Session 20 Energy Out - Systems and Metabolism

Transcription

1 Session 20 Energy Out - Systems and Metabolism Australian Institute of Fitness 1 / 42

2 SETTING THE SCENE For the human body to function, it needs energy. In this session, we identify the energy pathways used to generate energy. We firstly explain what energy in the body actually is the energy rich molecule, adenosine triphosphate (ATP). We explore the aerobic and anaerobic pathways and examine the three energy systems to see how they provide energy for a variety of activities. At a micro level, we look inside a muscle cell and learn about the mitochondria. We also determine which muscle fibres the different energy systems rely on. Finally we discuss daily energy expenditure and metabolism and the factors that affect our metabolic rate and fat loss and fat gain. Remember that exercise physiology is just how the body functions during exercise! Australian Institute of Fitness 2 / 42

3 Fabulous Facts about ATP The Breakdown of ATP to ADP When people talk about energy, they are talking about ATP. It is an energy-rich molecule and the energy currency for everything in the body from mechanical energy for muscular activity (doing stuff), synthesis (making stuff) and electrical energy for nerve impulse transmission (sending messages within the body) plus many other metabolic processes. Energy cannot be created or destroyed it can only change its form (this explains why we can get fat if we eat too much energy!). The energy from the food that we eat is used to generate ATP which is then used as energy for various uses in the body. For the purposes of this session, we will discuss ATP as the energy-rich molecule used by skeletal muscle fibres for muscle contraction during exercise. In fact, it is the only molecule that can provide energy directly for muscle contraction; hence, it is very important in exercise. Energy is released when ATP is broken down into ADP and a free phosphate ion (P). To release energy, the third phosphate bond is broken. This energy powers the cross bridges during muscular contractions. At rest, your muscles only use a modest amount of ATP whereas during exercise, the muscles are contracting and using ATP at a rapid pace. Not only do the muscles need ATP; it is also used to power the metabolic reactions. We only have enough ATP stored inside the muscle fibres to last a few seconds of exercise so ATP needs to be continually synthesised or replenished this is where the energy pathways and energy systems come into play! Australian Institute of Fitness 3 / 42

4 ENERGY PATHWAYS ATP can be produced via two energy pathways: 1. The aerobic pathway (with oxygen) muscle fibres produce ATP in the presence of oxygen; we use this pathway the most. 2. The anaerobic pathway (without oxygen) muscle fibres produce ATP in the absence of oxygen; this pathway is used when the energy demand is greater than oxygen supply. Our muscles have got the ability to switch into anaerobic metabolism when the aerobic pathway cannot keep up with the energy demand. This depends on the capacity of the cardiorespiratory system to deliver oxygen to the cells. A fit person can deliver more oxygen to their cells, so they will switch into the anaerobic pathway at a higher intensity than a less fit person. Let s use a car analogy to explain the pathways. Anaerobic pathways are used for short, fast trips around the block, whereas aerobic pathways are used for longer, slower journeys. What about the terms aerobic and anaerobic exercise? Aerobic exercise is an activity that uses large muscle groups, can be maintained continuously and is rhythmical in nature (American College of Sports Medicine). It is where the body can provide sufficient oxygen to meet demand and relies on the aerobic pathway. Anaerobic exercise is short lasting, high-intensity activity, where the energy demand for oxygen exceeds the oxygen supply. So we use the anaerobic pathway to supply the energy. Australian Institute of Fitness 4 / 42

5 Energy Systems There are two energy pathways as described above and three energy systems as follows: Phosphate system (up to ten seconds) provides immediate energy used for activities of a high to very high intensity e.g., shotput Lactate system (15 seconds to three minutes) provides short-term energy for activities of a moderate to high intensity e.g., 200m sprint Aerobic system (>3mins) provides long-term energy for activities of a lower intensity e.g, long distance run. Before we get into the detail, let s use a simple analogy of the 'little house in the mountains' story. You have a little house in the mountains and when you go to stay in this house, the first thing you do is start the fire in the lounge room. To start the fire you use the kindling and paper right next to the fireplace. This will get your fire roaring, however, the fuel source will run out rapidly. This is like the phosphate energy system. To keep the fire going, you walk to the woodbin on the patio, grab some sticks, walk back and put it on the fire and it s hot. It took a little longer to get this fuel but you now have enough fuel for several minutes. This is like the lactate energy system. To keep the fire going for a long period of time you have to drive to the forest, cut up some wood, bring it back, stoke up the fire and put wood stores in the woodbin. There is now plenty of wood to keep you toasty warm but it did take a long time to get it. Australian Institute of Fitness 5 / 42

6 Snapshot Here is a quick overview before we dive in and explore the systems in detail! There are two anaerobic energy systems: 1. Phosphate (also known as phosphagen or alactic): ATP and creatine phosphate (CP) are broken down to release energy for short, intense bursts of activity lasting between 10 to 15 seconds. The energy is produced without the presence of oxygen. 2. Lactate (also known as lactic anaerobic glycolysis): Glycogen is broken down without oxygen this is called anaerobic glycolysis to produce energy for intense activities lasting up to two minutes. As a by-product, a build up of hydrogen ions develops acidosis in the muscle causing muscle discomfort and fatigue, the burn. For many years, lactate or lactic acid was thought to cause muscle fatigue but recent research states that the accumulation of hydrogen ions is responsible. Lactate is present but is not responsible for muscle fatigue. There is only one aerobic energy system providing our third system: 3. Aerobic Energy System: Fats, glycogen and protein (in emergencies) are broken down to produce large amounts of energy in the presence of oxygen. Carbon-dioxide (CO 2 ) and water (H 2 O) are by-products of this process. Definitions Aerobic production of energy with oxygen Anaerobic production of energy without oxygen Force production the strength of the muscle contraction Australian Institute of Fitness 6 / 42

