Body position determines propulsive forces in accelerated running

Rapid accelerations during running are crucial for the performance in a lot of sports. While high propulsive forces are beneficial to forward acceleration, vertical forces have to be small to attain high stride frequencies. However, propulsive and vertical force components cannot be altered independently, because the resultant force vector affects the angular momentum of the body. Therefore we hypothesized that propulsive forces in accelerated running mainly depend on body position regardless of performance level. In our cross-sectional study 28 male and 13 female physical education students performed submaximal and maximal accelerations. Ground reaction forces and whole body kinematics were recorded. Higher accelerations were generated by lower, but more forward oriented forces. The orientation of the maximum force vector strongly correlated with the forward lean of the body at toe-off (r = 0.93, p <= 0.001). All subjects demonstrated similar propulsive forces at equal body positions. This indicates an external constraint of propulsive force application by the mechanical requirement of running to maintain a stable body posture. Faster subjects utilized a more posterior foot plant or a longer ground contact time. Both strategies facilitated greater forward leans of the body which finally resulted in greater propulsive forces. Consequently, maximizing forward propulsion requires optimal, not maximal force application. (C) 2009 Published by Elsevier Ltd.