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.

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.

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.

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.

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.

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.

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.