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Family
Doctor Books |
Preview of Understanding Sports and Exercise
Medicine
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Regular exercise leads
to the heart becoming more efficient. The volume of blood pumped with
each heart beat is increased, so the heart rate becomes slower, and therefore
is more capable of speeding up during exertion. This is called the training
effect. Top athletes have been known to have resting heart rates lower
than 40 beats per minute (the average for an untrained person is about
72 beats per minute).
The training effect is one aspect of overall fitness,
which enables the cardiovascular system and the lungs to supply blood
and oxygen to the muscles more effectively and efficiently. These changes
in fitness can be measured scientifically. The muscles show increased
stamina, become stronger and contract more firmly, thus increasing
their capacity for work and activity. Tendons surrounding joints become
stronger, so that the joints are more stable and are thus less liable
to injury
– this is especially important in older people. The joints themselves
become capable of an improved range of movement and are more flexible.
Finally, the effects of age and chronic diseases including coronary
artery disease are reduced.
What is fitness?
Total fitness is defined in the Oxford Dictionary
of Sports Science and Medicine as: ‘The ability of an individual
to live a happy and well balanced life. It involves not only physical
but also intellectual, emo-tional, social and spiritual aspects.’
Physical fitness by itself has several components.
These are strength, speed, flexibility and endurance. Skill is also important
in sports and exercise. It is something you learn, usually by repeating
a range of pre-set movements. The skill factor is important in avoiding
injury.
The training
effect enables the cardiovascular system and the lungs to supply
nutrients and oxygen to the muscles and vital organs more efficiently. |
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Strength
This is defined as the maximum force that a muscle
or group of muscles can exert in a single contraction. In practical terms,
the person who can lift 200 pounds (90 kilograms) has twice the strength
of someone who can lift 100 pounds (45 kg). Tests for muscle strength
can be performed using special equipment such as strength-dynamometers,
free weights or isokinetic strength-testing machinery. Weight-lifting
and shot-putting are good examples of how strength is tested on the sports
field. In other sports, certain movements are repeated over and over
again; this also requires strength in the right places, for example,
in the arms of tennis players.
Strength – the
maximum force that a muscle or group of muscles can exert in
a
single contraction. |
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Speed
This is defined and measured as the time taken to move
a single limb (limb speed) or the total body (body speed) between two
fixed points. It can be recorded simply as a time (in seconds) or, if
the distance is known, it can be given units of velocity, that is, feet
per second, metres per second, or whatever. The time taken for a short
sprint (less than 60 metres) or a karate chop is an example of a speed
test. These can be measured by equipment such as lasers and light sensors.
Power – the combination
of speed of movement and strength. |
Speed – the time taken
to move the body or a limb between two fixed points. |
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Muscular power
Muscular power is strength and speed of movement combined. Mathematically,
power equals strength times speed or force times velocity. For example,
two people may be able to lift a 150-pound (68-kg) weight, but the one
who can do it in half the time of the other has twice the power of the
slower person. Power is a key component for most athletic performances.
It determines, for example, how effectively a tennis player can serve,
how far a golfer can drive a golf ball or whether a penalty kick beats
the goalkeeper.
Flexibility – the
ability to bend the body and limbs easily. Flexibility is crucial
to avoid injury, especially to muscles and ligaments. |
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Flexibility
This is an aspect of fitness that is sometimes mistakenly
overlooked. It is crucial in preventing injury – especially to muscles
and ligaments – and should be an essential part of the training
programme of all top-class performers. Training experts, both in the
USA and in the UK, believe that a degree of overall body flexibility
is desirable for everyone. Some of the more appealing aspects of flexibility
are:
- It can be improved by practice
- It does not use up much energy
- It is safe and does not require apparatus
- It is enjoyable.
At least 15 to 20 minutes of flexibility and stretching
exercises two to three times per week would be a valuable part of your
overall fitness programme. There are two types of flexibility exercises:
ballistic and static. Static flexibility exercises are preferable because
they are much less likely to lead to injury.
Ballistic exercises involve activities such as bobbing
or bouncing up and down, stretching the inner aspects of the thighs and
rapid rotational exercises of the trunk for the abdominal and back muscles.
Although they are effective, you have less control over them than in
static stretching and they can cause a muscle reflex which may limit
your movements.
