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The Science of Exercise

          Knowing what takes place when you exercise could help you plan and perform a workout routine.  You may also know more about what to expect for a result.

Three Energy Systems

Exercise basically involves three energy systems of the human body:  1) the Phosphagen (pro-creatine) system, 2) Glycolic (anaerobic) system, and 3) the Aerobic system.  All exercise involves working – or contracting – the muscles which need a substance called Adenosine Phosphate (or ATP) as a source of energy.  In fact, ATP is needed by every cell in the body to keep things going.  In the muscle, as the work increases the more ATP is needed to maintain the muscle.  Principal sites of ATP synthesis are mitochondria, spherical or rod shaped parts of the cell containing genetic material.  The three energy systems work together in phases, overlapping to maintain the vital supply of adenosine phosphate. 

At the beginning of exercise there is some free-floating ATP about the muscle cell which can be used immediately but only lasts three seconds.  (This is thought to be the energy available in an emergency situation and allows a person to react quickly and strongly to a sudden threat of danger or disaster).  To replenish this level of energy, the muscles contain a high-energy phosphate compound called creatine phosphate which work with ATP to comprise the Phosphagen system lasting 8 to 10 seconds, about the time it takes to run a 100m dash. 

A sprinter uses the phosphagen energy system to run 100 meters.

The muscles also contain major reserves of glycogen, a complex carbohydrate which is split to make glucose.  The cells use anaerobic metabolism – anaerobic meaning without the presence of oxygen – to make ATP and a byproduct called lactic acid.  This is done in about 12 chemical reactions, so it is slower than the Phosphagen system, but since it doesn’t involve the heart and lung (considered “additional machinery” at this point) it can still act rapidly and lasts about 90 seconds.  This is the time used to swim 100 or 200 meters or to run 200 or 400 meters. There is a limit to the anaerobic system because of the byproduct of lactic acid which makes the muscles hurt, or fatigue all of a sudden.                                                                  

After about two minutes, the body responds by supplying the muscles with oxygen which completely break down glucose into carbon dioxide and water – called Aerobic Respiration.  The glucose can come from what remains in the muscle, from the liver, or from food in the intestine.  But aerobic respiration can also use fatty acids from fat reserves (in the muscles and throughout body) to produce ATP.  Aerobic respiration will use carbohydrates first, then fats.  In an extreme condition of starvation, even proteins can be used as the last source of fuel.  Since the number of chemical reactions are even more than the other two systems, the aerobic system produces ATP at the slowest rate but can continue for several hours or more, as long as the fuel supply lasts. 

A swimmer uses both glycolic and aerobic energy systems.

Aerobic Capacity 

To become “aerobically strong” you will need to get oxygen to the blood and have the muscles use this oxygen.  This will depend on how well one can get blood to the muscle and then extract the oxygen from the blood into the muscle tissue.  In general, the working muscle takes oxygen out of the blood three times as much as the resting muscle.  In addition, the body has several ways of increasing oxygen-rich blood to the working muscle: increased local blood flow of the working muscle, diversion of blood from nonessential organs to the working muscle, increased flow from the heart (cardiac output), increased rate and depth of breathing, and increased unloading of oxygen from hemoglobin to the working muscle.  These mechanisms can increase the blood flow to your muscles by five times, which means the amount of oxygen made available to the working muscle can be increased a total of fifteen times during exercise.  

As you exercise regularly, vasodilation takes place within small thin-walled blood vessels of the muscle, called capillaries, which when dilated will deliver more oxygenated blood.  Also as you exercise, a remarkable diversion occurs where blood that once went to your stomach for instance, now goes to your muscles.  The sympathetic nervous system plays a role in constricting blood vessels to organ tissues diverting additional blood flow to the muscles.  The heart gets a workout too during exercise, which is also a muscle, and so it can become stronger and healthier.  Its job is to pump blood to the muscles according to a rate that will sustain the working muscle, or heart rate, but also the volume of blood it can move in one beat, called stroke volume.  In a resting heart rate the cardiac output (Heart Rate times Stroke Volume) is about 5 liters per minute.  When the heart is pumping at its maximum (Maximum Heart Rate, or MHR) the output is about 20 to 25 liters a minute.  The rate and depth of breathing with the lungs will affect the amount of oxygen that can enter the bloodstream.  The sympathetic nerves stimulate the lung muscles to increase the rate of breathing, and the increased rate and force of each heartbeat opens blood flow to more air sacs (alveoli).  The increased ventilation allows more oxygen to enter the blood.  A protein called hemoglobin is the means of exchanging oxygen and carbon dioxide.  During metabolic activity, a lower pH allows hemoglobin to release more oxygen than usual, and when the body is burning energy and producing waste, hemoglobin can carry away carbon dioxide.  

Training the body can make the muscles perform better and improve the efficiency of delivery of oxygen to the muscles.   

A means of measuring the aerobic capacity has been devised in Sport Medicine as the ‘VO2max Test.’  The ‘V’ is for volume, the ‘O2 is oxygen, and ‘max’ is for maximum.  The test measures the maximal oxygen consumption (also called “maximal oxygen uptake”) which is the maximum capacity of an individual’s body to transport and use oxygen during exercise.  VO2 max is expressed as an absolute rate in litres of oxygen per minute (liter/min) or as a relative rate in milli-litres of oxygen per kilogram of bodyweight per minute (ml/kg/min).  The latter is used to compare the performance of endurance athletes. 