7 PHOSPHATE ENERGY SYSTEM Adenosine Tri Phosphate (ATP) and Creatine Phosphate (CP) are stored in the muscle and are used directly for muscular contraction. Hence, the body has an immediate supply of energy for short intense bursts of activity. Once the immediate supply of ATP has been depleted, we need to replenish it. The phosphate energy system will give you energy very quickly but only for a short period of time, commonly around 5-10 seconds but up to 15 seconds in individuals highly-trained in this system. Examples include 100m sprint and a 1RM (one repetition maximum) bench press or squat. Muscle fibres? This system will have a great reliance on fast twitch muscle fibres. So what happens? The breaking of P away from ATP releases energy for mechanical energy. Heat is also produced as a by-product of energy production. Creatine Phosphate (CP) is used to rebuild ADP back to ATP. This is replenished through the aerobic system where oxygen is available. The aerobic system breaks down carbohydrates, fats and protein and the energy generated is used to rebuild the ATP molecules. The ATP-CP (phosphate) system is dependent on the aerobic system for the rebuilding of energy. This is why one breathes with an elevated rate after an anaerobic burst (oxygen debt). There are no metabolic by-products from the ATP-CP system, as every molecule that is broken down is replenished. The limitation in this system to produce energy is not the amount of ATP or CP available, but the ability of the body to rebuild it to keep pace with the energy requirement. This is why an unfit person cannot exercise at very high-intensity; they get puffed or do not have the ability to use oxygen to break down substrates very effectively to rebuild their energy stores (ATP). After a maximum high-intensity anaerobic burst, approximately 50% of the ATP stores have been rebuilt after 30 seconds, 75% after 60 seconds, 87% after 90 seconds and over 90% after two minutes. After approximately three minutes the phosphate system is almost totally replenished. How the phosphate system produces energy? CP present in the muscle cell is broken down to creatine + phosphate and energy is released. This energy is used for resynthesis of ATP. ATP is broken down to release energy for muscle contraction. Hence, only one step is involved to resynthesise ATP which is why it is so quick and powerful. The ATP CP System Australian Institute of Fitness 7 / 42

8 Snapshot Here is a summary of the key points. For short bursts of energy High force production High energy production and immediate (up to 10 seconds) Fast twitch muscle fibres Fuel creatinephosphate (CP) Poor endurance Quick recovery (2 to 5 minutes) No metabolic by-products Australian Institute of Fitness 8 / 42

9 LACTATE ENERGY SYSTEM The lactate energy system produces energy through a process called anaerobic glycolysis which is the incomplete breakdown of glucose. Anaerobic glycolysis uses glucose (and stored glycogen) as an energy source which is broken down in the absence of oxygen to form pyruvate or pyruvic acid. When one molecule of glucose is broken down anaerobically (without oxygen), it yields two molecules of ATP. This system provides sufficient energy for 2-3 minutes of high intensity activities such as weight training and sprinting over m. Inside the muscle fibre, there is a substantial amount of glycogen (stored glucose) ready to provide the fuel for muscle contraction. Muscle fibres? The lactate energy system will have a greater reliance on fast twitch muscle fibres. What about muscle fatigue? In the short term, high-intensity exercise, the problem is not the lack of fuel but rather the ability to produce energy at a rate great enough to keep pace with the energy requirement, and the ability to tolerate the build-up of hydrogen ions which leads to muscular fatigue. An individual who has developed a good base of aerobic fitness is able to rely on the aerobic energy system even at higher levels of intensity to replenish their energy. Australian Institute of Fitness 9 / 42

10 What About Lactic Acid? Well, here is the science. When someone is respiring anaerobically, using the lactate system, the by-product of the ATP resynthesis is lactic acid. However, this is such a strong acid that it immediately splits into two parts lactate and hydrogen ions (refer to diagram). For many years, exercise scientists have been able to measure blood lactate levels and as lactate levels were high when athletes were reaching fatigue in exercise, lactate was blamed for muscular fatigue. However, lactate is not the baddie the hydrogen ions are. Research now reveals that it is the hydrogen ions (H + ) that build up to make an area (like a muscle) highly acidic which can interfere with muscular contraction in the area the H + has been produced and cause muscular discomfort, fatigue and a burning sensation in the muscles. If the acidosis (lowered ph) in the muscle interferes with the muscular contraction process, it will cause an exercise participant to lower the intensity of exercise or to stop altogether due to fatigue. The fate of lactate Lactate is important as it facilitates the pathway of hydrogen through the cellular membrane and out of the cell, and then it is converted back into pyruvate to be used by the aerobic system. The fate of hydrogen ions The hydrogen ions are released into the blood where they meet bi-carbonate ions, which buffer the H + and convert it to CO 2 to be exhaled out. As an exercise participant works at increasing intensities, they produce more hydrogen ions, subsequently, the body sends more bicarbonate to neutralize the rising acidosis. The participant s breathing rate will rise to get rid of the CO 2, which lowers the acidity of the working cells, so the participant can continue to exercise. With training, this process of hydrogen ion clearing becomes more efficient, and the participant will be able to work harder for longer at a high intensity. Important points You do not have lactic acid hanging around in your muscles it is broken into lactate and hydrogen ions. It is the hydrogen ions, not lactate that interferes with muscular contraction and causes fatigue. Lactate helps remove the hydrogen ions. Hard breathing is a good indicator that you are working anaerobically. What about removal of lactate and hydrogen ions? After exercise, the hydrogen ions and lactate are removed from the tissues and blood. A good aerobic recovery cool-down is essential to keep oxygen consumption and blood flow slightly elevated for waste product removal. With the assistance of the aerobic energy system (think active recovery), it is believed that: 20% of the lactate is actually converted to glycogen in the liver 60% is oxidised through the aerobic system to H 2 O and CO 2 15% is converted to protein Australian Institute of Fitness 10 / 42

