if the moment is generated by changing the point of action (the position of the footprint relative to the body).
