EXERCISE PHYSIOLOGY

At an athletics meeting in Oxford in 1954, Roger Bannister became the first man to run a mile in under 4 minutes. In 1999, the Moroccan Hicham El Guerrouj ran the same distance in 3 minutes 43 seconds and he still holds that record in 2009. Genetically, humans today are no different from their predecessors. Yet year upon year, games upon games, athletes get better and world records keep tumbling. How is this being achieved?

Pixabay

First, there are advances in science and technology. A substantial support team now surrounds top athletes: sports physicians, sports physiotherapists, sports dieticians, etc. Training can now be tailored to an individual’s needs. Their muscle types can be analysed so that they do just the right balance of different training methods and intensities. Training machines can isolate a particular muscle group and/or reproduce movement patterns such as a javelin throw or tennis serve. Movement can be filmed and analysed with a computer so that a biomechanics expert can target areas for improvement.

Diet can also play a big part both before and after the event. Endurance athletes prepare with a mainly carbohydrate diet so that their glycogen stores are topped up. Following an event, athletes need to rehydrate and replace lost fuels so that they can recover and begin training again as soon as possible.

Another important aspect in preparing a world-class athlete is the prevention of injury. Modern sports physiotherapists have an array of tools at their disposal to diagnose potential weaknesses. If an injury does occur, there are variety of techniques available to speed the healing process.

Lastly, and crucially, there has been a change from amateur to professional status. Roger Bannister was a doctor who fitted his athletics around his medical school training. Many of today’s young athletes see their talent as a route to fame and fortune. They can devote all of their energies to excelling at their particular event or sport, knowing that success can set them up for life.

WHAT IS FITNESS?

What do we mean by fitness? Overall, it refers to a person’s ability to perform well in their chosen activity. But there are several different aspects to the concept of ‘fitness’.

There is strength and speed. Both aspects depend largely on the power in the muscles. Some men can lift 600 kilograms – about the weight of a small car – above their heads, and there are top sprinters who can cover 100 metres in less than 10 seconds – by running at a speed of about 36 kilometres per hour. A person’s strength depends to some extent on the cross-sectional size of their muscles, but other factors such as the shape of their skeleton, good technique and motivation are all important. So is skill. Some people are naturally gifted; their brain is able to control their muscles in just the right way to perform a particular activity. Of course, skill levels are further improved through coaching, training and practice.

Yet another factor is suppleness, the flexibility of the body. Often an underestimated aspect of fitness, most sports require a degree of suppleness, and performance can be improved by incorporating suppleness work into a training programme.

In this article, I will mainly focus on a fourth element of fitness: stamina – the body’s ‘staying power’. I’ll look at the way in which the body gets its energy, and the different types of ‘energy currency’ that are important in different sports.

GEARING UP FOR EXERCISE

As you sit and read this article, your body is ticking over nicely. Oxygen demand is low, and can be met comfortably by relatively shallow breathing and low pulse rate. Blood is delivering oxygen and glucose to your cells and waste products are being taken away.

The levels of these chemicals remain relatively constant, and homeostasis, that all-important steady state, is being maintained with relatively little fuss. There is, literally, ‘no sweat’.

All of this is rudely interrupted when you begin to exercise. The metabolic rate of the muscles increases by up to 20 times, or by 2000 per cent. To fuel this frantic activity, and to maintain some sort of stability, your body must adapt. The overall response of the body to exercise is an excellent example of how the different systems work together to carry on maintaining homeostasis.

Pixabay

HOMEOSTASIS AND EXERCISE PHYSIOLOGY

Exercise physiology is the study of the responses of the body to exercise. The effects are familiar: in the short term we sweat, pant and go red, while in the long term we ‘get fit’, improving our muscle tone, strength, stamina and general well-being. This chapter builds on your knowledge of the human body, pulling together many different areas to show you that homeostasis is a whole-body process involving many different systems.

Before we look at the ways in which the body responds to exercise, we need to set the scene by looking at some basic principles:

·         The process of cell respiration releases the energy in organic molecules such as glucose and lipids, and transfers the energy to a chemical called adenosine triphosphate, ATP.