Static exercises are controlled stretching movements.
To achieve the desired effect, you need to do them for only 20 to 30
minutes two or three times a week. They require little energy and are
safe. You simply move your body slowly to the stretched position until
you feel a slight discomfort, then hold the stretch position for up to
20 seconds. Remember to stay relaxed, keep breathing and focus your attention
on the part that you are stretching.
What happens to the body during exercise?
Muscles need energy to contract, and this comes from
food that is broken down to form energy stores in the muscles, liver
and fat. The body produces energy from these stores in two different
ways:
- by aerobic means (requiring oxygen)
- by anaerobic means (not requiring oxygen).
The process involved is called metabolism.
Anaerobic energy is used for short bursts of activity
such as sprinting, lifting or jumping. Once anaerobic energy is used
up, it cannot be replenished quickly. There are two anaerobic pathways:
for the first few seconds of high intensity activity (for example, a
100-metre sprint), what is called the phosphogen system is used. Beyond
these first few seconds, two other systems take over to release energy
for further muscular con-tractions and relaxations. These are known as
anaerobic glycolysis and the oxidative system (aerobic energy production).
Anaerobic glycolysis will supply energy in all-out sprint events for
one or two minutes, but it leads to an accumulation of lactic acid in
the muscle and body fluids, halting the process of glycogen breakdown
that is responsible for supplying anaerobic energy. After that, aerobic
energy is required because without this our ability to exercise would
be limited to only a few minutes.
Aerobic energy is used for longer sessions of exertion,
such as medium- and long-distance running, swimming and brisk walking.
This energy can be continuously produced to meet the body’s needs
during exercise, but not at the high rate supplied by anaerobic metabolism.
The term ‘anaerobic threshold’ is used to define the point
at which the body cannot meet the energy demands placed upon it by the
aerobic pathways. The body then starts to work anaerobically and produces
lactic acid and other metabolic poisons.
It is the way the body produces and uses both these
kinds of energy that is changed by increasing fitness. When you are fit,
all the systems in your body are adapted towards producing and using
energy in the most efficient way possible.
During prolonged exercise, anaerobic energy is used
first. However, your body starts to adapt to the exertion by trying to
maintain a good supply of oxygen to the muscles to allow aerobic metabolism
to take place. For this, your heart has to beat faster and your lungs
have to take in faster and deeper breaths so that the red blood cells
carrying oxygen to your muscles are pumped faster towards their destination.
Various other changes also occur to ensure that your
muscles get the blood supply they need – for instance, blood flow
to your intestines and kidneys is decreased so that your muscles get
a bigger share of the blood supply. Fitness and training reduce the need
for such changes (which are stressful for the body) to take place early,
so they will be needed only after you’ve been exercising for a long
time. Instead, your heart, lungs, muscles and blood supply are all adapted
to ‘make light’ of the exercise – they have a reserve
capacity for exertion, and you don’t start to experience discomfort
and fatigue until much later.
If you rarely exert yourself beyond a certain point,
you may kid yourself that you are fit because you only ever do things
in short bursts, and so always use anaerobic energy. If, however, you
were to run for longer, your heart rate would increase uncomfortably,
you would quickly become out of breath, your muscles would weaken and
tire, and you would soon have to stop. Someone who is fit, however, would
be adapted to keep producing aerobic energy for their muscles, and their
heart and lungs would work more efficiently to keep supplying the oxygen
required for this.
Although it supplies energy more slowly, unit for unit,
aerobic energy production releases almost ten times more energy than
anaerobic means. Aerobic energy is also ‘cleaner’ – it
produces mainly water and carbon dioxide as by-products. Aerobic metabolism
produces less lactic acid – it is thought that the accumulation
of lactic acid in the muscle is one reason for muscle fatigue and pain,
the rising acidity progressively inhibiting further metabolism.
Muscles
need energy to contract, and this comes from food that is broken
down and stored in the muscles, liver and fat. |
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Energy sources
For competitive athletes, a correct fitness training
programme is crucial because they have different patterns of energy requirements
and need particular combinations of muscle use, depending on their particular
sport. If, however, you just want to maintain your level of fitness,
you can adopt a more relaxed programme which combines aerobic and anaerobic
fitness, some strength and flexibility training, and relaxation (see
later under ‘All about training’).