11 1-2% excreted in the urine and sweat. Half (50%) of the by-product is removed within 30 minutes, 75% within 50 minutes, 87% within 75 minutes, 95% within 100 minutes and is almost fully removed within two hours. Australian Institute of Fitness 11 / 42

12 Exercise Example The lactate energy system is used in sustained high-intensity anaerobic events lasting around three minutes in total. A great example of this is Ice Hockey a hockey team will have up to three different lines, i.e., combinations of players who will be on the ice for around 180 seconds, and then trade off for another line to take their place. This sport is entirely designed around the lactate system - athletes come onto the ice, work at a really high intensity until they fatigue their lactate system, and then come off for a rest to replenish ATP supplies and clear cellular H +. Once this replenishment is done, they can go straight back onto the ice and keep performing at the high intensity. Hence, for an athlete, it is obviously highly beneficial to be able to clear hydrogen ions quickly. Some elite athletes, such as Michael Phelps (American Olympic swimmer), are genetically gifted; their bodies produce disproportionate amounts of lactate to speed up clearing of hydrogen ions. This is what gives them their competitive edge. For everyone else, application of specific training protocols will help us to improve the clearing rate of hydrogen ions, enabling us to work harder, for longer. We will discuss this in a later session on cardiovascular training. Australian Institute of Fitness 12 / 42

13 Snapshot Here is a summary of this system. For short sustained bursts of energy High force production High energy production but short term (2 to 3 minutes) Fast twitch muscle fibres Average endurance Fuel glycogen Significant recovery (30 to 120 minutes) Metabolic by-products hydrogen ions can inhibit muscle contraction, lactate helps remove the hydrogen ions Performing interval training consisting of multiple, short bouts of high intensity activity (using this system) interspersed with recovery intervals can be advantageous for the fit person wanting fat loss. Why do you need to be fit? Remember that this sytem is utilised for high intensity activity, usually greater than 80% effort, hence a less fit person would need to gradually increase their fitness and training intensity in order to safely sustain lactate training. Australian Institute of Fitness 13 / 42

14 Nice to Know - Inside a Muscle Cell All muscle cells have an outer membrane and multiple nuclei which controls the cell s activities. The cytoplasm is all the cellular contents between the membrane and the nucleus put simply, it is the insides of the muscle cell. Obviously, there are numerous cell parts but we are only going to focus on a few of them to discuss where all this metabolic action is taking place! All the chemical reactions in anaerobic metabolism occur in the cytoplasm of the muscle cell/fibre. The chemical reactions in aerobic metabolism occur in the mitochondrion (mitochondria is plural) of the muscle cell/fibre. It is where the oxygen and fuel combine to create ATP and produces most of the cell s ATP; hence, they are referred to as the powerhouses of the cell. The number of mitochondria range from a few hundred to a few thousand depending on the rate of ATP production the more metabolically active, the more mitochondria! Active cells such as muscle fibres have a large number of mitochondria and the number and size can increase with fitness which is a cardiovascular training adaptation. Good news! Australian Institute of Fitness 14 / 42

15 AEROBIC ENERGY SYSTEM The aerobic energy system produces energy by breaking down substrates (predominantly fat and glucose) in the presence of oxygen with the by-products of carbon dioxide and water. Aerobic metabolism is the greatest source of energy production. The process of aerobic metabolism of substrates occurs through a complex series of chemical reactions within the mitochondria of the cell. In the aerobic metabolism of glucose, pyruvate or pyruvic acid is formed by glycolysis and then converted with oxygen present into acetyl coenzyme A. This then enters a metabolic pathway called the Krebs cycle or Citric Acid cycle. After being through the Krebs cycle, active bodies are created that provide the energy to replenish ATP molecules. Aerobic metabolism provides a large amount of energy. When a glucose molecule is broken down aerobically, enough energy is produced to replenish ATP molecules. When a free fatty acid (FFA) molecule is broken down aerobically, enough energy to replenish 129 ATP or more is produced depending on the fatty acid. The complete breakdown of a molecule of triglyceride (which consists of glycerol and FFAs) can yield up to 460 molecules of ATP, especially in the very fit individual who has a high mitochondrial density. Hence, aerobic metabolism provides large amounts of energy to the human body; however, the process involves numerous chemical reactions which takes time, hence it is a 'slower' system of producing ATP. This system will have a greater reliance on slow twitch muscle fibres. The aerobic energy system is the only system where fat is broken down (as well as glucose) to produce energy; oxygen is required to break down fat as an energy source and it can only be broken down in the presence of glucose. How does the aerobic system produce energy? The number of ATP molecules produced in the diagram below are just examples. Australian Institute of Fitness 15 / 42

