Oxygen Consumption Results





Oxygen Consumption

The figure to the right shows VO2 plotted as a function of power for our eight subjects. Notice first that all of the subjects, regardless of size, expended nearly the same amount of energy for a specific level of work. This is expected with a bicycle ergometer. Cranking the pedal in each case required about the same number of cross-bridge cycles per minute, and thus about the same amount of ATP.

The line in the figure is not derived from our data, but is taken from the standard reference work on exercise tolerance tests. In other words, with different equipment at different locations, the same amount of oxygen is consumed at each watt level on a bicycle ergometer.

If we had measured the subjects walking up stairs or a similar type of work, we would have found that the VO2 was different for individuals of different sizes doing the same exercise.

Note that our resting energy expenditures vary quite a bit. Mainly this is because points at zero watts were taken both at complete rest and with the subject peddling against zero resistance. But also this is because the resting energy expenditure depends on body size. As a result there is a small effect of size on the data.

VO2max

Given that all of our subjects gave an excellent effort, the highest VO2 recorded for each individual is close to the VO2max for that individual. And looking at population averages, the numbers are as expected for fit young adults. The absolute VO2max is strongly dependent on the size of the individual.  To compare VO2max numbers between individuals of different sizes, the number is divided by the body weight (usually this is expressed in units of ml O2 per kg).  

In this type of protocol, it is typically not possible to see the points reach a plateau. Finer gradations around the VO2max would be required.

Factors Influencing VO2max

The maximal oxygen uptake also depends on a person's genetics and level of training for endurance exercise. The key determinant of VO2max is the ability of the cardiovascular system to pump blood to the exercising muscles.  We will examine this more in the spring quarter when we study cardiovascular physiology.

A recent clinical study with older patients found that VO2max is one of the best predictors of mortality, especially if cardiovascular disease is present.



Now let's focus on the results from one subject (Tuesday, 8:30). To the right are the VO2 measurements for this subject, who we will continue analyzing below. The blue line is the same as in the graph above for all lab sections. 

Ventilation During an Exercise Tolerance Test

The graph to the right shows the ventilation (Ve) for the subject as a function of watts. Notice that the points follow a linear relationship at lower levels of work. This, of course, is as expected, since proportionately more ventilation is required to deliver more oxygen to the lungs.

But above about 160 watts there is a change, with ventilation increasing considerably more than expected based on the earlier relationship. As we discussed in lab, this is because the subject has passed the lactate threshold (anaerobic threshold), in which lactic acid begins accumulating in the body. This causes a disproportionate increase in breathing through stimulation of the peripheral chemoreceptor. The exhausted subject is breathing harder in order to remove CO2 from the blood.  Remember this key chemical reaction:

Increasing ventilation to remove CO2 from the blood causes respiratory alkalosis. The above reaction is shifted to the left, thereby causing a net loss of H+ and HCO3-. Respiratory alkalosis helps to raise the blood pH to compensate for the acidosis caused by lactate accumulation.


Respiratory Quotient (R.Q.)

The R.Q., which is the ratio of the CO2 exhaled to the O2 consumed, is shown to the right for the same subject as in the graph above of Ve. If a person is burning pure carbohydrate the ratio will be 1.0.

At low levels of exercise, our subject had an R.Q. of about 0.8-0.9. This is an expected value in a resting person burning a mix of fats and proteins as well as carbohydrates.

Note that the initial R.Q. value is above 1.0. This is due to hyperventilation as the subject anticipates the upcoming exercise. With hyperventilation, the R.Q. is higher than the actual resting level, because the subject is blowing off more CO2 than expected. Then as the subject begins to exercise at lower rates, the R.Q. settles down to the expected resting level as physiology, rather than anticipation, controls the breathing.

But above the lactate threshold we expect the R.Q. to steadily increase as H+ stimulates the peripheral chemoreceptor and as H+ reacts with bicarbonate in the blood to liberate CO2. This is what we see above about 160 watts. The final R.Q. at exhaustion was above 1.30, which is what we would expect for a good effort with lots of lactate produced near the end.

Converting Oxygen Consumption to Energy

As discussed in the lab manual, the oxygen consumption can be converted directly to kcal/min by multiplying by 4.8 kcal/l O2. One kcal, of course, is equal to one dietary calorie, such as are used on food packages.

Another energy unit is the metabolic equivalent (MET). This is useful, since it takes into account the size of the subject. One MET is defined to be 1.0 kcal/hr per kg. A person expends, for example, about 1.0 MET watching TV. Clinical studies looking at the effects of exercise typically express levels of exercise in terms of METs.

Inspecting the definition of a MET above, you will see that at 1.0 MET your energy expenditure in kcal/hr is equal to your weight in kilograms. For other activities, you find your MET level by multiplying your expenditure at 1.0 MET by the number given for the activity in the table on page 46 of the lab manual. Approximately what is your expected energy expenditure on a typical day? This converts directly to dietary calories burned during that day.

In the lab, our first subjects did different exercises, and their exhaled gas was collected in a Douglas bag.  This allowed us to measure the exhaled volume and determine their oxygen consumption and rate of energy expenditure for the exercise. Be sure you understand how we determined the MET values for each exercise.

Calculations (for the lab practical)

See the review sheet for the first lab practical. You should be able to calculate VCO2, energy expenditure, and METs as we did in the first part of the lab.  As well, you should be able to calculate R.Q., and know how to calculate VO2 if you know the R.Q. and the VCO2.  Finally, know how to determine energy expenditure for a subject from MET values, as in the table on p. 49.

On the lab practical, a calculation question might begin with a partially completed table from the lab manual. You might then be asked to fill in another square based on a one- or two-step calculation. The entire laboratory exercise will be available for reference.