The test for VO2max involves physical effort which is enough in duration and intensity to fully tax the aerobic energy system.  The intensity of exercise is progressively increased while measuring ventilation and oxygen and carbon dioxide concentration of the inhaled and exhaled air.  VO2max is reached when oxygen consumption remains constant despite an increase in workload.

A V02max test at an ergospirometry lab.

VO2max has been widely accepted as the single best measure of cardiovascular fitness and maximal aerobic power.  Absolute values of VO2max are typically 40-60% higher in men than in women.  The average untrained healthy male will have a VO2 max of approximately 35-40 ml/kg/min.  The following are relative VO2max (ml/kg/min) per sport: 1

Sport Male Athlete Female Athlete
Baseball/Softball 48-56 52-57
Basketball 40-60 43-60
Bicycling 62-74 47-57
Canoeing 55-67 48-52
Football            42-60  
Gymnastics 52-58 36-50
Ice Hockey 50-63  
Jockey  50-60  
Orienteering 47-53


Racquetball 55-62 50-60
Rowing 60-72 58-65
Skiing, Alpine 57-68


Skiing, Nordic 65-94


Ski Jumping 55-63  
Soccer  54-64 50-60
Speed Skating 56-73 44-55
Swimming 50-70


Track & Field, Discus 42-55  
Track & Field, Running 60-85    50-75


Weightlifting 38-52  
Wrestling 52-65  

Anaerobic Capacity 

To understand an individual’s anaerobic capacity is by understanding the significance of lactate threshold, the exercise intensity at which lactic acid begins to accumulate in the bloodstream.  The blood acidifies at high exercise intensities because high rates of ATP hydrolysis in the muscle release hydrogen ions and also bicarbonate in the blood is used up.  When exercising below this threshold, lactic acid (or lactate) produced by the muscles is removed by the body without building up.  With increased intensity, the lactate threshold (or anaerobic threshold) will be reached which is the onset of blood lactate accumulation (OBLA).  

The anaerobic threshold can vary among individuals and can be increased with training.  Interval training has shown to temporarily exceed the lactate threshold and then recover below the threshold while still exercising.  

Measuring lactate threshold may involve taking blood samples with a pinprick to a finger or earlobe when exercise intensity is progressively increased.  It can also be measured noninvasively by using gas-exchange methods, or by tests strategically devised to measure the mechanical efforts of athletes on a relative energy-power scale. (e.g. The Wingate Test).


A person may train very hard and still not be able make their muscles perform as well as another person’s.  This is because athletes can be born, and not just made.  This could be a grim realization for an aspiring athlete, so it is best to compare oneself to oneself in measuring one’s progress, and compete with a class or category of athletes that are much like yourself.  The reason is that strength, power, and endurance may be due in part to the distribution of fiber types within an individual's muscles. 

Muscles have a combination of two basic types of fibers, fast-twitch and slow-twitch.  Fast-twitch fibers are capable of developing greater forces and contract faster and therefore have a greater anaerobic capacity.  Slow-twitch muscle fibers, on the other hand, develop forces slowly and can maintain contractions longer and therefore have a higher aerobic capacity.  Whether one has more of one kind of muscle fiber over the other is mainly determined by the genes. 

Sprinters will have more fast-twitch fibers and marathon runners more slow-twitch fibers.  Generally, most people have an equal amount of both types, and it is not understood yet whether training can change the distribution of fiber types within an individual. 

Resistance training can improve the strength and endurance of muscle performance which increases the size of the muscle fibers.  It is not understood yet if training can increase the number of muscle fibers.  Muscle fibers can increase in size by having more protein content in the muscle.  This is achieved by making new proteins and decreasing the rate at which existing proteins break down.  These proteins include contractile proteins and enzymes related to metabolic reactions.

Exercise and Age 

Many agree that the physiological benefits of regular exercise can be a means of slowing the aging process, and are fairly certain that persons who are more physically active live longer than those who are sedentary.  We therefore speak in terms of a physiological age as well as a chronological age.  It has been pointed out that metabolic fitness training can reduce a person’s comparative chronological age by as much as thirty years, and that it is possible for a person who is 55 years-old to have the health and performance capability of an average 25-year-old. 

Proper exercise maintains lean body mass needed to sustain youthfulness along with sustained (or improved) cardiac output and the ability to perform aerobically and anaerobically.  Although most any type of exercise will help sustain – or even improve – youthfulness, exercise that engages all or most of the muscles simultaneously seem to give the best result, such as cross-country skiing, swimming, or triathlon.  These involve a certain amount of aerobic intensity which maintain the cardiovascular system consisting of heart and lung and all the life sustaining blood vessels (including capillaries) throughout the body, organs and muscles.  Another approach is to participate in a combination of various recreations such as tennis, golf, swimming, running, bicycling, etc. - which will exercise all the muscles along with cardio-conditioning.

1.       Sport Fitness Advisor – A Guide to Vo2max  (http://www.sport-fitness-advisor.com/VO2max.html)

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