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Heartline Magazine July - September 2005
What is exercise?
By Kim Clarke-Grant
Definition: Any activity that results in contraction of
skeletal muscle.
The term is usually used in reference to any activity that
promotes physical fitness. Although muscle contraction is the common element of
all forms of exercise, many other organs and systems are affected; for example,
the heart and lungs. During exercise the heart pumps increased volumes of blood
to supply oxygen and nutrients and remove carbon dioxide and metabolic wastes,
the respiratory system handles an increased workload, exchanging oxygen and
carbon dioxide between the blood and the atmosphere.
The nervous system and various hormones have important roles
as well, integrating the body’s response to exercise and regulating the
metabolic changes that occur in muscles and other tissues. Another critical role
of the cardiovascular and respiratory systems during exercise is to get rid of
the heat produced by increased metabolism. Many people also find that regular
exercise enhances their sense of mental well being along with their physical
health.
Physiology of Exercise
Contraction of skeletal muscles, the muscles under conscious
control, is the primary physiological event during exercise. At the molecular
level, muscle contraction occurs when large proteins called Actin and Myosin
slide together to shorten muscle fibers. The energy for contraction and
relaxation of skeletal muscle is provided by Adenosine Triphosphate (ATP), a
high-energy molecule formed during the break down of glucose or fats. Glucose
can be stored in muscle as glycogen, and enters exercising muscle via the blood.
The break down of glycogen or glucose to provide energy for
exercise occurs in one of two ways, depending on the presence of available
oxygen to the muscle, which in turn depends on the type of exercise being
performed. If oxygen is not available (Anaerobic activity), glycogen or glucose
will be broken down by Glycosis. If oxygen is available, glycogen or glucose
will be broken down by the Citric Acid Cycle, i.e. glucose will react with
oxygen to produce water and carbon dioxide. A portion of the energy released
from one molecule of glucose is utilized to produce ATP.
In the Anaerobic process the breakdown of glucose stops at
an early point, producing lactic acid and two molecules of ATP. This process
results in an oxygen deficit, which is repaid when oxygen becomes available.
When skeletal muscle is heavily worked, the acute soreness
that results is due partly to a build-up of lactic acid. The presence of lactic
acid may also present as a burning sensation in the muscles. Unlike anaerobic
exercise, aerobic exercise uses oxygen to keep large muscle groups moving
continuously at an intensity that can be maintained for at least 20 minutes.
Aerobic exercises use several major muscle groups throughout the body resulting
in greater demands on the cardiovascular and respiratory systems to supply
oxygen to the working muscles. Aerobic exercise includes walking, riding,
dancing and jogging, and is the form recommended for reducing the risk of heart
disease and increasing endurance (activity tolerance).
People of all ages, both male and female, benefit from
regular physical activity. Significant health benefits can be obtained by
including a moderate amount of physical activity, e.g. 30 minutes of brisk
walking or raking leaves, on most, if not all, days of the week. Additional
health benefits can be gained through greater amounts of physical activity.
Those persons who can maintain a regular regimen of activity that is of longer
duration or of more rigorous intensity are likely to derive greater benefits.
Many people exercise to lose weight. A calorie is a unit
that measures the energy expenditure of the body.
When the daily calorie intake from food is the same as
calories expended from exercise, weight remains the same. The number of calories
burned during exercise varies greatly with the type of physical activity, but
the key to successful weight reduction is to exercise regularly, without
increasing food intake proportionally. One way to reduce calories is to eat
healthier foods that contain more fibre and less fat, and subsequently fewer
calories. Regular exercise, along with a generally healthy lifestyle is
beneficial.
Exercise is characterized in terms of four variables:
Frequency, intensity, duration and mode.
When planning an exercise programme, the personal fitness
objectives and the exercise regimen that will best meet the objectives, age and
existing health conditions should be considered. Individuals over the age of 40,
who suffer from serious health problems or physical limitations, should first
consult a physician for recommendations about the best exercise programmes to
adopt. If overall fitness or prevention of heart disease is a primary goal, 20
to 30 minutes of moderate intensity, daily aerobic exercise – such as walking,
jogging, swimming or dance aerobics should be considered. Initially you would
begin with shorter exercise sessions and gradually work up to 20 to 30 minutes.
If improving muscle strength is the primary consideration, regular high
intensity workouts with weights are more appropriate. Studies have shown that
even older people can benefit greatly from a weight lifting workout. Those
benefits include increased bone density, prevention of muscle atrophy and
improvement in general strength and co-ordination. A structured, supervised
weight programme after consultation with a physician is recommended. A good
exercise programme always includes stretching exercises, warm up and cool down
periods to prevent muscle pulls and other injuries.
When an aerobic programme is adopted, in addition to doing
the 20 to 30 minutes on a daily basis or several times a week, the intensity of
the exercise should be considered. The intensity can be determined by evaluating
the heart rate attained during exercise. The maximum heart rate for an
individual is approximately 220 minus age. To improve aerobic capacity (VO2
maximum), exercise should be performed at an intensity that produces a heart
rate of at least 70% of this maximum. Example: for a 30 year old, maximum heart
rate equals 220 - 30, or 190bpm. The heart should rise to at least 133bpm (70%
of 190). You can determine your heart rate by placing two fingers over the
radial artery in the wrist, or carotid artery of the neck.
The most important aspect of getting in shape is to make
exercise an integral part of one’s lifestyle. Exercising to stay as physically
fit and healthy as possible should be a lifelong commitment and is especially
important to people who perform little physical work in their daily lives.
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