Continuous change in spring-mass characteristics during a 400m sprint

Abstract 

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.

Keywords: Biomechanics, Stiffness, Athletes, Fatigue