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KIN 143Chapter 6.docx

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Biomedical Physio & Kines
BPK 143
Tony Leyland

Chapter 6 ENERGY PRODUCTION - Energy: the capacity or the ability to perform work. Energy is required for muscle contraction and other biological work, such as digestion, nerve conduction, secretion from glands, and so on - Power: the rate of change of energy or how quickly you can perform work. Power output is the rate at which working muscles can produce energy - Adenosine triphosphate (ATP): the only immediate energy source for muscle contraction - The phosphate bonds in ATP are high-energy bonds and energy is released when one of these bonds is broken - As ATP is the ONLY FUEL a cell can use, the body must be able to rebuild ATP as fast as it is broken down if muscle contraction and other processes are to continue - In general, complex organic molecules ( such as carbohydrates, proteins, and fats) will large amounts of energy stored in their chemical bonds are broken down into other molecules - ^ in this process energy is made available to produce ATP - Metabolic processes for producing ATP can be divided into two categories 1) Anaerobic processes- chemical processes that do not require the presence of oxygen delivered by the blood 2) Aerobic processes- processes that do require the presence of oxygen delivered by the blood Phosphagen System - Referred to as the immediate energy system, also known as the alactic energy ststem because there is no lactate by-product in this system - Also called the ATP-creatine-phosphate system - Creatine phosphate (CP) = phospho-creatine (PC) - ATP is broken down into adenosine disphosphate (ADP) and inorganic phosphate (Pi), which releases energy for work - As ATP is broken down to ADP+Pi during muscle contraction, it is almost simultaneously resynthesized using the energy released when the chemical bonds of CP are broken - Special proteins molecules called enzymes catalyze all chemical reactions in the body - - The two-way arrows highlight the principle of coupled reactions, whereby the energy released by one chemical reaction is used to drive another chemical reaction - The body has limited amount of ATP available for the muscle cells to use immediately - Body has a higher amount of CP that can be used ALMOST as quickly to release energy, which can resynthesize ADP and Pi into ATP - * we must continually resynthesize ATP to supply our cells with energy* - Both ATP and CP are stored in the muscle fibres, providing an immediate supply of energy - The phosphagen system is in high-power, short-duration activities Glycolytic system - An anaerobic system and it must use glucose as a fuel, also called the lactic acid energy system - The term lactic acid is misleading because the acidic form of lactate (lactic acid) cannot be formed in human tissues under normal circumstances - Glucose is a simple sugar that cells use as a source of energy and metabolic intermediate - Anaerobic glycolysis: the process of breaking down glucose - Glycogen: the form in which humans and other mammals store glucose - Glycogen is stored in liver and muscle tissues - Glucose s accounts for 99% of all sugars circulating in the blood - Blood glucose comes from the digestion of carbonhydrate and the breakdown of liver glycogen - Glysolysis: the chemical breakdown of glycogen or glucose; this metabolic pathway uses only carbohydrates - Glycolysis produces a net of two molecules of ATP for one molecule of glucose - If glycogen is used, there a net production of three ATPs as energy is realeased when splitting the glucose molecule from the glycogen cluster - The glycolytic system uses glucose molecules to obtain the necessary energy to produce ATP - The glucose can be make available in the muscle cells for breakdown to lactate by two principal methods : 1) Glucose molecules pass from the blood through the muscle cell membrane into the cell interior 2) The glucose splits from glycogen stores in the muscle cell itself - Anaerobic glycolysis can produce ATP rapidly to help meet energy requirement during severe exercise, when oxygen demand is greater than oxygen supply - High rates of ATP production by glycolysis cannot be sustained very long (60-90 seconds) because the acidity in the muscle cells (low muscle PH) associated with lactate accumulation inactivates a few key enzymes in the glycolytic metabolic pathway and interferes with the process of muscle contraction causing muscle fatigue, referred to as local muscle fatigue - The glycolytic system can provide a relatively rapid supply of ATP, but not as rapid as the phosphagen system - The glycolytic system can resynthesize a greater quantity of ATP than the phosphagen system, but the total amount of ATP that can be produced is still limited - Exercises that are performed at maximum rates for between one and three minutes, such as sprinting 400 metres, depend heavily on the glycolytic system for ATP energy OXIDATIVE SYSTEM - Also referred to as the aerobic energy system, it can be used when there is enough oxygen present to allow the required ATPs to be produced - It predominates in the majority of daily situations ( exercises that are in lower-intensity, longer duration) - Unlike the phosphagen and glycolytic systems, which are limited in the fuel they can use - The aerobic system can use carbohydrates, fats, and proteins - Sarcolemma: the cell membrane of a muscle cell - Sarcoplasm: a gelatin-like substance fills in the space between the myofibrils, where the enzymes for glycolysis are located - Mitochondira: sub-celluar structures (organelles) where most of the ATP in all cells are produced. It is referred as the powerhouses of cells - The oxidative (aerobic) production of ATP from glucose involves three processes: 1) Glycolysis: The same process in both aerobic and anerobic conditions. In the presence of oxygen, however, the pyruvate molecules are converted to acetyl-coenzyme A , which means they are not shunted to lactate 2) Kreb’s cycle (citric acid cycle): a series of chemical reactions occurring in mitochondira in which carbon dioxide is produced and hydrogen ions and electrons are removed from carbon atoms (oxidation). Acetyl-CoA molecules pass from the sarcoplasm into the mitochondira, where they enter Kreb’s cycle and electron transport chain (ETC), and are ultimately broken down to carbon dioxide and water 3) Electron transport chain (ETC)- Kreb’s cycle is coupled with the ETC. The hydrogen ions and electrons released during glycolysis and Kreb’s cycle are passed through the ETC and ATP is produced. A chain of chemical reactions occurs in which electrons and hydrogen ions combine with oxygen to form water, and ATP is resynthesized. Because this process requires oxygen, it is called oxidative phosphorylation. - In this process, up to 39 molecules of ATP can be generated from one molecule of glycogen - Aerobic breakdown of carbonhydrates yields many molecules of ATP and does not result in a build0up lactate and hydrogen ions, which can interfere with muscle function - The breakdown of glycogen can be summarized as:  Glycogen glucose pyruvate Acetyl – CoA Kreb’s cycle electron transport chain + O 2 CO 2 H O2+ ATP. - Aerobic breakdown of glucose yields much more energy than anaerobic breakdown Oxidation of fat (aerobic lipolysis) - Stored fat represents the body’s greatest source of available energy, and the quantity is almost unlimited - Although some fat is stored in all cells, the most active supplier of fatty acide molecules is adipose tissue - Fatty acids stored in the muscle are an important supplier of energy to the muscle during low power endurance activities - The breakdown of fat can be summarized as :  Fat fatty acids beta oxidation Acetyl-CoA Krebs cycle electron transport chain + O 2 CO 2 H O2+ ATP. - Complete metabolism of the three macronutrients (food compounds that provide us with energy) provides us with the following amount of energy: - 1 gram of fat produces 9 kcal of energy. - 1 gram of carbohydrate produces 4 kcal of energy. - 1 gram of protein produces 4 kcal of energy. - As oxygen delivery is limited by the oxygen transport system, carbonhydrate is the preferred fuel during high-intensity exercise - Glucose is the only source of fuel for the central nervous system (CNS) under non-starvation conditions Protein Metabolism - Endurance activities also cause the muscle breakdown and a small amount of protein is used as fuel during endurance exercise - Although carbohydrates and fatty acids are our preferred fuels, we can metabolize protein as an energy source - Digestion breaks protein down into amino acids, which are the building blocks used to repair tissues, make enzymes, and so on - If amino acids > body’s biological requirement, they are metabolized to glycogen or fat and subsequently used for energy metabolism - If amino acids are to be used for energy, their carbon skeletons are converted to acetyl-CoA, which enters the Kreb’s cycle of oxidation, producing ATP. - The final products of protein catabolism include carbon dioxide, water, ATP, urea, and ammonia - Protein breakdown during exercise of long duration can account for up to 5-10% of energy expenditure , because limited carbonhydrate stores become very low during events of long duration and some protein can be converted into glucose - Fat, cannot be converted to glucose - Adequate carbonhydrates in the body will “spare” protein breakdown and specifically conserve muscle protein, which is more readily utilized for energy than other proteins - The breakdown of protein can be summarized as follows:  Protein amino acids deamination Krebs cycle electron transport chain + O 2 CO 2 H 2 + ATP + urea + ammonia. - We tend to burn most fat at rest and during recovery rather relying on it during exercise, due to ie being a slow way to produce energy SUMMARY OF THE THREE ENERGY SYSTEMS - You never use ONLY one energy system during activity - When determining which energy system are being used, intensity (power output) is the key - The only exception to the rule is when starting exercise from a resting state, where even at moderate power outputs, individuals need to work anaerobically until the cardiorespiratory system “ramps up” to deliver enough oxygen to the working muscles - Cannot just attain the cardiac output immediately, this is why warm ups are important - “ you do not want to start working anaerobically during a race before you absolutely have to” - Energy continuum- energy systems work together, not just exclusively - - Power is the rate at which the energy system can produce ATP - Capacity is the total amount of ATP that system can produce - Fast glycolysis: breakdown of glucose in the absence of oxygen - Slow glycolysis: breakdown of glucose when oxygen is present - The efficiency of converting metabolic (chemical) power to mechanical power (output) is assumed to be 23% - Once you are working in the aerobic energy system, you theoretically can sustain that power oouput for hours Percentage of maximal power output expended during various activities ENERGY PRODUCTION IN SPORT Predominant energy pathways - - - Athletes who have the highest participate in sports where the predominate snergy system is the aerobic system - The best anaerobic work to improve are internals in the 30-90 second range - Because it stresses the aerobic system for a considerable amount of time and ensures a very high oxygen debt - TRAINING ALL THREE METABOLIC PATHWAYS - For complete fitness, we should train in all three metabolic pathways - Aerobic and anaerobic training must include very different modes of activity OXYGEN CONSUMPTION DURING RECOVERY: THE OXYGEN DEBT - The oxygen uptake during recovery from exercise is called the excess post-exercise oxygen consumption (EPOC) - EPOC is a measurably increased rate of oxygen uptake following exercise intended to erase the body’s “oxygen debt” - More oxygen is consumed at recovery time than at rest level - At the start of exercise, oxygen consumption increase progressively until the body reaches a steady state - Oxygen supply = oxygen demand - Oxygen deficit: the period of time during exercise when oxygen uptake does not meet the exercise needs - You cannot push your respiratory rate and heart rate to the required levels immediately at the start of exercise - In heavy exercise, oxygen consumption increased until is reached, but a steady-state situation is noto achieved - Oxygen demands continues to exceed oxygen supply throughout the entire workout and the oxygen deficit is quite large - The accumulation of hydrogen ions causing high muscle acidity means that the duration of such exercise is limited before muscle fatigue sets in - EPOC is not simply repaying the oxygen deficit - EPOC during recovery from exercise (oxygen debt) is caused by the following factors 1) Replenishment of muscle phosphagen stores (ATP and CP) and reloading hemoglobin and myoglobin with oxygen. This results in the rapid recovery phase. 2) Body temperature remaining elevated for a long time after cessation of strenuous exercise. This has a stimulating effect on the rate of chemical reactions in the cells of the body. Some studies have shown a close relationship between post-exercise oxygen consumption and return to normal levels of body temperature. This effect of elevated temperature on metabolism probably accounts for the greater part of the slow recovery phase of oxygen debt. 3) The residual effects of hormones such as epinephrine and thyroxine released during exercise. This may continue to increase the metabolism for a long time during recovery. 4) The energy needed for tissue repair and redistribution of ions (sodium, potassium, calcium) in the body. 5) The extra oxygen needed for heart and respiratory muscles, since heart rate and minute ventilation remain elevated during recovery. - The major portion (approx 75%) of the lactate produced during exercise is oxidized during recovery to provide energy in organs such as the heart, liver, kidneys, and skeletal muscle - The main source for re-establishing pre-exercise glycogen levels is the post-exercise carbohydrate in the diet, NOT resynthesized lactate RECOVERY TIME - Time varies - In extended, non-steady-state, intense exercise, the glycolytic system is activated to a significant extent - The return of muscle acidity can be accelerate to normal levels with active aerobic recovery exercise - Balance between carbohydrate and protein/fat is crucial BLOOD LACTATE - During moderate levels of exercise, energy demands are adequately met by reactions that use oxygen, and muscle acidity remains relatively normal - Although lactate is used as a fuel, it still signals that you are working anaerobically and that muscle acidity is rising, till it reaches a point called onset of blood lactate accumulation (OBLA) - Lactate threshold: the highest intensity of exercise that is not associated with a rise in blood lactate above resting levels - Lactate threshold is always used interchangeably with OBLA - For untrained subjects, lactate threshold occurs at 60% or more of the , and 75% for trained subjects - The difference response could be caused by an endurance athlete’s genetic endowment ( a high percentage of slow-twitch fibres) or by specific local adaptations within the muscle that help buffer the hydrogen ions and reduce muscle acidity - The quicker you remove lactate ( the liver converts it to blood glucose), the longer it iis likely for the subject to continue that exercise intensity BIOCHEMICAL ADAPTATIONS TO EXERCISE CONDITIONING Changes Due to Phosphagen System training The metabolic changes that occur due to activities that demand a high level of anaerobic metabolism (such as sprint, heavy weight training, and power-type training) include:  Increases in resting levels of anaerobic substrates, such as ATP and CP, and to a lesser extent glycogen content.  Increases in the quantity and activity of key enzymes controlling the phosphagen system (ATPase and creatine kinase). The largest alterations in anaerobic enzyme function occur in Type IIx fast-twitch muscle fibres.  Selective hypertrophy of fast-twitch fibres (particularly Type IIx). Changes Due to Gylcolytic System training The metabolic changes that occur due to activities that demand a high level of sustained anaerobic metabolism (such as 30-90 seconds of intense exercise) include:  Increases in resting levels of anaerobic substrates, such as ATP, CP, and glycogen.  Increases in the quantity and activity of key enzymes controlling the anaerobic phase of glucose breakdown. The largest alterations in anaerobic enzyme function and increases in size occur in Type IIa fast-twitch muscle fibres.  Selective hypertrophy of fast-twitch fibres (particularly Type IIa).  Increased ability to tolerate high muscle acidity during all-out exercise,
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