Spring-mass model characteristics during sprint running: correlation with performance and fatigue-induced changes.

Spring-mass model properties of eight non-specialized male runners were measured during four straight 100-m sprints on an athletics track. A recently developed simple measurement method allowed to calculate leg and vertical stiffness, vertical displacement of the center of mass, and stride temporal characteristics. Changes in these mechanical parameters were studied and correlated with those of sprint performance. During the first 100 m, forward velocity showed significant variations (mean value of 8.10 +/- 0.31 m x s(-1) over the entire 100-m), while leg and vertical stiffnesses (19.5 +/- 4.3 kN x m(-1) and 93.9 +/- 12.4 kN x m(-1), respectively) remained constant. No significant link was found between mechanical and performance parameters over this first sprint. During the following three sprints, vertical stiffness, step frequency, and contact time significantly decreased (20.6 +/- 7.9%, 8.03 +/- 3.34%, and 14.7 +/- 7.2% of the first 100-m value, respectively) with decreasing maximal and mean velocities (10.9 +/- 2.0% and 7.30 +/- 5.23%, respectively), whereas leg stiffness and maximal force remained constant. Furthermore, changes between these mechanical and performance parameters were significantly related, showing the clear relationship between impairment in spring-mass model properties of the runners' lower limbs and the decrease in performance in fatigue conditions induced by the repetition of these all-out efforts.