Oxygen cost

The oxygen cost of performing work (V
O2) is primarily dependent upon the
work rate. As work rate increases V·
O2 increases linearly for moderate work
rates (those below the lactate threshold). The oxygen cost is reflective of the
metabolic cost of the exercise (see Section B) and is usually measured in
the laboratory using indirect calorimetry. Expired air is collected in Douglas
bags, or sampled directly with an on-line gas analyzer, and the fractions of
expired oxygen (FEO2) and carbon dioxide (FECO2) in the expired air measured.
The volume of air expired per minute is usually measured via a dry gas meter,
and is then recorded as the minute ventilation, or volume expired (VE). Dry
gas meters usually possess internal thermometers in order that the temperature
of the expired air can be recorded.
In the laboratory an experiment might involve a subject cycling at 100 W for
10 minutes whilst expired air is collected during the final minute of exercise.
When the air in the Douglas bag has been sampled for O2 and CO2, and
the quantity of air, and temperature of air in the bag have been measured,
all the student now requires in order to complete the calculation of oxygen
uptake is the room temperature and atmospheric temperature, pressure and
relative humidity.

As any volume of air is influenced by atmospheric temperature, pressure and
water vapor, it is important that volumes are reported as a standard against
which all laboratories can compare results. So, oxygen uptake is calculated
using the volumes that were recorded at atmospheric (A) temperature (T) and
pressure (P). These volumes would also be saturated (S) with water vapor. Thus
they are said to be measured at ATPS. In order to calculate oxygen uptake these
volumes need to be corrected to a standard (S) temperature (T: 273 K or 0°C),
standard pressure (P: 760 mmHg), and the volume that they would be in dry
air (D). Thus the corrected volume is said to be recorded as STPD.

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