16 Nice to Know - The Fate of Fats Fats or lipids, come in many forms for the various functions that fat performs. For example, we have lipids to transport cholesterol (lipoproteins) and others that contribute to the myelin sheath that surrounds nerves and speed up nerve impulses. It is the triglycerides that are broken down to produce ATP. Triglycerides are stored in adipose (fat) tissue and must first be split into glycerol and fatty acids. These fats are then broken down via different pathways and as a result, the number of ATP produced does range. For example, a 16-carbon fatty acid yields 129 molecules of ATP whereas the complete breakdown of a molecule of triglyceride yields 460 molecules of ATP. Regardless, fat provides much more energy than glucose and hence, is extremely important for the endurance athlete. However, be aware that fat oxidation relies on the presence of glucose so if glucose levels fall then so does the ability to break down fat to produce energy. Hence the science tells us that fat burns in a carbohydrate flame. Australian Institute of Fitness 16 / 42

17 Snapshot Here is a summary of this system. For sustained energy Low force production Low energy production but long term (2 minutes onwards) Great endurance Slow twitch muscle fibres Fuel glycogen, fat, protein minimally Long recovery time to replace fuels (glycogen) Metabolic by-products CO 2 and H 2 O Limitation fuel supply (glycogen) Australian Institute of Fitness 17 / 42

18 SUMMARISING THE SYSTEMS Below is a snapshot of the three energy systems: Intensity Phosphate Lactate Aerobic Explosive Very high intensity % of max effort Duration 1-15 seconds of explosive activity High intensity 60-95% of max effort pending fitness level 10 seconds to 3 minutes Low intensity Up to 60-80% of max effort pending fitness level At low intensity there is no limit (pending glycogen stores) Fuel Creatine phosphate Glycogen/glucose Glycogen/glucose Fats (Proteins in emergency) Waste Product No waste products Recovery 100%-2 minutes plus (50% every 30 seconds) The Little house in the Mountains Hydrogen ions Lactate 30 mins-2 hours to remove waste products We introduced this analogy earlier let us now revisit it to confirm our understanding. These types of stories are also useful when educating your clients. You have a little house in the mountains and when you go to stay in this house, the first thing you do is start the fire in the lounge room. To start the fire you use the kindling and paper right next to the fireplace. This will get your fire roaring, however, the fuel source will run out rapidly. This is like the phosphate energy system. Carbon dioxide and water Time to replace fuel stores To keep the fire going, you walk to the woodbin on the patio, grab some sticks, walk back and put it on the fire and it s hot. It took a little longer to get this fuel but you now have enough fuel for several minutes. This is like the lactate energy system. To keep the fire going for a long period of time you have to drive to the forest, cut up some wood, bring it back, stoke up the fire and put wood stores in the woodbin. There is now plenty of wood to keep you toasty warm, but it takes a long time to get it. The paper next to the fireplace is the phosphate system it burns quickly. Likewise, the phosphate system gives you energy quickly but only for a short time so if you want to continue the activity you will have to find an alternative energy provider. The sticks in the woodbin on the patio is the lactate system; it takes a bit longer to produce the energy and at a slightly slower rate (so you will have to slow down) and it will only last a couple of minutes. The wood in the forest is like the aerobic system; it provides massive amounts of energy, Australian Institute of Fitness 18 / 42

19 however, it takes longer to get it! It takes time to transport substrates to the mitochondria in the cell, where aerobic metabolism takes place and the substrates to go through the stages of the Krebs cycle. The big picture You use all of your energy systems constantly; however, the predominant energy system during a given activity or time period is determined by the rate at which you require the energy. Australian Institute of Fitness 19 / 42

20 Energy Systems Working Together In many activities, especially sport, the muscles are frequently switching from aerobic to anaerobic metabolism to accommodate for the differing levels of intensity. The graph below is a good illustration of our Forrest Gump example: Australian Institute of Fitness 20 / 42

21 What is the Anaerobic Threshold? Depending on which book you pick up, you may get several definitions and explanations about what the anaerobic (or lactate) threshold is. For example, researchers, Shield and Young state that the anaerobic threshold is the highest intensity of steady state exercise at which lactic acid does not accumulate. Powers and Howley however, state that the point of a systematic rise in blood lactic acid during exercise is the anaerobic threshold. Traditionally in exercise, the anaerobic threshold was thought of as that point at which your muscles switched from aerobic to anaerobic metabolism to produce energy. In an untrained person, this could be as low as 50-60% of the maximal aerobic capacity, and in a highly trained individual, as high as 80-90% maximal capacity. This difference is a result of the person s ability to tolerate and remove the by-products of anaerobic metabolism. The term lactate threshold rather than anaerobic threshold has become increasingly popular since the primary definition for this threshold is where blood lactate begins to accumulate past resting levels. The other term that the fitness professional must be aware of is onset of blood lactate accumulation (OBLA) which as the name suggests, is where blood lactate significantly accumulates and in practice, is when the energy demands of exercise are no longer being met by the aerobic system and the lactate system plays a predominant role. Note: for athlete development this threshold is usually portrayed as a percentage of VO 2 max. The big picture When the training intensity is at a level where the accumulation of lactate is higher than its removal, this leads to increases in blood lactate concentrations. This is indicative of high levels of hydrogen ions which cause muscle fatigue. Analogy: If a boat in the ocean is taking on more water than it can pump out... it's going to sink. Hydrogen ions represent the water coming aboard your ship, so if your bilge pumps are not working effectively in rough seas, your boat will sink! Australian Institute of Fitness 21 / 42

