Physiological Aspects of Exercise and Sport - Essay Example

Physiological Aspects of Exercise and Sport Introduction ATP is the ultimate source of energy for all living forms. Numerous pathways lead to continuous and uninterrupted production of this chemical 'store-house' of energy. The chemical energy of food material is stored in the terminal phosphate bond of ATP and enzymatic (hydrolytic) cleavage of ATP to ADP and inorganic phosphate by action of membrane-bound enzyme complex, termed ATPase, yields high energy (ca. 30.5 kJ per mole) to carry out vital physiological reactions, which include voluntary and involuntary muscle contraction. Human body utilizes about 15-30% of dietary energy for physical activity at a rate of 1-1.5 kcal min-1 at rest and 15-36 kcal min-1 during exertion. On an average, human body can store 86 g of ATP. As ATP is fast utilized during intense exercise, unless it is equally fast regenerated, there would be acute depletion of energy. The re-synthesis of ATP by way of phosphorylation of ADP is an endothermic process. Acquisition of energy in muscle There are three important systems in muscles that produce ATP (1): ATP-creatine phosphate (CP), Lactic acid and aerobic. The former two systems operate anaerobically while the third needs O2, derived from respiratory intake of air and subsequent flow through blood stream for breakdown of food stuffs. All the systems can simultaneously operate or, depending on the exercise regime, can sequence each other. Further, the intensity and time-course of given exercise will determine which system predominates over others. CP is an energy rich bonded compound present in muscle sarcoplasm whose level is about 5-times higher than ATP. This difference ensures that once available ATP depletes, CP rapidly phosphorylates ADP by enzyme, Creatine kinase. This is the most efficient ATP production system in muscle (rate = 73 mmol ATP s-1). It takes about only 4-10 s to consume most of the CP and concomitantly anaerobic glycolysis of storage muscle glycogen starts to feed ATP through substrate level phosphorylation (rate = 39 mmol s-1). A series of reactions first convert glycogen to glucose which is then cataboliszed to pyruvate and then lactate. These reactions release sufficient energy to re-synthesize two ATP per molecule of glucose. Other factors like stress hormone adrenaline also stimulates glycolysis thus generating lactic acid. Excess lactic acid brings about acidic pH and can lower glycolysis by inhibiting a participating enzyme, phosphofructokinase (2). Excess lactate also interferes with Ca2+ binding with the troponin-tropomyosin complex, a pre-requisite for muscle contraction. Before the problem aggravates some of the lactic acid diffuses in to blood stream for further oxidative breakdown in liver by lactate dehydrogenase. A portion is assimilated as glycogen by gluconeogenesis and remains stored in liver. Lactic acid system lasts for ca. 100 s before oxidative breakdown of remaining glycogen, involving, besides glycolysis, Kreb's cycle and respiratory electron transport pathways set in. Though the process yields more ATP (30-32 molecules) per glucose molecule, the net production is reduced to ca. 6 mmol s-1. Here, muscle protein myoglobin plays a critical role to capture oxygen from haemoglobin and to transport to mitochondria, the organelle responsible for oxidative phosphorylation. There is a continuum of ATP production after muscle glycogen exhaustion first by partial utilization of liver glycogen, and subsequently carbohydrates and stored lipids. Among lipids, triglycerides present in adipose tissue are mainly hydrolysed by enzyme, lipase into glycerol and free fatty acids; the later enters muscles and further gets oxidized to acetyl co-enzyme A. Lipids are capable to aerobically support about 60% of energy demand of prolong sub-maximal exercise. An interesting connection is seen between lipid breakdown by Kreb's cycle and glycolysis of sugars. Build-up of acetyl coenzyme A derived from triglycerides would inhibit pyruvate dehydrogenase, and citric acid, an intermediate of Kreb's cycle, would inhibit 6-phosphofructokinase, both glycolytic enzymes, resulting in alleviation of glycogen/glucose breakdown by either aerobic or anaerobic pathway. A large lipid composition of over 15% of body weight can provide sufficient energy to extend the exercise regime for long duration. However, when the exercise intensity is high, carbohydrates are required and first stored glycogen and subsequently blood glucose start to get aerobically utilized. Storage protein in quite rare cases can support muscle energy demand. One crucial factor regulating aerobic system is the component of O2 responsible for replenishing PC and glycogen, and lactic acid breakdown during