Body & Mind Training Center

Human Body - Responses to Exercise


When you begin to exercise your body must immediately adjust to the change in activity level. Energy production must increase to meet demand with changes to the predominant energy system and fuel source occuring throughout the exercise in order to maintain the required level of performance.

Responses to Anaerobic Exercise

In order to immediately meet the sudden higher energy demand, stored ATP is the first energy source. This lasts for approximately 2 seconds.
When stored ATP is broken down into ADP + P, the rising ADP level stimulates Creatine Kinase to begin the breakdown of Phosphocreatine.
As discussed on the energy systems , the ATP-PC system can only last 8-10 seconds before PC stores are depleted.
The lactic acid system (Anaerobic glycolysis) must then take over as the predominant source of energy production. High intensity (but sub-maximal) exercise can last for between 3 and 5 minutes using this system
If the exercise continues at a high intensity, and so Oxygen is not available at a fast enough rate to allow aerobic metabolism to take over, the production of lactic acid will reach the point where it interferes with muscular function. This is called the Lactate threshold.
Muscles begin to fatigue when ATP resynthesis can no longer match demand.

Responses to Aerobic Exercise

Due to the necessity of Oxygen being present for aerobic metabolism, the first few minutes of low to moderate intensity exercise are powered by anaerobic metabolism.
Continued low to moderate intensity exercise is then fuelled by carbohydrate and fat stores using aerobic metabolism.
The intensity and duration of exercise determines which fuel source is used. Fat metabolism is a slow process and so can only be used as fuel for exercise at less than 60% VO2 max.
Carbohydrate is a much faster fuel source and so can be used for exercise up to 80% (in trained individuals).
Carbohydrate stores within the muscle and liver can fuel exercise for up to 80 minutes. As carbohydrate stores get lower, the body has to rely more and more on fat stores.
The intensity of exercise which can be maintained drops as fat cannot supply the required amount of energy.

Short-term Effects of Exercise


When we begin to exercise the body has to respond to the change in activity level in order to maintain a constant internal environment (homeostasis). Here are the changes which must take place within the muscles, respiratory and circulatory system:

Circulatory System :

The release of adrenaline (often before exercise even begins) causes the heart rate to rise
This increases Cardiac Output
Venous return increases due to the higher Cardiac Output and the skeletal muscle pump and respiratory pump
Increases in Lactic Acid (produced during the early anaerobic phase of exercise), Carbon Dioxide (due to increased rates of energy production) and temperature all act as stimuli to the cardiac control centre which responds by further increasing the heart rate
Oxygen levels within the blood decrease which causes increased diffusion at the lungs
Blood pressure increases, thus increasing flow rate and the speed of delivery of O2 and nutrients to the working muscles
Vasodilation and vasoconstriction ensure blood is directed to areas that need it (muscles, lungs, heart) and away from inactive organs


Respiratory System :

Changes in the concentration of CO2 and O2 in the blood are detected by the respiratory centre which increases the rate of breathing
The intercostal muscles, diaphragm and other muscle which aid the expansion of the thoracic cavity work harder to further increase the expansion during inhalation, to draw in more air.

Muscles :

The higher rate of muscle contraction depletes energy stores and so stimulates a higher rate of energy metabolim.
The bodys energy stores are slowly depleted
Myoglobin releases its stored Oxygen to use in aerobic respiration. O2 can now be diffused into the muscle from the capillaries more quickly due to the decreased O2 concentration in the muscle.


Long-term Effects of Exercise :


Regular exercise results in adaptations to the circulatory, respiratory and muscular systems in order to help them perform better under additional stress. Here are the changes which must take place within the muscles, respiratory system and circulatory system:

Circulatory System :

The cardiac muscle surrounding the heart hypertrophies, resulting in thicker, stronger walls and therefore increases in heart volumes. The more blood pumped around the body per minute, the faster Oxygen is delivered to the working muscles.
The number of red blood cells increases, improving the bodies ability to transport Oxygen to the muscles for aerobic energy production.
The density of the capillary beds in the muscles and surrounding the heart and lungs increases as more branches develop. This allows more efficient gaseous exchange of Oxygen and Carbon Dioxide.
The resting heart rate decreases in trained individuals due to the more efficient circulatory system.
The accumulation of lactic acid is much lower during high-levels activity, due to the circulatory system providing more Oxygen and removing waste products faster.
Arterial walls become more elastic which allows greater tolerance of changes in blood pressure.


Respiratory System and Exercise :

The respiratory muscles (Diaphragm/intercostals) increase in strength.
This results in larger respiratory volumes, which allows more Oxygen to be diffused into the blood flow (VO2 max)
An increase in the number and diameter of capillaries surrounding the alveoli leads to an increase in the efficiency of gaseous exchange.


