The physiology of physical effort


Intense work and competition cause physical and psychological stress. General knowledge of the physiological changes produced by physical effort can help owners and handlers to understand their dogs and thus better train and prepare them for competition or work, preventing any pathological conditions which might arise.

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Cardiovascular and respiratory changes

© Diffomédia/Royal Canin

During effort, changes in the cardiovascular and respiratory systems occur to ensure the provision of oxygen and nutrients to the muscles and the removal of waste – especially carbon dioxide and heat – produced by muscle metabolism. These changes are absolutely necessary not only in the course of completing a work task or sporting endeavour, but also afterwards. The body responds to effort in two ways:

- There is a direct response adapted to the body’s immediate needs, occurring concomitantly with effort.

- There is a longer-term response which anticipates the body’s needs linked to adaptations produced by training.

During physical exercise

Changes to the circulatory system during effort play an essential role in increasing blood flow and consequently the provision of oxygen to the tissues with increased metabolism, especially muscles. The body does this by increasing the cardiac output and directing more blood to active areas. The blood’s capacity to carry oxygen is also increased by the contraction of the spleen, which releases a large number of red blood cells into the blood, increasing the haematocrit (the ratio of red blood cells to the total volume of the blood) and the quantity of haemoglobin (a chemical pigment in the red blood cells responsible for transporting oxygen).

Cardiac output can increase considerably – up to ten times the resting level – which significantly increases the heart rate.

Depending on the intensity of the effort, it can rise to 300 beats per minute in a racing dog and 200 in a sled dog. The blood vessels in the muscles that are working dilate to a very high degree so that blood flow can increase. Minute respiratory volume – the total amount of new air moved into the respiratory passages each minute – also increases in various stages during effort:

- Ventilation increases sharply during the first three or four seconds.

- This is followed a few seconds later by a second, slower, increase.

- The curve then levels off, which continues until the end of the effort.

- During recovery, the breathing rate falls slowly from over 300 to about 30 breaths per minute in just a few minutes.


Cardiac output can increase considerably – up to ten times the resting level – which significantly increases the heart rate.”


Effect of training

After four to five weeks daily training, the dog’s cardiovascular and respiratory systems will have undergone significant changes. Changes to the heart and circulatory system due to repeated physical exercise tend to minimise the energy the heart needs to work, as well as improving the heart’s pumping capacities. Properly trained dogs have a lower resting heart rate than sedentary dogs and their respiratory arrhythmia is more pronounced. Their plasma volume is higher and they have better venous blood return, which increases overall cardiac output.

Intensive training sometimes leads to enlargement of the heart muscle (often seen in sled dogs). Training will also cause an increase in the number and density of muscle capillaries. Contrary to what many people think, regular physical exercise produces very little if any change in the respiratory system in healthy dogs. It is the body’s overall ability to consume oxygen (VO2max or maximum oxygen consumption) that is significantly increased by endurance training.

These changes can occur optimally only in generally healthy individuals. Any alteration or failure of these functions will curb the dog’s ability to adapt to physical effort, and so limit its sporting or working performance.

Consequences of biological stress during effort

© Diffomédia/Royal Canin

In sporting dogs, stress induced by environmental conditions or physical effort causes behavioural changes (barking, lack of drive), changes to the autonomic nervous system (salivation, tachycardia, mydriasis), digestive problems (vomiting, diarrhoea, gastric ulcers) and anaemia (there’s even such a thing as sports anaemia). Many of these outward signs reported by dog handlers during intense work or sporting competition are due to a physiopathological process, but they can be prevented with the right training.

Overtraining is often the leading cause of such changes, but it is important to acknowledge that, at the psychological level, dogs can very easily differentiate between training situations and competition or proper work. Generally speaking, stressful situations often cause anxiety in animals, which is expressed in species-characteristic behaviour. In dogs, anxiety and fear is expressed in well-known responses, such as repeated bowel and bladder evacuation, barking and howling and other stereotypical repetitive behaviour, such as biting objects and digging.

For these reasons, reactions of a behavioural, autonomic, digestive or other nature can be regarded as stress-induced. Clearly, in the context of physical tests, which often place high demands on the metabolism, digestive disorders (stress diarrhoea, for instance) are among the most harmful in physical terms, especially in combination with loss of water and electrolytes or insufficient water and food intake.

An appropriate training programme, a calm psychological environment and the right type of food are key to the prevention of problems due to physical stress or overtraining.


Energetics of effort

© Diffomédia/Royal Canin

The chemical energy used in muscle contraction comes only from the high-energy phosphate bonds in a fundamental molecule known as adenosine triphosphate (ATP). The ATP concentration in the muscle cells must be immediately replenished as it falls during muscular effort so that the dog can sustain its exertions. Three processes achieve this. The role and relative importance of each process depends on the type of effort required.

Anaerobic mode without lactic acid production: During very brief, very intense effort (lasting a few seconds), ATP is replenished from muscle phosphocreatine reserves, without the need for oxygen (anaerobic) and without the production of lactic acid.

Anaerobic mode with lactic acid production: During intense efforts lasting at least two minutes (racing, agility), the energy is replenished from glycogen stored in the muscle and blood glucose. Again, no oxygen is consumed, although this time lactic acid, a waste product of metabolism, is produced. In very general terms, it is often the accumulation of lactic acid that causes muscle fatigue and cramps.

Aerobic mode: This metabolic process covers the energy requirements of the dog when sustained effort is required (effort of relatively low intensity but lasting between several minutes and several hours). Initially, blood glucose is oxidised by the oxygen provided to the muscle in the red blood cells, but, unlike in humans, fats very quickly become the preferred energy source in dogs.

Without going into too much detail, it should be noted that proper training and nutrition is reliant on knowledge of these specific metabolic processes.

© Diffomédia/Royal Canin
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