the recovery (resting) phase in between the vigorous exercise phases. Furthermore, the levels of hormones - adrenaline, nor-adrenaline and glucagons also increase, and insulin level decrease. These hormonal dynamics help greater lipid oxidation (by adrenaline/nor-adrenaline) and diversion of lipid derivatives to glucose synthesis (glucagons). Energy pathways during sport specific-exercises Performances of short duration and high intensity such as football, field events, weightlifting, baseball, volleyball etc. demand brief yet maximum effort. CP-ATP system seems to be the most efficient process of energy in such events (1). Stored carbohydrates and fats mainly replenish the phosphate pool. Events that demand a last bout of energy over the sustained basal energy level, such as 400 m run or 100 m swim, basically depend on lactic acid system. The accumulated lactic acid can be sustained for over 180 s and subsequent aerobic metabolism in rest brings down this build-up. Long-term and less rigorous events like marathon depend on athlete's ability to deliver and utilize oxygen and sustain aerobic system of energy. Adequate training of athletes significantly improvises performance, but depending of the nature of sport and mode of energy acquisition, specific training methods have been devised (3,4). For improving CP-ATP energy system, the recommended training involves 5-8 s of high intensity workouts like 3-10 ' 30 m sprint with 30 s recovery periods, and for lactic acid/anaerobic glycolysis system 5-8 ' 300 m with 45 s recovery. Most spectacular changes in aerobic metabolism is training for aerobic energy system, which depends on various intensity runs (20 ' 200 m to 1 ' 5 km) as long slow runs at 50-70% max heart rate, or continuous runs at 60-90% max heart rate with varying recovery periods. Relief periods depend on the required time for replenishment of key energy metabolites. Trained athletes are capable to perform 80-90% of their maximum capabilities for aerobic metabolism. Even during anaerobic system the level of lactic acid accumulation goes 20-30% higher compared to un-trained subjects. The increase by about 20% of phosphofructokinase and other glycolytic enzymes in trained state justifies greater ability to derive energy. The total count and size of mitochondria per cell also increase upon training, enabling more membrane surface area for ATP production. Myoglobin content rises substantially to cope with excess oxygen transfer to mitochondria. In long-term sub-maximal exercise like mild jogging, oxygen consumption steadily increases to meet the aerobic system and reaches a plateau, which is faster in case of the trained athlete. Since at this state, oxygen uptake (VO2 in L min-1) is a balance between ATP production by aerobic metabolism and energy requirement of muscle to replenish ATP, theoretically exercise could continue indefinitely. This is of course not possible as other factors such as electrolyte depletion, heat generation, tissue damage and reserve fuel status determines the endurance of the person. Essentially, oxygen deficit occurs during exercise. This leads to build-up of lactic acid and depletion of energy-rich phosphates including ATP. In general, oxygen consumption during mild or moderate exercise is similar for both trained and un-trained persons, but the former reaches steady state more rapidly and has much reduced oxygen deficit. One other important aerobic index is maximum VO2, representing the highest consumption rate of oxygen at an additional workload over sub-maximal exercise, simulated by elevation of treadmill or increasing resistance to paddling a cycle ergometer. Blood lactic acid levels peak at this state. The max VO2 provides quantitative statement on the person's ability to aerobic energy transfer, which after attaining peak gradually reduces to another plateau, VO2, corresponding to sub-maximal exercise intensity. As a result, the velocity of exercise (say a sprint) would automatically reduce. Training can improvise max VO2 and lactate-tolerance. By composition skeletal muscles may have two distinct fibre types: fast twitch and slow twitch. Energetically, fast twitch muscles are tuned for anaerobic glycolytic metabolism of glycogen yielding quick energy, whereas the slow twitch depends more on long-term aerobic ATP re-synthesis mechanisms. We find direct correlation between the relative compositions of slow twitch fibres and max VO2, especially in those athletes trained for aerobic endurance capacities, for e.g. marathon running, skiing etc. On the other hand, weight lifters and sprinters who depend more on anaerobic pathways for deriving ATP have greater composition of fast twitch fibres, and have relatively low max VO2. The length of the fibres also varies depending on the