| Bioenergetics |
| - Energy o The ability or capability to perform work - Bioenergetics (flow of energy in biological system) o Conversion of food – large carbohydrate, protein, and fat molecules, which contain chemical energy – into biologically usable forms of energy - Catabolism o Breakdown of large molecules into smaller molecules associated with release of energy - Anabolism o Synthesis of lager molecules from smaller molecules can be accomplished using the energy released from catabolic reactions - Exergonic reaction o Energy-releasing reactions and are generally catabolic - Endergonic reactions o It requires energy and include anabolic processes and the contraction of muscle - Metabolism o Total of all the catabolic/exergonic and anabolic/endergonic reactions in biological system - Adenosine triphosphate (ATP) o Allows for the transfer of energy from exergonic to endergonic reactions |
| Adenosine Triphosphate - ATP provides the energy for muscular contraction and thus human movement. - ATP is classified as high-energy molecule because it stores large amounts of energy in chemical bonds of the two terminal phosphate groups |
| Biological Energy Systems - 3 energy systems exist in mammalian muscle cells to replenish ATP o Phosphagen system (anaerobic process) o Glycolysis § Fast glycolysis § Slow glycolysis o Oxidative system (aerobic process) - Of the 3 main food components – carbohydrates, fats, and proteins – only carbohydrates can be metabolized for energy without direct involvement of oxygen. Therefore, the importance of carbohydrates in anaerobic metabolism cannot be underestimated. All 3 energy systems are active at a given time; however, the extent to which each is used depends primarily on the intensity of the activity and secondarily on its duration The Phosphagen System - Phosphagen system provides ATP primarily for short-term, high-intensity activities and is active at the start of all exercise regardless of intensity o ATP – Myosin ATPase ? ADP + Pi + Energy o ADP + Creatine phosphate – Creatine kinase ? ATP + Creatine - Type II (fast-twitch) muscle fibers contain greater concentrations of phosphagens than Type I (slow-twitch) fiber o 2 ADP – Myokinase ? ATP + AMP o This reaction provides an immediate source of ATP. It is also important because AMP is a powerful stimulant of glycolysis Control of the Phosphagen System - An increase in the sarcoplasmic concentraction of ADP promotes creatine kinase activity; an increase in ATP concentraction inhibits it Glycolysis - Glycolysis is the breakdown of carbohydrates – either glycogen stored in the muscle or glucose in the blood – to produce ATP - The enzymes for glycolysis are located in the cytoplasm of the cells (in muscle cells this is referred to as the sarcoplasm). The glycolytic system supplements the energy supply from the phosphagen system for high- intensity muscular activity - During fast glycolysis pyruvate is converted to lactic acid, providing ATP at a fast rate compared with slow glycolysis, in which pyuvate is transported to mitochondria for use in oxidative system Fast Glycolysis - Fast glycolysis occurs during periods of reduced oxygen availability in the muscle cells and results in the formation of lactic acid. - Muscular fatigue experienced during exercise is often associated with high tissue concentrations of lactic acid.Lactic acid accumulation in tissue is the result of an imbalance between production and utilization. Lactic acid is converted to its salt, lactate, by buffering systems in the muscle and blood. Unlike lactic acid in the muscle, lactate is not believed to be a fatigue-producing substance. - Instead. Lactate is often utilized as an energy substrate, especially in Type I and cardiac muscle fibers. It is also used in gluconeogenesis, the formation of glucose from lactate and nonbarbohydrate sources, during extended exercise and recovery. - Lactic acid production increases with exercise intensity and appears to be dependent upon muscle fiber type. - Blood lactate concentrations reflect lactic acid production and clearance and exercise intensity. Clearance of lactate from blood reflects return to homeostasis and thus person’s ability to recover. - Light activity during the postexercise period has been shown to increase lactate clearance rates, and aerobically trained and anaerobically trained athletes have faster lactate clearance rates compared with untrained people. - Peak blood lactate concentrations occur approximately 5 min after the cessation of exercise, a delay frequently attributed to time required to buffer and transport lactic acid from tissue to blood. - Glucose + 2Pi + 2ADP ? 2 Lactate + 2 ATP = H2O Slow Glycolysis - If oxygen is present in sufficient quantities in the mitochondria (specialized cellular organelles where the reactions of aerobic metabolism occur), the end product of glycolysis, pyruvate, is not converted to lactic acid but is transported to the mitochondria. Also transported to there are two molecules of reduced nicotinamide adenine dinucleotide (NADH) - When pyruvate enters the mitochondria it is converted to acetyl-CoA by the pyruvate dehydrogenase complex. Acetyl-CoA can then enter the Krebs cycle for further ATP production. NADH molecules enter electron transport system, where they can also be used to produce ATP. - Glucose + 2Pi + 2 ADP + 2 NAD+ ? 