Aerobic Exercise (cont.)
How aerobically fit can we be?
The average sedentary adult will reach a level
of oxygen consumption close to 35 ml/kg/minute during a maximal treadmill test
(where you're asked to walk as hard as you can). Translated, that means the
person is consuming 35 milliliters of oxygen for every kilogram of body weight
per minute. That'll get you through the day, but elite athletes can reach values
as high as 90 ml/kg/minute! How do they do it? They may have good genes for one, but they also
train hard. And when they do, their bodies adapt. The good news is that the
bodies of mere mortals like the rest of us adapt to training too. Here's how.
What are the fitness benefits of aerobic exercise?
How our bodies adapt
Here's what happens inside your body when you do aerobic
- Your heart gets stronger and pumps more blood with each
beat (larger stroke volume). Elite athletes, as I just mentioned, can have
stroke volumes more than twice as high as average individuals. But it's not just
that. Conditioned hearts also have greater diameter and mass (the heart's a
muscle too and gets bigger when you train it), and they pump efficiently enough
to allow for greater filling time, which is a good thing because it means that
more blood fills the chambers of the heart before they pump so that more blood
gets pumped with each beat.
- Greater stroke volume means the heart doesn't
have to pump as fast to meet the demands of exercise. Fewer beats and more
stroke volume mean greater efficiency. Think about a pump emptying water out of
a flooded basement. The pump works better and lasts longer if it can pump larger
volumes of water with each cycle than if it has to pump faster and strain to get
rid of the water. High stroke volume is why athletes' hearts don't pump as fast
during exercise and why they have such low resting heart rates; sometimes as low
as 40 beats per minute, whereas the average is 60-80 beats per minutes.
- Downstream from the heart are your muscles, which get more efficient at consuming oxygen when you do regular aerobic exercise (remember, "consuming" oxygen means that the muscles are taking the oxygen out of the blood). This happens because of an increase in the activity and number of enzymes that transport oxygen out of the bloodstream and into the muscle. Imagine 100 oxygen molecules circulating past a muscle. You're twice as fit if the muscle can consume all 100 molecules than if it can only consume 50. Another way of saying it is that you're twice as fit as someone if your VO2 max is 60ml/kg/min. and theirs is 30ml/kg/min. In terms of performance in this scenario, you'll have more endurance because your muscles won't run out of oxygen as quickly.
- Mitochondria inside the muscle increase in number and activity.
Mitochondria are the powerhouses of your cells. They do all the heavy-duty work
to keep you moving. They use the oxygen to burn the fat and carbohydrate that
makes you go. The good news is that they
increase in number and activity, by as much as 50%, in just a matter of days to
weeks in response to regular aerobic exercise in adults of all ages.
Burn, baby, burn
I mentioned that fat and carbohydrate are the fuels our muscles burn. The
difference between them is that fat is high-test; it contains 9 calories per
gram whereas carbohydrate has only 4, and so you get more energy and can go
farther on a gram of fat than on a gram of carbohydrate. You want to burn fat
because it's such an efficient fuel, plus it's nice to lose some of your excess
fat! The catch is that you need more oxygen to burn fat because it's denser than
carbohydrate. The good news is that your body gets better at using oxygen and
burning fat when you do regular aerobic exercise; like I described, your heart
pumps more blood, your muscles consume more oxygen, and you have more