nature of the sport. By training it is now possible to partially change the muscle types from one mode of metabolism to the other. There are associated cardiovascular and respiratory changes as well. An example to improve max VO2 by training is by increasing tidal volume of air intake and breathing frequencies (high minute ventilation). The metabolic changes during training can be partially reversed by 1 or 2 week-detraining sessions. Physical training and dietary need for footballers In team events like football, players run 10-11 km at very modest speed, sprint for average 1000 m at sub-maximal intensity and accelerate for 40-60 times, covering ca. 20 m with changing direction every 5 s (5). Typically, contribution of CP-ATP, lactic acid and aerobic pathways for energy is 50, 20 and 30. Training for football players, known as interval training, falls within parameters of short duration, high intensity and moderate recovery protocols. Typically, these involve progressively increasing repetitions of 10-20 s rigorous work bouts and relief periods, initially longer duration but gradually reduced to 30-40 s or even shorter. Most of the workout accounts for 1.8-2.5 km distance. From two non-consecutive days per week it can intensify to three. Running up to 150 m in straight line followed by sharp foot plants and cuttings and subsequently position-specific skilful movements/running with short sprints are the part of different exercises. Goalkeeper hardly needs any long running but for conditioning, plyometrics, agility drills, reaction training and 8-m sprints would be required. More-or-less same practice session is for Fullbacks and Defensive backs. Halfbacks have to do most of the running and thus should be practiced by long runs and quick sprints. Forwards have to be better trained for even longer duration of jogging. During heavy exercise regime, 8-10 g carbohydrate intake per kg body weight would be enough to sustain basal glycogen level. A week before an event carbohydrate intake should increase and during exercise, supplementation of glucose-polymer beverages, are good. In an interesting finding it was shown that during football game supplementation of liquid carbohydrates neither improvise performance nor help in post anaerobic recovery of glycogen, as expected (6,7). Possible explanation is that excess glucose oxidation generates free radicals leading to muscle cell damage and loss of contraction. Antioxidants like vitamin E can prevent this to happen. For long, creatine from meat extracts has been used as supplement for athletes who perform brief repeated bouts of intense exercise, in order to give muscle strength and endurance. Due to side-effects like muscle cramps, soreness, diarrhoea, gastrointestinal disorders and kidney and liver problems its use has been banned for athletes. However, it has been found that for footballers, supplementation of permissible amount of creatine in diet has shown no complications (8). In conclusion, energy metabolism plays and important role in muscular activity related to a sport event, and proper training and diet can help better fitness and performance of the players. References 1. McArdle, W. D., Katch, F. I. & Katch, V. I. 1991, Exercise Physiology: Energy, Nutrition, and Human Performance, Lea & Febiger, Philadelphia/London. 2. Newsholme, E. A. 1984, 'Metabolic control and its importance in sprinting and endurance running', Physiological Chemistry of Training and Detraining, Karger AG Basel, pp 1-8. 3. Brian, M. 2007, Energy Pathways, http://www.brianmac.co.uk/energy.htm. 4. Goninon, A. 2001, Energy systems and the energy continuum, http://www.cranbrook.kent.sch.uk/site/pe/ALevel/Docs/Physiology/Energy%20Systems%20and%20the%20energy%20continuum.ppt. 5. Anderson, O. Nutrition for Football Players | Football Diet; Diet and nutrition for football players: How soccer players can overcome the second-half slump, http://www.pponline.co.uk/encyc/football-nutrition.html. 6. Criswell, D., Powers, S., Lawler, J. M., Tew, J., Dodd, S., Iryiboz, Y., Tulley, R. &. Wheeler, K. B. 1991, 'Influence of a carbohydrate-electrolyte beverage on performance and blood homeostasis during recovery from football', International Journal of Sport Nutrition and Exercise Metabolism, vol. 1 no. 2, pp. 178-191. 7. Marjerrison, A. D., Lee, J. D. & Mahon, A. D. 2007, 'Preexercise carbohydrate consumption and repeated anaerobic performance in pre- and early-pubertal boys', International Journal of Sport Nutrition and Exercise Metabolism, vol. 17 no. 2. 8. Mayhew, D. L., Mayhew, J. L. & Ware, J. S. 2002, 'Effects of long-term creatine supplementation on liver and kidney functions in American college football players', International Journal of Sport Nutrition and Exercise Metabolism, vol. 12 no. 4 Read More