Muscle :

Increased numbers of mitochondria (the cells powerhouse) means an increase in the rate of energy production.
The muscles, bones and ligaments become stronger to cope with the additional stresses and impact put through them.
The amount of myoglobin within skeletal muscle increases, which allows more Oxygen to be stored within the muscle, and transported to the mitochondria.
Muscles are capable of storing a larger amount of glycogen for energy.
Enzymes involved in energy production become more concentrated and efficient to aid the speed of metabolism.

Oxygen Debt & Recovery

What is it all about then ?

When you have a short intense burst of exercise such as sprinting you generate energy for this anaerobically or without oxygen. When you stop exercising you are still breathing heavily.

This is your body taking in extra oxygen to 'repay' the oxygen debt. Well, that is the simple solution but there is a little more to it if you want to look a bit deeper.

True, your body has worked anaerobically and will have produced energy without some of the oxygen it would normally have used performing low intensity exercise such as slow steady running. The difference between the oxygen the body required and what it actually managed to take in during the sudden sprint is called oxygen deficit.

When you stop sprinting and start to recover you will actually need more oxygen to recover than your body would have liked to use had enough been available. This is called Excess Post Exercise Oxygen Consumption.

So why does it take more oxygen to recover then?

You needed to replace the oxygen the body needed but couldnt get (oxygen deficit).
Breathing rate and heart rate are elevated (to remove CO2) and this needs more oxygen.
Body temperature and metabolic rate is increased and this needs more oxygen.
Adrenaline and Noradrenaline are increased which increases oxygen consumption.
So after exercise there are other factors causing an increase in oxygen needs as well as repaying the lack of oxygen during exercise.

The chart below is often seen and shows how the amount of oxygen used by the body changes over time. At the beginning the body works anaerobically leaving an oxygen deficit. Over time the oxygen consumption levels out to a steady state. After exercise the oxygen is pain back (oxygen debt). Notice the area of oxygen debt is greater than the area of oxygen deficit for the reasons stated above.






What has Lactic Acid got to do with it ?

Lactic acid is a by product of exercising without using oxygen (anaerobially). It is essential this is removed but it is not necessarily a waste product. It is recycled into other useful chemicals:

During prolonged intensive exercise (e.g. 800m race) the heart may get half its energy from lactic acid. It is converted back to pyruvic acid and used as energy by the heart and other muscles.
It is thought that 70% of lactic acid produced is oxidised, 20% is converted to glucose (energy) in the liver.
10% is converted to protein.
How long does it take to remove lactic acid ?

About 1 hour if cooling down with gentle exercise.
It can take 2 hours or more if you dont warm down with gentle exercise.

Sports Nutrition :

The subject of nutrition is complex and varied. It encompasses everything from weight loss and gain to sports performance and supplements. Here we aim to give you a basic understanding of the requirements of a balanced diet and the importance of this in sport.

A balanced diet for most people should consist of:

60% Carbohydrates
30% Fat
10% Protein
Vitamins, minerals and water
This does vary depending on the activity levels, type of exercise and health status of the individual.

Carbohydates :

These are found in starchy and sugary foods. Complex carbohydrates are mainly starchy foods, including potatoes, rice, bread and pasta and have additional nutritional value as they contain many other vitamins, minerals and fibre. Simple carbohydrates are the sugary ones, found in cakes, biscuits and sweets which are sometimes termed empty calories, as they provide no other nutritional benefits.

Carbohydrates are our main source of energy, in fact, energy from the breakdown of carbs is the only type of energy the brain can use. Carbohydrates are broken down in the liver and muscles, by a process known as glycogenesis. It is then stored as glycogen until it is needed.

Sports people, especially those involved in endurance events often require higher than the normal 60% carbohydrate intake in order to maintain large stores of glycogen and resist fatigue.

Fats :

Fats serve several important purposes. They provide energy and when stored, provide protection to our vital organs.

There are two types of fats, saturated and unsaturated. Saturated fats are 'the bad fats' which are normally solid at room temperature, such as butter and meat fat. Unsaturated fat is more difficult to breakdown and so is mainly stored within the body. Unsaturated fats are generally better for us and are often liquid at room temperature, for example olive oil and sunflower oil, although they can also be found in avacados and nuts.

A healthy diet should not contain more than 30% fat, and a maximum of 10% should be saturated fat. Fat provides a secondary source of energy and once the relatively small carbohydrate stores are exhausted, fat metabolism becomes the primary source of energy.

Proteins :

Proteins are large compounds consisting of amino acids. There are 20 amino acids which the body requires. 12 of these can be synthesised within the body, and the other 8 (essential amino acids) must be consumed through our diets.

Proteins are found in abundance in meats, eggs, fish, dairy products, nuts and seeds. Protein is essential for growth, repair and maintenance of our body tissues and for this reason, many athletes (mainly those requiring strength or size) will increase the amount of protein they consume, in order to help their muscles grow and develop strength.

