Aerobic exercise and training - Extreme physiology

Demands of Aerobic Exercise

Ability to deliver large amounts of oxygen to working muscles efficiently and rapidly to be able to generate ATP.

1. Air to blood

2. Blood to interstitial fluid

3. Interstitial fluid to surrounding muscle fibres

VO2 max = Maximal cardiac output x arteriovenous difference

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Exercise Demands

Oxygen consumption increases x 13

Acheived by:

1. 1.5x increase in cardiac stroke volume

2. 3 x increase in heart rate

3. 3 x increase in arteriovenous difference

More blood flow to working muscles, less to digestive organs, kidneys, bones, brain etc.

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Cardiac Output

Increases with oxygen consumption in linear relationship

Can increase from about 5L.min to 20-25L.min

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Heart rate

Rises linearly with work rate up to about 180-200bpm

Max = 220 - age

Decreased parasympathetic and increased sympathetic stimulation of pacemaker cells

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Stroke volume

Increases then decreases with work rate

Increase partly acheived by an increase in filling pressure (increasing EDV)

And partly by increase in ventricular contractility (which increases ejection fraction)

Almost all increase in stroke volume occurs at low work rate

At high work rates - reduced filling time compromises EDV

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Distribution of blood flow

5 x increase in CO can result in 20 x increase in muscle blood flow.

Vasodilation - occurs in beds of active muscle, decreased resistance, increased flow

Vasoconstriction - occurs in non essential vascular beds e.g. renal and splanic, mediated by sympathetic nerves and diverts greater proportion of blood to active muscles

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Arteriovenous difference

Reflects amount of oxygen that is taken up in lungs and used in peripheral tissues

3 x increase during exercise

This is not due to increases in arterial O2 content - its due to decrease in venous O2 content

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Physical training

Fitness training stimulates a series of circulatory adaptions that are important for endurance athletes.

Limiting factor is rate of O2 transport from lungs to mitochondria

Limited by:

1. Maximal attainable CO

2. Extracellular resistance to diffusion between red blood cells and mitochondria

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Cardiac response to training

Before:

Max = about 200bpm

After 4 months = 199bpm

Top endurance athletes = about 190bpm

Having a lower resting heart rate ensures there is a proportionally larger increase when exercise begins so CO is able to increase significantly

Therfore CO can be up to 7x resting

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Diffusion of O2 from blood to muscle

Adaptions occur to improve rate of transport from the blood to muscle:

1. Capillarisation - more vascular beds reduces distance for diffusion

2. Muscle mitochondria increase in size - especially at sites close to capillaries

3. Muscle myoglobin concentration increases - more o2 storage

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Blood volume and training

Endurance training - increases blood volume

There are more plasma proteins, therefore plasma volume initially increases due to osmotic concentrations 

Later an increase in RBC increases too

This means that haemocrit usually decreases a little bit due to the increase in plasma volume

This reduces blood viscocity and means that the blood is able to flow more easily, therfore oxygen delivery is more effective

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Muscle Myoglobin

Myoglobin - what oxygen binds to when it enters the muscle

The stores release it to the mitochondria when O2 availability becomes limited during periods of muscle activation

The oxygen reserve is often used during the lag period - at the onset of exercise before the body has time to increase VO2

Training has shown to increase myoglobin by 75-80%

This suggests it plays a very important role

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Mitochondrial function

Ability to produce ATP depends on number of ATP, size and efficiency

Number and size have been shown to increase with exercise training

Also increases activity of key mitochondrial enzymes - succinate dehydrogenase and citrate synthase

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Consequences of aerobic training

Muscles use CO more effectively

Muscle demands less blood flow at submaximal exercise intensities

Peak blood flow is enhanced 

The use of oxygen is also more effective with increased mitochondrial capacity to generate ATP

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