22 Quick Quiz Using your knowledge of the energy systems, what would be the predominant energy system and substrate used during each of the activities below? High jump Answer Marathon Answer 400m sprint Answer What would be the physiological reason for fatigue, in relation to the predominant energy system, for the same activities? High jump Answer Marathon Answer 400m sprint Answer Australian Institute of Fitness 22 / 42

23 MUSCLE FIBRES AND ENERGY SYSTEMS In this section, we revisit the muscle fibres and their connection to the energy systems. This is extremely important as there are two interconnected elements we look at in program design to achieve results: 1. What energy system do we need to train? 2. What muscle fibres do we need to recruit? (Further to this we would consider what muscle groups need to be targeted.) Why you need to know about muscle fibres How our muscles respond and adapt to exercise depends on the muscle fibre types present in the muscle. There is a reason why people can run really fast over short distances, and others can run for much longer periods of time without getting fatigued. Understanding the differences between muscle fibre types can assist us to design programs that best suit our individual clients. We can now discuss the metabolic aspects with our new-found knowledge of energy systems. Definitions Mitochondrial density concentration of mitochondria in the muscle cell; the powerhouses where oxygen and fuel combine to produce energy for muscle contraction. Capillary density concentration of blood capillaries in the muscle. Oxidative capacity ability to use oxygen. Glycolytic capacity ability to use glycogen which is the stored version on glucose from the carbohydrates we eat. Remember that there are two types of fibres slow twitch and fast twitch that make up muscles. As you ll recall, our type II or fast twitch fibres can be further broken down to type IIa and type IIb. As such, we can refer to three types of fibres: Type I (Slow Oxidative) Type IIa (Fast Oxidative Glycolytic) Type IIb (Fast Glycolytic) What s with the colour? Slow twitch muscle fibres are also known as red fibres due to their colour under a microscope. This is indicative of large amounts of myoglobin a red-coloured, iron-containing protein. When oxygen enters the muscle cell, it attaches onto the myoglobin. When oxygen is needed to Australian Institute of Fitness 23 / 42

24 produce energy for muscle contraction, the myoglobin releases the oxygen to be used by the mitochondria. The fast twitch fibres have less reliance on oxygen, have less myoglobin and hence look pale in comparison (termed white but really lighter shades of red). Australian Institute of Fitness 24 / 42

25 Type I - Slow Twitch Fibres Slow twitch fibres are small, aerobic fibres that have a low force output (weaker) and a high endurance capacity (don t get fatigued). Structurally, they have a high mitochondrial and capillary density, and a high myoglobin content. They have a low supply of creatine phosphate (a high-energy substrate used for quick, explosive movements). Functionally, slow twitch fibres are used for aerobic activities requiring low-level force production, such as walking and maintaining posture. Most activities of daily living use slow twitch muscle fibres. Snapshot Small motor nerve Small muscle fibres Low frequency of firing Weak contractions Aerobic in nature High endurance High mitochondrial density High capillary density Australian Institute of Fitness 25 / 42

26 Type II - Fast Twitch Fibres Fast twitch fibres are large, anaerobic fibres that have a high force output (strong) and a very low endurance capacity (get tired quickly). These fibres can be further divided into Type IIa and Type IIb. Type IIa - Fast Oxidative Glycolytic Fibres (FOG) These are known as the chameleon fibres as they can change or adapt depending on the type of training. They have a moderate resistance to fatigue and represent a transition between the two extremes of the slow twitch and fast twitch type IIb fibres. They are termed intermediate fibres, as they have the characteristics of both slow type I and type II fibres and depending on the type of training performed will adapt (like a chameleon fibre that changes or adapts dependant on the environment). Structurally, they have a high mitochondrial density, a medium capillary density, and a medium myoglobin content. They have a high supply of creatine phosphate and glycogen. Functionally, they are used for prolonged anaerobic activities with a relatively high force output, such as running the 400 metres. Snapshot Large motor nerve Intermediate muscle fibres Fast frequency of firing Strong contractions Both aerobic and anaerobic in nature Intermediate endurance High mitochondrial density Intermediate capillary density Type IIb - Fast Twitch Glycolytic Fibres Type IIb fibres are very sensitive to fatigue and are used for short anaerobic, high force production activities, such as sprinting, hurdling, jumping and throwing a shot put or javelin. These fibres are also capable of producing more power than slow twitch fibres. Unlike type IIa fibres, type IIb fibres have a low mitochondrial and capillary density and myoglobin content. Snapshot: Very large motor nerve Very large muscle fibres Very high frequency of firing Very strong contractions Anaerobic in nature Low endurance Low mitochondrial density Low capillary density Australian Institute of Fitness 26 / 42