2 Pyruvate + 2 ATP + 2 NADH + 2 H2O Energy Yield of Glycolysis - Glycolysis produces a net two molecules of ATP from one molecule of glucose. However, if Glycogen (storage form of glucose) is used, there is net production of 3 ATPs because reaction of phophorylating (adding a phosphate group to) glucose, which requires one ATP, is bypassed. Control of Glycolysis - Glycolysis is stimulated by ammonia, inorganic phosphate, and ADP and a slight decrease in pH and is strongly stimulated by AMP. - Regulation of any series of reactions is the rate-limiting step, that is, the slowest reaction in the series. The rate-limiting step in glycolysis is the conversion of fructose-6-phosphate to fructose-1, 6-biphosphate, a reaction catalyzed by the enzyme phosphofructokinase (PFK). Lactate Threshold and Onset of Blood Lactate - Lactate threshold o Exercise intensity or relative intensity at which blood lactate begins an abrupt increase above the baseline concentration. - Onset of blood lactate accumulation (OBLA) o Second increase in the rate of lactate accumulation has been noted at higher relative intensities of exercise o Generally occurs when the concentration of blood lactate is near 4 mmol/L. - Training at intensities near or above LT or OBLA pushes the LT and OBLA to right (i.g., lactate accumulation occurs later at higher exercise intensity) This shift allows the athlete to perform at higher percentages of maximal oxygen uptake without as much lactate accumulation in blood. Some types of weight training may modify the LT and OBLA, allowing greater endurance. The Oxidative (Aerobic) System - Oxidative system uses primarily carbohydrates and fats as substrates. Protein is normally not metabolized significantly, except during long-term starvation & long bouts (>90min) of exercise - Oxidative metabolism of blood glucose & muscle glycogen begins with glycolysis. If oxygen is present in sufficient quantities, end product of glycolysis, pyruvate, is not converted to lactic acid, but is transported to mitochondria where it is taken up & enters the Krebs cycle, or citric acid cycle. - Reduced flavin adenine dinucleotide (FADH2) o Transport hydrogen atoms to electron transport chain to be used to produce ATP from ADP. - Electron transport chain o Use the NADH and FADH2 molecules to rephosphorylate ADP to ATP. - Oxidative phosphorylation o One molecule of NADH can produce three molecules of ATP, whereas one molecule of FADH2 can produce only two molecules. o The oxidative system, beginning with glycolysis, results in the production of approximately 38 ATPs from the degradation of 1 glucose molecule - Fats can also be used by the oxidative energy system - Free fatty acid enter mitochondria, where they undergo beta oxidation, series of reactions in which free fatty acids are broken down, resulting in which formation of acetyle-COA & hydrogen atoms. - Protein can be broken down into its constituent amino acids by various metabolic processes. Control of the Oxidative System - The rate-limiting step in the Krebs cycle is the conversion of isocitrate to ?-ketoglutarate, a reaction catalyzed by the enzyme isocitrate dehydrogenase. - The electron transport chain is inhibited by ATP and stimulated by ADP Energy Production Power and Capacity - Activities such as weight training, that are high-intensity, thus having high power output, require rapid rate of energy supplied; they rely almost entirely on energy supplied by phosphagen system. - Activities that are of low intensity (but long duration), such as marathon running, require a high capacity for energy supplied; they rely on the supply of energy from the oxidative energy system - The primary source of energy for activities between these two extremes shifts depending on the intensity and duration of the event. - Duration of the activity influences which energy system is being used. Athletic events range in duration from 1 to 3s (e.g, snatch & shot-put) to greater than 4 hr (e.g., triathlons & marathons). - At no time, during either exercise / rest, does any one energy system provide complete supply of energy. |
| Substrate Depletion and Repletion - Fatigue experienced during many activities is frequently associated with the depletion of phosphagens and glycogen. Phosphagens - Phosphagen muscle concentrations are more rapidly depleted as a result of high-intensity anaerobic exercise than of aerobic exercise. - Dynamic muscle actions, which produce external work, use more metabolic energy and typically deplete phosphagens to a greater extent than do isometric muscle actions. - Post-exercise phosphagen repletion can occur in a relatively short period; complete resynthesis of ATP appears to occur within 3 to 5 min and complete creatine phosphate resynthesis can occur within 8 min. Glycogen - Resting concentrations of liver and muscle glycogen can be influenced by training and dietary manipulations. - Muscle glycogen is a more important energy source than is liver glycogen during moderate- and high- intensity exercise; liver glycogen appears to be more important during low-intensity exercise. - Increases in relative exercise intensity of 50, 75, and 100% of maximal oxygen uptake result in increases in the rate of muscle glycogenolysis (broken down of glycogen) - Repletion of muscle glycogen during recovery is related to postexercise carbohydrate ingestion. Repletion appears to be optimal if 0.7 to 3.0 g of carbohydrate per kg body weight are ingested every 2 hr following exercise. |
| Bioenergetic Limiting Factors in Exercise Performance - Anerobic activities are the possible effect of lactic acid and increased tissue hydrogen ion concentration in both indirectly and directly limiting contractile force. Oxygen Uptake and Anaerobic Contribution to Work - Oxygen deficit o Anaerobic contribution to the total energy cost of exercise - Oxygen debt (Excess post-exercise oxygen consumption <EPOC>) o Post-exercise oxygen uptake o Oxygen uptake above resting values used to restore the body to the pre-exercise condition - Power output decreases as the contribution of energy from aerobic mechanisms increases - Different types of training may enhance either anaerobic or aerobic capacity |
| Metabolic Specificity of Training - Appropriate exercise intensities and rest intervals can permit the “selection” of specific energy systems during training for specific athletic events Interval Training - Allows appropriate metabolic systems to be stressed Combination Training - Aerobic training is added to the training of “anaerobic athletes” to enhance recovery because recovery primarily relies on aerobic mechanisms. However, aerobic training may reduce anaerobic performance capabilities, particularly high-strength, high-power performance. - Extensive aerobic training to enhance recovery from anaerobic events is not necessary and may be counterproductive in most strength and power sports. |