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Concord Field Station, Museum of Comparative Zoology, Harvard University, Bedford, Massachusetts 01730
We twice tested the hypothesis that
top running speeds are determined by the amount of force applied to the
ground rather than how rapidly limbs are repositioned in the air.
First, we compared the mechanics of 33 subjects of different sprinting
abilities running at their top speeds on a level treadmill. Second, we
compared the mechanics of declined (
6°) and inclined (+9°)
top-speed treadmill running in five subjects. For both tests, we used a
treadmill-mounted force plate to measure the time between stance
periods of the same foot (swing time, tsw) and
the force applied to the running surface at top speed. To obtain the
force relevant for speed, the force applied normal to the ground was
divided by the weight of the body (Wb) and
averaged over the period of foot-ground contact (Favge/Wb). The top speeds of
the 33 subjects who completed the level treadmill protocol spanned a
1.8-fold range from 6.2 to 11.1 m/s. Among these subjects, the
regression of Favge/Wb on top
speed indicated that this force was 1.26 times greater for a runner
with a top speed of 11.1 vs. 6.2 m/s. In contrast, the time taken to
swing the limb into position for the next step
(tsw) did not vary (P = 0.18).
Declined and inclined top speeds differed by 1.4-fold (9.96 ± 0.3 vs. 7.10 ± 0.3 m/s, respectively), with the faster declined top
speeds being achieved with mass-specific support forces that were 1.3 times greater (2.30 ± 0.06 vs. 1.76 ± 0.04 Favge/ Wb) and minimum
tsw that were similar (+8%). We conclude that
human runners reach faster top speeds not by repositioning their limbs
more rapidly in the air, but by applying greater support forces to the ground.
locomotion; running performance; running mechanics
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