27 Muscle Fibre Type Summary Find below a summary of the characteristics of the three muscle fibre types type I, type IIa and type IIb. Fibre Type Structural Characteristics Muscle fibre diameter Myoglobin content and colour Mitochondrial density Type I slow twitch oxidative fibres Type IIa fast twitchoxidative glycolytic fibres Smallest Intermediate Largest Large amount; red Large amount; red/pink High High Low Capillary density High Intermediate Low Type IIb fast twitchglycolytic fibres Small amount; white/pale Size of motor nerve Small Large Very large Functional Characteristics Oxidative capacity High Intermediate Low Glycolytic capacity Low Intermediate High Contraction time (firing frequency) Force production (force of contraction) Resistance to fatigue (level of endurance) Major storage fuel Activity used for Slow Fast Very fast Low High Very high High Intermediate Low Fatty Acids, Glycogen Aerobic, endurance activities and maintaining posture CP, Glycogen Long term anaerobic such as running, weight training CP, Glycogen Short term anaerobic explosive movements such as a short sprint or shotput Australian Institute of Fitness 27 / 42

28 BURNING FUELS In an exercise physiology laboratory it is possible to determine what fuels or energy substrates (fatty acids and glucose) are being used during various intensities and duration of exercise. This is determined by analysing the expired gas during exercise and looking at the ratio of carbon dioxide to oxygen. Nice to know Respiratory Quotient (RQ) This measure is known as the Respiratory Quotient (RQ) or Respiratory Exchange Ratio (RER): It is measured with complex and expensive equipment and represented as a number: RER = 1.0 represents carbohydrate utilisation (glucose) RER = 0.71 represents fat utilisation (fatty acids) RER = 0.86 represents 50% fat and 50% carbohydrate utilisation Although the majority of us will never set foot in an exercise physiology lab we can apply the results found during such testing in our exercise prescription; we can use the science to aid in writing exercise programs that are safe, efficient and effective. Burning fuels at rest The first thing we need to understand is that at rest, we burn both fat and carbohydrate. This makes sense as we are functioning aerobically (with oxygen) where both fat and carbohydrate are burned in the presence of oxygen. At rest under normal circumstances, most people burn about 50:50 fat carbohydrate (hence a RER value of about 0.86). A fitter individual may burn a greater percentage of fat at rest (perhaps 55-60% hence a lower RER) and will also burn more fat during exercise than an unfit person. Fit people are better fat-burning machines! It would be fantastic for fat loss if we could use 100% fat (RER of 0.71) but this cannot happen in normal exercise circumstances. In fact, we need glucose in order to burn fat as you learnt in the energy systems. Hence, there is no exercise that is totally 100% fat burning. What we need to do is understand how fitness level, the energy systems (aerobic and anaerobic) and intensity and duration affect the fuels we use. Australian Institute of Fitness 28 / 42

29 Do Fit People Burn More Fat? Yes, fit people are more efficient fat-burning machines. The advantages of fitness include: Fit people burn more fat at rest and at all workloads Fit people have an earlier onset of fat oxidation (burn more fat sooner) Fit people have higher anaerobic thresholds so can exercise longer at a higher level and still burn fat Reasons for this include: Larger and more mitochondria Greater muscle mass Greater capillarisation More fat stored on the myofibril Australian Institute of Fitness 29 / 42

30 How Hard Should I Exercise to Burn Fat? When we start to exercise, the body s preferred fuel is glucose (carbohydrate) for both fit and unfit individuals. This is because carbohydrate provides more energy per litre of oxygen than fat. Hence, when we are breathing harder to increase oxygen delivery to meet the energy demand of exercise, it is understandable that the body becomes more reliant on carbohydrate than fat. Hence, the closer the RER is to 1.0, the greater the percentage of glucose being utilised. An RER of 1.0 would resemble very high-intensity exercise of an anaerobic nature. While this type of training is great for developing anaerobic fitness, it burns glycogen/glucose rather than fat and can only be maintained for a short duration. Also be aware that for the less conditioned person, high-intensity exercise can be very stressful on the joints and muscles thus requiring several days of recovery. This may interfere with a person s fat loss program of exercising most days of the week. A lower RER where more fat is utilised occurs with lower intensity exercise. So, for an activity to be fat burning, it must use the aerobic energy system and be of a low to moderate intensity. The intensity of exercise must consider the fitness level of the person and what they are actually capable of. The average person wanting fat loss may not be fit enough to sustain long duration, high-intensity exercise of an anaerobic nature, but can participate in long duration low to moderate exercise. Australian Institute of Fitness 30 / 42

31 What About the Fitter Individual? High-intensity short intervals have been shown in the research to have a positive impact on fat loss. Read on. Researchers at the University of NSW and the Garvan Institute studied 45 overweight women over 15 weeks, putting one group of 15 through a 20- minute cycling regime in which they sprinted on a stationary bike for eight seconds followed by 12 seconds of cycling lightly. This totals 60 intervals and requires the participants to have the ability to sprint. This group experienced more fat loss than the other group who exercised at a continuous, regular pace for 40 minutes. This is great news but does require your clients to train with heart rates around 85-95% of the maximum. So always build your clients up to this by starting lighter and gradually increasing intensity and fitness. So if you don t have the fitness level or musculoskeletal strength for interval training, the exercise prescription for fat loss is to choose predominantly aerobic activities of a low to moderate intensity that can be sustained for a longer duration and can be performed at least every second day. If the goal is to burn calories, it is important to choose exercise that can be performed many times per week, can be sustained and has a low risk of injury. A balanced program which consists of cardiovascular exercise (great for burning energy and improving health), resistance training (improves muscular function, boosts metabolism and keeps bones strong) and flexibility training (keeps muscles supple) is the way to go. Australian Institute of Fitness 31 / 42