·         ATP provides the energy for muscular contraction; it allows the fibres to slide over each other.

·         When ATP splits by hydrolysis into ADP and P_i (phosphate), energy is released.

·         The purpose of cell respiration is therefore to resynthesise ATP from ADP and phosphate.

·         ATP splitting is a coupled reaction. When we exercise, ATP hydrolysis is coupled with muscular contraction.

·         When ATP is split, some of the energy is used to power the muscle but as no energy transfer is 100 per cent efficient, some is always lost as heat. This is why vigorous exercise produces large amounts of hear that must escape from the body.

ENERGY AND EXERCISE

If the movement of muscles requires ATP, it follows that the ability of an athlete to move his or her muscles for any length of time requires a continuous supply of this essential chemical. Muscles have three sources of ATP:

·         The ATP already present: This provides instant energy and allows us movement on demand. When we contract our muscles as hard and as fast as possible, we use ATP far faster than it can possibly be made. So we rely on the ATP that has accumulated during periods of relative rest. During maximum effort there is only enough ATP for about three seconds, but there is a back-up chemical, creatine phosphate, CP. The energy in CP can be used to instantly resynthesize more ATP, allowing maximal exercise to continue for up to 10 seconds. This is called the ATP/CP system, or the alactic anaerobic system. Alactic means there is no build-up of lactic acid, or more accurately, lactate ions and anaerobic means that this system does not require oxygen. All events that require explosive bursts of energy, such as weightlifting or short sprints, use the alactic anaerobic system.

·         The ATP provided by glycolysis, the first phase of respiration: Glycolysis provides two ATP molecules per glucose molecule. This might not seem a lot compared with the 36 or so available from complete respiration, but it has two big advantages: it is relatively quick and it does not need oxygen. Thanks to this system, exercise can continue at near-maximum levels for up to one minute. However, there is a price to pay – the accumulation of lactate ions. Lactate lowers the pH in the muscles, causing fatigue and interfering with enzymes in muscle cells. The body can tolerate only limited levels of lactate. This system is known as the glycolytic or lactic anaerobic system, and is the main energy source for events that last between 10 and 60 seconds, such as the 400 metres hurdles.

·         The ATP provided by aerobic respiration: This is the complete breakdown of glucose, when each molecule yields about 36 molecules of ATP. The problem is that this system takes time to provide energy, and then it still has its limits. However, provided that the level of activity stays within those limits, the aerobic system can fuel exercise for a couple of hours of more. This is the main energy system for many sports: all those that last for longer than one minute. Prolonged exercise that increases heart rate and breathing rate, but that is sustainable for hours rather than minutes, is commonly known as aerobics.

structure of ATP tetraanion, the majority ATP species in solution. Smokefoot - Own work, Public Domain

The energy continuum

From what I have explained so far, it might appear that there are three separate energy systems. But, as we all know, we don’t have to stop exercises after 10 seconds and then again after 1 minute to wait for the next energy system to cut in and give us some ATP. All three energy systems blend smoothly together, one taking over from the other. This phenomenon is known as the energy continuum.

FUELS FOR EXERCISE

Respiration is the release of the energy contained in organic molecules such as glucose. All foods – carbohydrates, lipids and proteins – can be respired when the need arises. As a rough guide, the body uses the following fuels:

·         Glucose: This supplies the normal, everyday energy needs when we are eating regularly (i.e. not dieting or starving). Anaerobic exercise always depends on glucose, because lipids cannot be used to fuel glycolysis.

·         Lipids: Most lipid is stored in fat-storage, or adipose tissue. It takes a while to mobilize the lipid stores, but once the process has begun, this reservoir can fuel the body for as long as it lasts (for some of us almost indefinitely). Lipids cannot be used for short-term exercise, but they can be used to fuel aerobic exercise. This is why doing an aerobics class regularly can help to get rid of stored fat, if the exercise is done at the right intensity.

·         Protein: The body usually gets around 5 per cent of its energy from protein. The proportion increases significantly only during starvation, when there is no other fuel available. Recent research suggests that protein can be used to fuel endurance events, but to what extent remains uncertain.

