Lecture 3 EnduranceEfficiency Energy System in exercise

Endurance - High level endurance performance depends on factors; 1) a high VO2 max 2) a high LT LT tells us something about how much of the CV capacity you can take advantage of in a sustained effort. It is determined by skeletal muscle characteristics and training adaptations. * To measure the effective size of your endurance engine = VO2max X LT

Efficiency - It is defined as the percentage energy expended by the body that is converted to mechnical work (another form of energy). - Work Efficiency = Mechanical work / Chemical energy expended - Mechanical work can be measure by using an ergometer (bicycle, treadmill, arm ergometer or rowing machine). Energy expended can be measured by the body indirectly via its VO2 at sub maximal workloads.

ATP- the Body’s energy currency - Energy can be defined as the potential for performing work or producing force. - It is required for all kinds of bodily processes including growth and development, repair, the transport of various substances between cells and of course, muscle contraction. - Development of force by skeletal muscles requires a source of chemical energy in the form of the compound ATP. In muscle, energy from the breakdown ATP to ADP and inorganic phosphate (Pi) by myosin ATPase activates specific sites on the contractile elements, causing the muscle fibre to shorten. The hydrolysis of ATP yields approx 31 kJ of free energy per mole of ATP degraded to ADP and inorganic phosphate (Pi): ATP + H2O —>ADP + H+ + Pi -31 kJ per mole of ATP

ATP is also required for the restoration of the muscle cell membrane potential via the action of the Na+/K+-ATPase and for the active reuptake of calcium ions by the sarcoplasmic reticulum. ATP must be resynthesised at the same rate as it is being used. There are three different mechanisms involved in the resynthesis of ATP for muscle force generation: Phosphocreatine (PCr) hydrolysis. Glycolysis which involves metabolism of glucose-6- phosphate, derived from muscle glycogen or blood-borne glucose, and produces ATP by substrate-level phosphorylation reactions in the sarcoplasm. The products of carbohydrate, fat and protein metabolism can enter the tricarboxylic acid (TCA) cycle (also known as Krebs cycle) in the mitochondria and be oxidized to carbon dioxide and water. This process is known as oxidative phosphorylation and yields energy for the synthesis of ATP.

- The purpose of these mechanisms is to regenerate ATP at sufficient rates to prevent a significant fall in the intramuscular ATP concentration. - An ATP molecule consists of adenosine and three (tri) inorganic phosphate groups. When a molecule of ATP is combined with water (a process called hydrolysis), the last phosphate group splits away and releases energy. The molecule of adenosine triphosphate now becomes adenosine diphosphate or ADP. To replenish the limited stores of ATP, chemical reactions add a phosphate group back to ADP to create ATP. This process is called phosphorylation. If this occurs in the presence of oxygen it is labelled aerobic metabolism or oxidative phosphorylation. If it occurs without oxygen it is labelled anaerobic metabolism

- Several energy sources or substrates are available which can be used to power the production of ATP. One of these substrates, like existing ATP, is stored inside the cell and is called creatine phosphate. - Creatine phosphate is readily available to the cells and rapidly produces ATP. It also exists in limited concentrations and it is estimated that there is only about 100g of ATP and about 120g of creatine phosphate stored in the body, mostly within the muscles. Together ATP and creatine phosphate are called the ‘high-energy’ phosphogens

The other substrates that can the body can use to produce ATP include fat, carbohydrate and protein. Fat: It is stored predominantly as adipose tissue throughout the body and is a substantial energy reservoir. It is less accessible for cellular metabolism as it must first be reduced from its complex form, triglyceride, to the simpler components of glycerol and free fatty acids.

Carbohydrate: - At rest, carbohydrate is taken up by the muscles and liver and converted into glycogen. Glycogen can be used to form ATP and in the liver it can be converted into glucose and transported to the muscles via the blood. -A heavy training session can deplete carbohydrate stores in the muscles and liver, as can a restriction in dietary intake. - Carbohydrate can release energy much more quickly than fat.

Protein: - It is used as a source of energy, particularly during prolonged activity, however it must first be broken down into AA before then being converted into glucose.

3 energy system 1) The ATP-PCr System - ATP and creatine phosphate (also called phosphocreatine or PCr for short) make up the ATP-PCr system. - PCr is broken down releasing a phosphate and energy, which is then used to rebuild ATP. Recall, that ATP is ‘rebuilt’ by adding a phosphate to ADP in a process called phosphorylation. - The enzyme that controls the break down of PCr is called creatine kinase. - The ATP-PCr energy system can operate with or without oxygen but because it doesn’t rely on the presence of oxygen it said to be anaerobic.

2) The Glycolytic System - Glycolysis literally means the breakdown (lysis) of glucose and consists of a series of enzymatic reactions. Remember that the carbohydrates we eat supply the body with glucose, which can be stored as glycogen in the muscles or liver for later use. - The end product of glycolysis is pyruvic acid. Pyruvic acid can then be either funnelled through a process called the Krebs cycle or converted into lactic acid. Traditionally, if the final product was LA, the process was labelled anaerobic glycolysis and if the final product remained as pyruvate the process was labelled aerobic glycolysis. - Alternative terms that are often used are fast glycolysis if the final product is lactic acid and slow glycolysis for the process that leads to pyruvate being funnelled through the Krebs cycle.

3) The Oxidative System - The oxidative system consists four processes to produce ATP: 1- Slow glycolysis (aerobic glycolysis) 2- Krebs cycle (citric acid cycle or tricarboxylic acid cycle) 3- Electron transport chain 4- Beta oxidation -Slow glycolysis is exactly the same series of reactions as fast glycolysis that metabolise glucose to form two ATPs. The difference, however, is that the end product pyruvic acid is converted into a substance called acetyl coenzyme A rather than lactic acid (5). Following glycolysis, further ATP can be produced by funnelling acetyl coenzyme A through the Krebs cycle.

The Krebs cycle: It is a complex series of chemical reactions that continues the oxidization of glucose that was started during glycolysis. - Acetyl coenzyme A enters the Krebs cycle and is broken down in to carbon dioxide and hydrogen allowing more two more ATPs to be formed. - However, the hydrogen produced in the Krebs cycle plus the hydrogen produced during glycolysis, left unchecked would cause cells to become too acidic.

Electron Transport Chain: - Hydrogen is carried to the electron transport chain, another series of chemical reactions, and here it combines with oxygen to form water thus preventing acidification. - This chain, which requires the presence of oxygen, also results in 34 ATPs being formed.

Beta Oxidation: - Unlike glycolysis, the Krebs cycle and electron transport chain can metabolise fat as well as carbohydrate to produce ATP. - Lipolysis is the term used to describe the breakdown of fat (triglycerides) into the more basic units of glycerol and free fatty acids. - Before these free fatty acids can enter the Krebs cycle they must undergo a process of beta oxidation... a series of reactions to further reduce free fatty acids to acetyl coenzyme A and hydrogen.