32 How Long Should I Exercise to Burn Fat? The longer the duration, the more calories being consumed which is good news for clients wanting fat loss. Continuous aerobic exercise would be the preferred type of exercise where both fat and glucose are being used. Now, clients can accumulate exercise periods over the day; for example, four 10-minute brisk walks to accumulate 40 minutes of exercise. However, research suggests that there is a gradual shift towards fat utilisation the longer you go in a single exercise bout. Science can explain this. Most people can store up to 2,000 calories of glycogen in their muscles and liver whereas even a lean person will have 100,000+ calories in the form of stored fat. Therefore, glycogen is a limited fuel whereas fat is not. The body cannot afford to run out of glycogen and when exercise duration is extended, the body will want to conserve glycogen. As a result, there is a gradual shift towards greater fat utilisation when exercising aerobically at a low to moderate intensity. This means that a slightly greater percentage of fat would be used at 60 minutes compared to 30 minutes compared to 10 minutes. There is no such thing as a magical time when fat burning starts. If the exercise is aerobic, we will be burning both fat and glucose from the onset of exercise. Remember, most people can exercise hard or long, but not hard and long together! Australian Institute of Fitness 32 / 42

33 Do I Burn Fat Doing Weights? Weight training, like all resistance exercise, is predominantly anaerobic, therefore glucose will be the main fuel utilised during the activities, not fat. Hence, it is not considered a fat-burning activity. We also need to be aware that you cannot spotreduce. What this means is that fat will not be removed from an area by working that area. For example, doing a biceps curl will not remove and use fat from the arm. Likewise, doing an abdominal curl will not remove the fat from the stomach. However, weight training can improve muscle mass which has a positive effect on metabolism. If we increase muscle mass (hypertrophy), it slightly increases our resting metabolic rate. It we lose muscle mass (atrophy), our metabolic rate will go down slightly and this would be counterproductive for someone wanting fat loss. Australian Institute of Fitness 33 / 42

34 Coach Tip Australian Institute of Fitness 34 / 42

35 DAILY ENERGY EXPENDITURE What is daily energy expenditure (DEE)? It is the total energy (calories/kilojoules) that your body uses in a 24-hour period. Major Components of DEE The circle below represents your DEE and the approximate typical percentages of each component of DEE for an average person. These percentages will of course vary, particularly if someone has a very active occupation or is an athlete training many hours of the day. Resting Metabolism Resting metabolism contributes approximately 70% to our DEE. This includes the energy required to stay alive during sleep and most hours of the day when we are sitting or standing but not physically active. Total lean mass, especially muscle mass, is largely responsible for the basal metabolic rate (BMR) so anything that increases lean mass, will increase BMR. If muscle mass is decreased then BMR will decrease stressing the importance of preserving muscle mass. For adults, BMR metabolism represents about 5,900kJ per day for the average female and 7,100kJ per day for the average male (approximately 1,400-1,700 Calories per day; to convert to Calories, divide kj by 4.184). Men have a higher BMR than women largely because they have a higher proportion of muscle cells that burn more energy. As resting metabolism is the biggest chunk and constitutes most hours of the day, it would be important to boost metabolism for the purposes of fat loss. Physical Activity This contributes about 15% to our DEE and is the most variable component as it depends on the physical activity levels of a person. This includes the energy used in any incidental exercise such as walking around and planned exercise. This could total up to only 500kJ (~100 Calories) per day for an inactive person or up to 8,000kJ (~2,000 Calories) for a professional athlete involved in several training sessions per day. For a 70kg person, a 30-minute brisk walk would burn about kJ (~100 Calories) whereas a 30-minute run would burn about 1,000kJ (~250 Calories). Exercise is a great way to boost your DEE! Australian Institute of Fitness 35 / 42

36 Thermogenesis Thermogenesis is the thermic effect of food and contributes about 15% to our DEE. You use energy to eat, digest, absorb and store the food you eat. The rise occurs soon after you start eating and peaks two to three hours later. The increase depends on the size of the meal and the types of foods eaten. It is highly variable as a professional athlete will be consuming a lot more food than an office worker will need to consume. Calorie vs kilojoule? Calories and kilojoules both define the energy value of food and the amount of energy produced during exercise - ever noticed the generation of heat during physical activity? In Australia, legislation was passed requiring all food labels to be labelled in kilojoules (kj) whereas Calories is still used in the USA. You may sometimes see both Calories and kilojoules used but the label MUST have kilojoules. In relation to exercise, both are used with Calories being popular due to a greater understanding of the numbers if you have grown up with calories, and also the 'smaller' number is often easier to work with. However, there are some important kilojoules to know such as 8,700kJ being the benchmark for average daily energy consumption for healthy weight Australian adults. Visit for more information. Good to know A person requires an energy intake of at least their BMR to maintain brain function. The above benchmark of 8,700kJ (2080 Calories) is what is recommended as the average daily consumption. Many low-calorie fad diets recommend less than 1,200 Calories per day (~ 5,000kJ) which is an extreme reduction in energy intake can you see how dangerous dieting could be for the brain? Australian Institute of Fitness 36 / 42

