Environmental Engineering Reference
In-Depth Information
this threshold may rise to 85% of the maximum power.
The maximum metabolic power of anaerobic glycolysis
is 1.8-3.3 kW for average persons and up to 8.3 kW for
trained persons; peak capacities average 75-100 kJ and
can go up to 205 kJ, as much as three to four times the
CP-derived total. Short (30-180 s) exertions are largely
energized in this way. For example, models of 100-m
runs (using speed curves of world champions) show an-
aerobic metabolism contributing 95% of all energy (Arsac
and Locatelli 2002).
All prolonged efforts are powered primarily through
aerobic (oxidative) recharge. Because the body's oxygen
store of about 1 L can support moderate exertion for
no more than 0.5 min, the subsequent demand requires
linear increases in pulmonary ventilation. Sustained hu-
man power is largely a function of maximum oxygen
intakes. Maximum aerobic power, sustainable for about
90 min, is only 10-20 mL/kg
min in people with
chronic pulmonary diseases, and its range is 20-55 ml/
kg
min, or 350-1350 W, in adults. A metabolic range
of 600-900 W would include most mildly active people,
but the rate goes over 90 ml/kg
min for elite endurance
athletes. This is the equivalent of more than 2 kW, a flux
25 times BMR, an impressive metabolic scope in com-
parison with most mammals (see section 4.1). Aerobic
capacities are slightly lower in women than in men of
the same age, and after the adolescent peak the annual
decline range is 0.4-1 mL/kg
min, so by the age of 65
aerobic are just capabilities 25-30 ml/kg
min. Peak aer-
obic capacities are 1.5-3.5 MJ for healthy adults and sur-
pass 10 MJ for trained athletes, with maxima at 45 MJ.
Both glycogen and fatty acids are the substrates of the
oxidative metabolism, with the acids' share rising to 70%
during prolonged activities. But it is important to note
that every sustained exertion also uses a small share of
anaerobic glycogen breakdown, and conversely, as the
duration of brief (predominantly anaerobic) exertions
increases, aerobic recharge supplies rising shares of en-
ergy for efforts lasting 30-120 s. Individual limits of
performance can change. Genetic endowment is a pre-
requisite of exceptional performance (Bouchard, Malina,
and Perusse 1997), but the aerobic power of average
individuals can go up by 20% with training. The concur-
rent increase of blood volume, roughly 1 mL/mL of aer-
obic capacity, raises cardiac filling pressure, output, and
blood delivery to muscles and skin for cooling.
Walking and running are the two physical activities
during which most individuals experience the limits of
their performance, whether ascending stairs, carrying
loads, or racing. In mechanical terms, walking can be
modeled simply as a motion of an inverted pendulum,
with the body mass center at its lowest point at heel
strike and at highest level at midstance. Theoretically, no
mechanical work is required to move a pendulum along
an arc, but work is needed for the transition from one
stance limb to the next, and experiments indicate that it
increases with the fourth power of step length (Donelan,
Kram, and Kuo 2002). This matters because a faster walk
is normally an equal combination of increased step length
and higher step frequency. In actual walking nearly half
of the metabolic cost goes into generating horizontal
propulsive force (Gottschall and Kram 2003).
Many studies have measured the energy cost of walk-
ing on the level, all indicating lower efficiencies at speeds
both below and above the optimum range of 5-6 km/h.
The minimum costs of walking on the level are about
1
:
5G0
:
5J/kg
m at the speed of 1.3 m/s. Higher
speeds and walking uphill bring linear cost increases
across a broad range of speeds, as much as 18 J/kg
m
on a 45
slope (Minetti et al. 2002). The gross energy
