oa Running mechanics and spring-mass behavior over 100-, 200- and 400-m treadmill sprints

Background & Objectives: Sprint races are extremely popular in athletic competitions. Although those events have hitherto mainly been studied from a split-time perspective, at present, the nature of the adjustments in sprint mechanics and the extent to which stride parameters are altered are yet to be fully elucidated. This study was undertaken to test the hypothesis that single, maximal treadmill sprints (100 m, 200 m and 400 m) modify running kinetic-kinematic variables and spring-mass characteristics, with larger magnitude changes as sprint distance increases. Methods: Eleven physically active males performed 100-, 200- and 400-m running sprints on a validated instrumented sprint ergometer (ADAL3D-WR, Medical Development - HEF Tecmachine, Andrézieux-Bouthéon, France), which allowed subjects to run and produce speed ''freely'', i.e. with no predetermined belt speed imposed. Vertical and horizontal ground reaction forces were measured continuously (averaged every 50 m distance intervals) and used to determine stride parameters and spring-mass behavior. Results: Compared with the initial 50 m, running speed decreased (P<0.001) by 8±2%, 20±4% and 39±7% at the end of the 100, 200 and 400 m, respectively. All sprint distances (with the exception of stride length in the 100 m) induced significant (P<0.05) increments in contact (+7±4%, +22±8% and +36±13%) and swing times (+12±15%, +16±15% and +16±9%) and decrements in stride lengths (-1±4%, -5±5% and -41±2%) and frequencies (-6±3%, -13±7% and -22±8%) at the end of the 100, 200 and 400 m, respectively. Running kinetics all significantly decreased (P<0.001): mean vertical and total ground reaction forces for each step were reduced by about 2, 7 and 17% in the 100, 200 and 400 m, while net horizontal forces decreased by 14±17%, 41±21% and 121±35% on average. On the 100 m, the only spring-mass parameters to change (P<0.001) were an increased center of mass vertical displacement and a decreased vertical stiffness (+13±7% and -12±6%, respectively), with no change in leg stiffness. All the leg-spring variables significantly changed (P<0.05) over the 200 and 400 m: peak vertical forces: -6±3% vs. -14±9%; center of mass vertical displacement: +36±20% vs. +48±23%; leg compression: +8±8% vs. +8±9%; vertical stiffness: -30±11% vs. -63±20%; leg stiffness: -12±8% vs. -23±18%, respectively. Multiple linear regression analysis with best subset approach showed that 96% of the variation in speed was explained by stride length (ß=1.7) and frequency (ß=4.5) during 100 m. Stride length (ß=3.7) was also important predictor for speed during 200 m along with flight (ß=-24.9) and contact (ß=-25.6) times (r2=0.98), while leg stiffness showed a negative association (ß=-0.08) for predicting speed in the 400 m sprint model together with stride length (ß=-24.8) and contact time (ß=1.9) (r2=0.92). Conclusion: Along with speed, the magnitude of changes in running kinematics-kinetics and spring-mass behavior over short (100 m), intermediate (200 m) and long (400 m) treadmill sprint increased with sprint distance. Stride length was positively associated with speed in all sprint distances. Future research should evaluate the effects of various individualized training interventions on optimizing musculo-skeletal stiffness regulation, and their effects on sprinting speed development and maintenance.