37 Metabolism We have discussed metabolism in general terms so far. It is often easier to understand metabolism if you say it is the energy required to stay alive. It is however, all the chemical reactions that occur in your body. There are two types of metabolism catabolism and anabolism. Catabolism includes the chemical reactions by which substrates are broken down and energy is stored and released. These substrates are carbohydrates, fats and proteins. Anabolism is the building of structural and functional components that the body needs. Metabolism is an energy-balancing act between catabolism and anabolism and the molecule ATP is the star of the show providing energy for: Maintenance and repair of body tissue Regulation of all the chemical reactions of cellular respiration The provision of energy for muscle contraction Conduction of nerve impulses Secretion by glands Synthesis of compounds which become part of the body s structures Growth, reproduction and milk production during pregnancy Metabolic Rate (MR) The speed or rate at which the chemical reactions use energy is termed the metabolic rate. It can be thought of as the amount of energy that an individual is expending at any given time. Your metabolic rate is constantly changing throughout the day; for example, if you are asleep your metabolic rate is very low, when we are inactive our metabolic rate is low, as the demand for energy is not great. However, if you were to carry your shopping up the stairs there would be a dramatic increase in your metabolic rate. Our metabolic rate is influenced by many factors as follows: Body size - larger bodies have a larger MR. Lean muscle tissue - muscle is more metabolically active hence increases MR. Body fat - fat cells are less metabolically active and burn less kilojoules than most other tissues and organs of the body. Growth - infants and children have higher energy demand per unit of body weight due to the energy demands of growth. Gender - generally, men have faster metabolisms than women because they tend to be larger and have more muscle. Age - metabolism slows with age, due to a loss in muscle tissue but also due to hormonal and neurological changes. Genetic predisposition - your metabolic rate may be partly decided by your genes. Hormonal and nervous controls metabolism is controlled by the nervous and hormonal systems; hormonal imbalances can influence how quickly or slowly the body burns kilojoules. Dietary deficiencies - for example, a diet low in iodine reduces thyroid function, which slows the metabolism. Environmental temperature - if temperature is very low or very high, the body has to work harder to maintain its normal body temperature. Australian Institute of Fitness 37 / 42

38 Infection or illness - MR increases because the body has to work harder to build new tissues and to create an immune response. Crash dieting, starving or fasting - eating too few kilojoules encourages the body to slow the metabolism to conserve energy; BMR can drop by up to 15 per cent. If there is also a loss of lean muscle tissue, MR drops further. Amount of physical activity - hard-working muscles burn energy and also need energy for repair and recovery. Drugs - some drugs, like caffeine or nicotine, can increase metabolism. Australian Institute of Fitness 38 / 42

39 How Do We Influence Our Metabolic Rate? As mentioned, boosting the metabolic rate is recommended for fat loss. It means that you will be burning more calories, every hour, every day and night, for weeks, months and years! Let s take a look at some of the factors which increase our metabolic rate: You can do more exercise drive the car more and it will use more fuel You can fidget more often You can be a male (generally more muscle) You can build more muscle mass You can consume caffeine not too much though! You can move to a colder climate burn more energy to keep warm You can make sure you eat on a regular basis, especially breakfast! You can eat some spicy treats You can become pregnant and lactate (ladies only) And some of the factors which decrease metabolic rate: You can be inactive if the car sits in the garage, it doesn t use fuel You can miss meals if we don t eat after 3-4 hours, our body knows it and will go into a semi-starvation state where it slows down the metabolism to conserve precious energy! You can go on a crash diet or period of fasting not only does the body slow down its metabolism but it may use protein in muscle to provide glucose for energy we then lose muscle which decreases metabolism further You can get older metabolic rate decreases 2% per decade after the age of 20 You can be female You can lose muscle mass You can keep warm we burn energy when we shiver Australian Institute of Fitness 39 / 42

40 Energy Balance There are numerous other factors affecting metabolism, fat gain and fat loss. These include hormones, climate, genetics, sleep, and nutrition to name a few. It is highly complex but there is a simple universal law the law of energy balance. Remember, energy cannot be created or destroyed; it can only change its form! It we eat too much energy, it has to go somewhere. Gaining and losing weight can be attributed to the following. Energy In < Energy Out = Decrease in Weight Energy In > Energy Out = Increase in Weight Energy In = Energy Out = Stable Weight Energy In refers to meals and snacks consumed including fluids. Energy Out includes factors such as metabolic rate, thermic effect of food, incidental movement, and exercise. Australian Institute of Fitness 40 / 42

41 ROUND UP The energy pathways and systems influence so many aspects of our training and the achievement of common fitness goals. Make sure you have a sound knowledge of the following key aspects of this session. Energy pathways Energy systems Phosphate energy system Lactate energy system Aerobic energy system Muscle fibres and energy systems Burning fuels Daily energy expenditure Metabolism Energy balance Australian Institute of Fitness 41 / 42