Fluids :

Fluids are vital in any sport to help prevent dehydration. When we exercise our bodies sweat to help cool us down. This results in a loss of water which must be replaced so as to not inhibit performance. Electrolytes such as sodium are also lost in our sweat. For this reason many sports drinks contain a mix of water and electrolytes. The presence of these electrolytes also helps the water to diffuse through the small intestine, back into the body.

Vitamin and Minerals :

Vitamins and minerals are vital in the diet for a wide range of functions, but only needed in tiny amounts. Providing you have a balanced diet as shown above, containing lots of fresh foods, there is no need for vitamin or mineral supplementation.

Supplementation :

There are so many types of supplements now widely available, but for most people, providing their diet is balanced and varied, supplementation is not necessary. Some athletes may which to supplement their diets to enhance their performance, especially when the difference between winner and runner-up can be a fraction of a second, or a single millimetre. The most commonly taken supplements amongst athletes are protein, and creatine.

Protein based supplements are taken by those athletes wishing to increase their size and strength. Creatine is used by a wider band of athletes. It is promoted as a muscle performance enhancer as it is designed to allow reapeated powerful muscle contractions. Creatine is naturally present within the body, it is used in the ATP-PC system of anaerobic energy production, where PC stands for Phospho-Creatine. Having excess stores of creatine enable this anaerobic energy systems (and so a higher level of performance) to continue for longer.

International sport ?

The 19th century saw the start of international competitions. These have continued to grow and become more popular ever since. Just about every sport is played on an international level, from football, to show jumping to chess! Here are some of the big ones:

The Olympic Games - The biggest modern day international competition. There are both summer and winter Olympics every four years

The Pan-American Games - These are also held every four years (but so they fall in a different year!) and are competed by countries in North, South and Central America

The Commonwealth Games - The commonwealth is the group of countries which used to make up the British Empire. These competitions are also held every four years

World Cups - A lot of sports have world cups, like Football, Cricket and Rugby

Most sports now have their own world championships like athletics and gymnastics.

These huge international competitions obviously have positives, but they also have their negatives:

Positives :

The mix of cultures and ways of life which occurs when lots of people from different countries get together
When the best athletes in the world compete against each other the standard improves - which is why you often see new records being set
International events encourage lots of people to get involved in sport

Negatives :

These big events are very costly which means many poorer countries cannot afford to host them
Even bigger countries need help from big businesses in the form of sponsorship which makes events like this very commercialised
Some countries want to be successful at sport to prove they're better than a rival country !

Attitudes :

Every country wants their sports people to succeed because it brings the country pride and status amongst the top sporting countries. It can also help bring a nation together and encourage people to start exercising and become healthier. However, different countries promote sport in different ways:

UK : Phisical Education is compulsory in schools and they have introduced sport for all campaigns to get more people involved after they leave school. For their top competitors there are grants and sponsorships available.

USA : Again Phisical Education is compulsory in schools and school and college sport has a very high profile which attracts sponsorship. There are a lot of scholarship schemes to help athletes get into college, from where they can be drafted into professional leagues.

Former Eastern Bloc (countries once dominated by the USSR) : Sport was controlled by the state and talented children started training for sport at a very young age. They were also given token jobs in the army or industry so they could spend more time training. Since the collapse of the USSR in 1989, sport has been more open.

Third World Countries : Sports which cost the least money are promoted, like football and athletics. If athletes succeed on an international stage this brings more money into the country. Athletes are often supported by giving them token jobs within the government

Age and Gender in Sport :

The fact that women rarely compete against men, children are not often involved in adult competitions and there are sometimes separate veteran events show that both a competitors age and their gender affects their sporting performance.


Effects of age in sport :

A competitors age will affect their performance in the following ways:

Strength :

Children are not as strong as adults. We do not reach our maximum strength until we are fully grown at around 20
In your 20's and 30's it is easy to build up strength and muscle mass
In your 40's and onwards strength decreases as protein levels and muscle mass falls

Flexibility :

We are most flexible as adolescents
As we get older we tend to loose this suppleness

Oxygen capacity :

Our capacity to transport and use oxygen so effectively falls as we age

Injury and disease :

As we age, weakness and inflexibility make us more prone to injury
It also takes us longer to heal
Diseases such as cancer and heart disease are more prominent in the older population

Reaction times :

As we age, we get slower to react to a stimulus

Experience :

Older athletes are often called experienced, and this experience can be an important tool
Experience helps us to know how best to deal with a situation
In some sports age isn't a disadvantage. Sports which do not require strength, flexibility or endurance capacity can be competed in by both young and old. Sports which require accuracy or concentration, such as bowls and darts.


Effect of gender in sport :

Men and women rarely compete against each other because of some significant differences in our physical make-up:

Men have longer, heavier bones
Women have a wider and flatter pelvis
Women carry more body fat than men
A womens menstrual cycle can affect her performance
Men have higher testosterone levels and so bigger, stronger muscles
Women tend to be more flexible due to a lower muscle mass
Girls reach their physical maturity at 16 or 17, 3-4 years earlier than boys