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There are two opinions on Frequency and
duration among the experts.
- First, It takes 15-20 minutes to get your
metabolism into the fat burning zone many people desire for
an aerobic workout. Once you have reached this level, your body
tends to obtain more of its energy from fat than carbohydrates.
Using this argument, a single 90-minute workout will allow you
to exercise in this "fat burning" zone for at least 70 minutes
while two 45-minute sessions would allow you to be in this zone
for at least 50 minutes (2 x 25). This logic supports a single,
longer workout.
- Second, For 6 or so hours after a
workout your body remains in "afterburn" mode, burning more
calories at rest than it would have if you had not worked out.
Using this logic, two sessions would produce two afterburn periods
and result in more fat being burned than would be for a single
workout session.
The real bottom line is that if you exercise
for 90 minutes a day, you're interested in more than just
fitness(don't get me wrong, this is perfectly OK). To stay
in reasonable cardiovascular shape, you need to perform aerobically
for 20-30 minutes at least 3 times a week. If you wish to
be in better than "the minimum acceptable" shape, remember
that the returns are not geometric (you won't be in twice
as good shape if you workout for twice as long). Therefore,
if you're going to workout for 90 minutes a day, splitting
the time between one or two sessions probably doesn't make
a significant fitness-level difference.
You are always burning fat at
one rate or another, but you burn most when your body is in its
aerobic range. A good rule of thumb is that after 20 minutes in
your aerobic zone, you will be burning more fat than carbos. Covert
Bailey, in Smart Exercise, states that you will
be burning fat after only twelve minutes of aerobic exercise. If
you can increase your aerobic activity to 30 minutes or longer,
you will be burning a larger percentage of calories from fat. There
is still some disagreement as to which is better - longer duration
at lower intensity, or shorter duration at higher intensity. If
you are limited in time, then the higher intensity will maximize
your aerobic benefits in a shorter amount of time. If you can work
for a longer duration at a lower intensity, you will decrease your
chance of injury. The object is to burn more calories than you take
in. 3,500 calories equals l pound of fat. Your muscles will continue
to burn fat after both aerobic and anaerobic (muscle training) exercise.
This is perhaps the most common question raised
by individuals exercising for the purpose of either weight loss
or simply weight control. This stems from the recognition that aerobic
exercise is a significant adjunct to any weight loss program, that
is diet plus aerobic exercise produces more weight loss than diet
alone. In addition, the weight lost with exercise tends to be a
higher percentage of fat.
Exercise can be grouped into three broad levels
of intensity, mild, moderate, and high. Mild intensity is a comfortable
walking pace and can be sustained almost indefinitely, moderate
intensity is equal to an average cardiovascular conditioning workout
(able to talk, but not sing) and can be sustained (in a trained
individual) for upwards of 3 - 4 hours, and high intensity is not
able to talk and can only be sustained for 30 - 45 minutes.
Based on recent and very detailed research studies,
in terms of absolute fat burning, a moderate intensity workout burns
the most fat. At a heart rate equal to about 75% of max, fat burning
will approach 0.5 grams - 1.0 grams of fat per minute. There is
a weight dependence with the lower end referring to a 100 pound
individual and the upper end to a 200 pound person. As the duration
continues (greater than 1 hour), fat burning can increase slightly
(another 10%).
At a mild intensity, the majority of calories
expended (85 - 90%) are fat calories, but the absolute level is
only about 60% of the moderate intensity. At high intensity levels,
fat burning declines to a level of about 65% of the moderate pace,
as sugar burning supplies the rest. The high rate of sugar burning
exhausts the limited sugar supply in muscles and causes muscular
failure.
The only caveats for the above burn rates are
that these numbers are derived from individuals who were already
aerobically trained and were conducted in the AM before breakfast.
Less fit individuals are known to burn less fat and more sugar (part
of aerobic conditioning is greater reliance on fat burning for energy).
Exercising after a meal will tend to promote more sugar burning.
Consumption of sugar during an exercise session will also tend to
retard fat burning in favor of the sugar. These numbers were derived
from cycling and so the absolute numbers can be increased if exercises
that involve more muscle groups are utilized (running, rowing, etc.).
From peak energy production rates for various exercises, rowers
might reach about 40% higher.
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