The purpose of the present study was to utilise a spring-mass model to (1) continuously measure vertical stiffness (K vert ) and leg stiffness (K leg ) over an entire 400m sprint, and (2) investigate the relationship between leg spring stiffness (K vert and K vert ) and the performance characteristics of mean forward running velocity (V forwad ), mean stride frequency (f stride ), and mean stride length (L stride ). Eight well-trained male athletes performed a 400m sprint with maximal effort on an outdoor field track. K vert was calculated from the subjects’ body mass, ground contact time and flight time at each step. V forwad , f stride and L stride were determined from video images. K vert and V forwad peaked at the 50–100m interval, and consistently decreased from the middle to the later part of the sprint. K leg peaked at first 50m interval, and remained constant from next 50m interval to finish. As compared with peak values, K vert and V forward in the last 50m decreased by about 40% and 25%, respectively. A significant positive linear relationship existed between the K vert and V forward . While K vert was significantly correlated with f stride , it had no correlation with L stride . Further, no significant positive linear relationship was found between K leg and V forward , f stride , or L stride . This result indicates that in order to keep V forward at later stage of a 400m sprint, maintaining the higher f stride through retaining a higher K vert would be necessary.