Purpose Acceleration phases require additional mechanical and metabolic power, over and above that for running at constant velocity. The present study is devoted to a paradigmatic example: the 100-m dash, in which case the forward acceleration is very high initially and decreases progressively to become negligible during the central and final phases. Methods The mechanical ( Pext ) and metabolic ( Pmet ) power were analysed for both Bolt`s extant world record and for medium level sprinters. Results In the case of Bolt, Pext and Pmet attain peaks of ˜ 35 and ˜ 140 W kg -1 after ˜ 1 s, when the velocity is ˜ 5.5 m s -1 ;they decrease substantially thereafter, to attain constant values equal to those required for running at constant speed (˜ 18 and ˜ 65 W kg-1) after ˜ 6 s, when the velocity has reached its maximum (˜ 12 m s-1) and the acceleration is nil. At variance with Pext , the power required to move the limbs in respect to the centre of mass (internal power, Pint ) increases gradually to reach, after ˜ 6 s a constant value of ˜ 33 W kg -1 . As a consequence, Ptot (= Pext + Pint ) increases throughout the run to a constant value of ˜ 50 W kg -1 . In the case of the medium level sprinters, the general patterns of speed, mechanical and metabolic power, neglecting the corresponding absolute values, follow an essentially equal trend. Conclusion Hence, whereas in the last part of the run the velocity is about twice that observed after ˜ 1 s, Pext and Pmet are reduced to 45-50% of the peak values.
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