Overall, the main fuel used during exercise depends on the intensity of the activity. If you take running as an example, the main fuel for a long, gentle jog is lipid. But, as the intensity increases, so does the proportion of energy that comes from glucose.

Diet for athletes

It has long been known that the correct diet can improve an athlete’s performance, and in recent years the diets of sports stars such as footballers have been increasingly in the spotlight.

The best approach is a holistic, or whole-body one. The most dedicated athletes pay close attention to their diet, taking in the right amount and types of food at the right time. In addition, they ensure that they have the right amount of sleep and minimize their alcohol intake, especially before important events.

How do you plan a diet to maximize performance? A widely used technique is known as carbo-loading or glycogen loading. About six days before an event, the athlete goes on to a low-carbohydrate, high-protein diet. This depletes the body’s glycogen reserves. Then, for the three days preceding the event the athlete goes on a high-carbohydrate diet, eating 8 to 18 grams of carbohydrate per kilogram of body weight per day. This significantly increases the amount of glycogen stored in the muscles, and this obviously helps during endurance events. It is of little help, however, if the event lasts for less than 90 minutes, and the effect wears off with repetition. Generally, athletes should aim for a diet in which 60 to 70 per cent of their energy comes from carbohydrate, increasing this figure in the day or two before an important event.

Dehydration and isotonic drinks

Long-term exercise leads to prolonged sweating, and this can easily dehydrate an athlete. By ‘dehydrate’ I mean that the blood and body fluids become too concentrated. In the correct biological jargon we say that the water potential is lowered. The practical consequence of this is that the blood becomes more viscous and cannot flow as easily. In addition, sweating loses vital ions such as sodium, potassium and chloride, collectively known as electrolytes. A loss of electrolytes can lead to muscular cramps and a greatly reduced performance.

Lucozade Sport, an example of isotonic drinks.

Pierre-Yves Beaudouin / Wikimedia Commons, CC BY-SA 4.0

But, it is not too serious; all the athlete needs to do is to replace the lost water, electrolytes and glucose. This is easy enough if the athlete happens to be at home, but dehydration can strike when you are three-quarters of the way through a sporting event. Getting a drink that can do the trick takes a bit of planning.

The fastest way to combat dehydration is to drink pure water. This way, water is absorbed as fast as possible by osmosis. However, if you add glucose and salts to the drink you lower the water potential and therefore slow down the water uptake. In fact, if the drink becomes too concentrated it actually makes matters worse by drawing water out of the blood, in the same way as drinking seawater would.

The best compromise is to take an isotonic drink, a mixture that has the same water potential as body fluids. This way, all three components are absorbed as rapidy as possible. So do you have to spend money on expensive isotonic drinks? The simple answer is no. It is easy to make up an isotonic mixture on the same principle as the oral rehydration therapy given to people suffering from diarrhoeal diseases. For events lasting less than 90 minutes, it is debatable whether isotonic drinks are any benefit at all. The fluid can be replaced by that cheapest of drinks, water, and the electrolytes and glucose are replaced later by any sensible meal.

In the next article, I’ll discuss on both the short and long term responses to exercise, the recovery process and the dangers of over-exercising.

Thanks for reading.

REFERENCES

https://www.physio-pedia.com/Exercise_Physiology

https://www.researchgate.net/topic/Clinical-Exercise-Physiology

http://www.css.edu/the-sentinel-blog/what-is-exercise-physiology.html

https://pdfs.semanticscholar.org/ba9c/e23190d4672c30815d6a914a647c7afdacd6.pdf

https://emedicine.medscape.com/article/88484-overview

https://study.com/directory/category/Biological_and_Biomedical_Sciences/Physiology_and_Related_Sciences/Exercise_Physiology.html

https://en.wikipedia.org/wiki/Exercise_physiology

https://biologydictionary.net/exercise-physiology/

https://en.wikipedia.org/wiki/Adenosine_triphosphate

https://en.wikipedia.org/wiki/Sports_drink