Environmental Engineering Reference
In-Depth Information
cost of walking also varies linearly with sex and age: as a
group, adult slim women have the lowest, and heavier
women the highest, requirements. Uneven surfaces
(meadows, stubble field, plowed land), muddy roads, or
deep snow will raise the costs of level walking by up to
25%-35%. Level walking with light loads is not too
demanding, but heavier loads can be very taxing, espe-
cially when carried uphill.
Slow to moderate walking speed is 1.1-1.2 m/s (4-
4.3 km/h), and 1.9-2.1 m/s (6.8-7.6 km/h) is the
speed at which walkers voluntarily switch to running.
Untrained runners manage speeds of 3-4 m/s. By 2005
the best men's marathon time was run at 5.6 m/s (com-
pared to about 4 m/s in 1910); world record speeds in
10-km and 5-km races were, respectively, 6.2 and 6.4
m/s, and for 100-m races, just over 10 m/s (IAAF
2006). Running usually requires power outputs between
700 W and 1400 W (about 10-20 times BMR for
adults). Compared to other mammals (see section 4.4),
the energetic cost of human running is relatively high,
but humans are unique in virtually uncoupling this cost
from speed (Carrier 1984; Bramble and Lieberman
2004). Quadrupeds have optimum speeds and hence dif-
ferent COT for different gaits (e.g., horses walk, trot,
and gallop). COT for human walking has a similarly U-
shaped curve, but the cost of human running is essen-
tially independent of speed between about 2 m/s and 6
m/s (fig. 5.7). Empirical data show that the metabolic
COT roughly doubles from 1 J/kg
m to 2 J/kg
mas
the walking speed goes from 1 m/s to 2 m/s (3.6-7.2
km/h), but the cost of running remains fairly stable,
about 4 J/kg
m (Minetti and Alexander 1997).
Two factors explain this extraordinary capability: effi-
cient heat dissipation (see section 5.3) and bipedalism.
In quadrupeds ventilation is limited to one breath per lo-
comotor cycle (the thorax bones and muscles must ab-
sorb the impact on the front limbs as the dorso-ventral
binding rhythmically compresses and expands the thorax
space), whereas human breath frequency can vary relative
to stride frequency. People thus have an option to run
at a wide variety of speeds, but quadrupeds are largely
restricted to structurally determined optima. Faster top
running speeds are achieved by applying greater support
forces to the ground, not by a more rapid repositioning
of legs in the air. Experiments show that support forces
to the ground rise with speed, whereas the time taken to
swing the limb into position for the next step does not
vary (Weyand et al. 2000). Peak running performance
has a structural basis. The highest ground support forces
exceed the body's weight fivefold, and hence more mus-
cle (to generate the forces) and tendon and bone (to
transmit them safely to the ground) are needed than in
endurance running (Weyand and Davis 2005).
Expectedly, a study of the world's 45 fastest athletes
showed that male body mass declines from more than
75 kg for sprinters to less than 60 kg for long-distance
runners. Human excellence in running is also illustrated
by steadily improving record speeds for every distance
(fig. 5.8). Since 1910 the annual rate of improvements
has averaged less than 1 m/min, a gain imperceptible in
sprints now run at speeds over l0 m/s but a reduction of
about 30 min in running the marathon. Actually, this
42,195-m endurance race is now run by many of the
world's best athletes at a faster pace than the record 10
km run as recently as 1945. Ryder et al. (1976) believed
that the historic rate of improvements could continue for
decades to come. Whipp and Ward (1992) predicted that
by 1998 women might run the marathon as fast as men.