ENERGY
SOURCES |
Energy
source
Anaerobic
- Intramuscular phosphogen
- Intramuscular glycogen
Aerobic
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Duration
of energy source
Few seconds
Up to 50 seconds
2 hours (continuous moderate-to- high intensity exercise)
Days
Protein can be an energy source at
extremes of exertion and endurance |
What is fatigue?
As mentioned above, muscle fatigue is thought to result
partly from the accumulation of lactic acid from anaerobic metabolism.
Other causes are the depletion of energy stores in the muscle (for aerobic
or anaerobic work). During aerobic exercise the waste products are easily
disposed of by the body. The limiting factors to energy production, and
consequently the onset of fatigue, are the availability of glycogen and
oxygen.
Training improves cardiovascular performance and oxygen
delivery to the muscles but there is a finite limit to the amount of
glycogen that can be stored in the body. The total stored is sufficient
for about two hours of intense aerobic exercise only. If you run for
more than two hours without feeding you will soon run out of fuel for
your muscles. This is what is known as ‘the wall’ or ‘the
knock’.
Yet another cause, which has more to do with ‘central
fatigue’ – the feeling of overall deep tiredness rather than
just muscle ache – is related to the level of a chemical called
tryptophan in the brain. Exercise increases the amount of tryptophan
entering the brain, which in turn increases the level of another chemical
called serotonin or 5-hydroxytryptamine. This chemical may be implicated
in the condition known as subjective or perceived fatigue, in which the
person feels the need for a rest. This may be a protective mechanism
to prevent us having to tolerate further pain or discomfort.
If you
run for more than two hours without eating or drinking, you will
soon run out of fuel for your muscles. |
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What is sweating?
Exercise produces body heat. Long-distance runners
may have body temperatures two to three degrees higher than normal. However,
human beings must maintain a constant internal body temperature to function
efficiently. This range is between 36°C and 38°C (97°F to
100°F). Outside these ranges, we can suffer from hyperthermia (overheating
– heat cramps, heat exhaustion and heat stroke) or hypothermia (drop
in body temperature – reduced heart rate, reduced respiratory rate,
frost-nip and frost-bite). If the body temperature rises, we respond
by sweating. As the sweat evaporates, it causes cooling. Similarly, in
cold conditions we will shiver to generate body heat, to maintain a constant
internal temperature.
The amount of sweat produced at high levels of activity
can be enormous. It has been calculated that a marathon runner who completes
the race in two and a half hours may lose more than five litres of fluid
during that time as sweat. This is necessary to keep the body temperature
from climbing, which would in turn impair the athlete’s ability
to function. However, if the fluid loss is too great, dehydration and
heat stroke may result.
To avoid this problem, remember to:
- drink early in your exercise session
- drink small quantities
- drink frequently.
What is muscle cramp?
Cramp is a sudden sustained and uncoordinated spasm
or contraction of a muscle. It may be caused by dehydration, fatigue,
muscle damage or low blood sugar levels, or a combination of these factors.
To date, there has been no complete explanation for its development.
Cramp often affects the hip, thigh and calf muscles during prolonged
exercise. Relief can be obtained by gentle stretching and rest.
How are sprinters different from marathon runners?
Having the right body composition can give an athlete
the edge. At the highest levels of competition, it is unusual to find
competitors who do not conform to the general physical attributes common
to that sport – for example, most gymnasts are short, light and
muscular, whereas height is an asset in basketball players.
The other significant variation is in muscle fibre
type. There are two: fast twitch and slow twitch. The proportion of each
in a particular muscle is inherited from your parents.
Fast twitch fibres
This type of muscle fibre reaches peak tension quickly,
fatigues quickly and so uses anaerobic metabolism. Muscles with a preponderance
of these fibres are powerful.
Slow twitch fibres
These have a slow contraction time, are low in power
and use aerobic metabolism predominantly. These fibres predominate in
the muscles of people who are good at long distance events.
Aerobic exercise such as jogging and cycling uses slow
twitch fibres, whereas activities that require bursts of energy use fast
twitch fibres. Marathon runners possess more than 90 per cent slow twitch
fibres in their calves or thighs, whereas sprinters may have only 25
per cent. Slow twitch fibres are adapted to utilise aerobic energy, whereas
fast twitch fibres use anaerobic energy stores.
