German scientists in the early 1940’s perfected a lightweight, portable system to determine indirectly the energy expended during physical activity. The activities included war-related operations traveling over different terrain with full battle gear, operating transportation vehicles, and tasks soldiers would encounter during combat operations. The person carries the 3-kg box-shaped apparatus (Figure 4) on the back. Air passes through a two-way valve, and expired air exits through a gas meter. The meter measures expired air volume and collect a small gas sample for later analysis of O2 and CO2 content, and thus determination of oxygen uptake and energy expenditure.
Carrying the portable spirometer allows considerable freedom of movement for estimating energy expenditure in diverse activities like mountain climbing, downhill skiing, sailing, golf, and common household activities. The equipment becomes cumbersome during vigorous activity; with rapid breathing, the meter under-records airflow measurements during heavy exercise.
Figure 5 depicts the bag technique. The subject rides a stationary cycle ergometer wearing headgear containing a two-way, high-velocity, low-resistance breathing valve. He breathes ambient air through one side of the valve and expels it out the other side. The air then passes into either large canvas or plastic bags or rubber meteorological balloons, or directly through a gas meter, which continually measures expired air volume. The meter collects a small sample of expired air for analysis of oxygen and carbon dioxide composition. Assessment of oxygen uptake (as with all indirect calorimetric techniques) uses an appropriate calorific transformation to convert measures of oxygen uptake to energy expenditure. Figure 6 illustrates oxygen uptake measured by the bag technique while lifting boxes of different weight and size to evaluate the energy requirements of a specific occupational task.
With advances in computer and microprocessor technology, the exercise scientist can accurately and rapidly measure metabolic and cardiovascular response to exercise. A computer interfaces with different instruments to measure oxygen uptake.
The computer performs metabolic calculations based on electronic signals it receives from the instruments. A printed or graphic display of the data appears during the measurement period. More advanced systems include automated blood pressure, heart rate, and temperature monitors, and preset instructions to regulate speed, duration, and workload of a treadmill, bicycle ergometer, stepper, rower, swim flume, or other exercise apparatus.