How does fitness affect the body?
Generally it takes eight to ten weeks of regular training
before you start to notice any benefits.
The cardiovascular system
Regular exercise changes your heart in several ways:
your heart rate becomes slower, and the amount of blood the heart beat
pumps (called the stroke volume) increases. The ventricles (the lower
chambers in the heart) also become bigger.
These changes work in conjunction with the changes
in your muscles to ensure that your body has the capacity to do more
exercise before it starts to feel the effects. As it is beating more
slowly, your heart has to work less hard and your blood pressure is also
affected. It is lower after a work-out in normal people (those who are
healthy and do not have high blood pressure), and regular work-outs make
the rise in blood pressure during exercise slower and smoother. In people
with mild hypertension (raised blood pressure), regular moderate exercise
also reduces blood pressure by about 10 millimetres of mercury (mmHg),
which is more effective than some medications.
There is also evidence that the electrical activity
of the heart becomes more stable with regular exercise, so that sudden
changes in heart rhythm (which sometimes lead to sudden death) are less
likely.
Muscles
Although the number of muscle fibres does not increase,
they become bigger and stronger. The blood supply to the muscle fibres
also improves, which means that more blood and oxygen can be transported
to and from the muscle than before. The enzymes that help convert energy
stores to energy for muscle contraction also become more efficient. They
make better use of the available oxygen supply, and produce less lactic
acid, so the muscle tires less easily. Overall, the performance of the
muscle improves.
Lungs
Lungs are responsible for bringing enough oxygen to
the body tissues and meeting the increased demand during exercise. Although
they do not change size, the amount of air they can take in and breathe
out increases. Breathing rate at rest also becomes slower, which allows
a comfortable increase during exercise.
Bone
Bone density is maintained and improved through exercise.
Osteoporosis – the thinning of the bones that is a common cause
of fractures in older people – is more severe in those individuals
who have led inactive lives. Studies have demonstrated that the density
of the hip bone and lower spine correlates closely with good stamina.
Good bone density prevents fractures and loss of mobility later in life.
Joints and tendons
Regular movement of the joints helps to increase flexibility
and prevents stiffness. The cartilage that lines the bones becomes thicker,
and both ligaments and tendons become more strongly attached to bone.
This, combined with stronger muscles and bones, means that exercised
joints are more stable and less vulnerable to twisting and other injury.
Other changes
Exercise changes the existing pattern of blood lipids
by increasing the levels of high-density lipo-proteins (HDL) and decreasing
the low-density lipoprotein (LDL) level. These are both types of blood
cholesterol, and it is thought that, whereas HDL protects against heart
disease, LDL promotes fatty deposit formation within blood vessels which
can lead to heart attacks. A favourable ratio of HDL to LDL is therefore
consistent with lowered coronary heart disease risk.
The way the body deals with insulin is also altered,
so that less insulin is required to deal with the blood glucose rise
that happens after you’ve eaten. This can be beneficial for people
at risk of diabetes as well as for those who already have the condition.
At maximal levels of training, the amount of oxygen
in the blood increases, because the body tissues have become more efficient
at extracting oxygen from the blood. The distribution of blood to active
tissues also improves.
Psychological changes
People who exercise are less prone to depression and
anxiety. What’s more, exercise is a good outlet for feelings of
stress, tension and aggression – people who exercise regularly sleep
better and gain a sense of well-being. Exercise can be used to treat
mild cases of anxiety and depression – in these situations it can
be as helpful as drugs or psychotherapy. Severe depression or mental
illness does not, however, respond to exercise.
It is still unclear why
exercise has mental effects, but it is likely that somehow brain chemicals
are altered. Two types of chemicals have been implicated: they are called
endorphins and central monoamines, but their exact role is still unknown.
They may also be responsible for ‘runner’s high’ – the
state of euphoria that some long-distance runners report.
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KEY POINTS
- Regular exercise produces the ‘training
effect’
- Strength, speed, flexibility and endurance
are the components of fitness
- Skill and flexibility are important in preventing
injury
3 After 8–10 weeks of regular training, benefits are noted:
muscles are stronger, lungs more efficient and bones strengthened,
and we feel psychologically better
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