Medical brain notes 44

  Sports Physiology There are few stresses to which the body is exposed that even nearly approach the extreme stresses of heavy exercise. In fact, if some of the extremes of exercise were continued for even moderately prolonged periods, they might be lethal. Therefore, in the main, sports physiology is a discussion of the ultimate limits to which several of the bodily mechanisms can be stressed. To give one simple example: In a person who has extremely high fever approaching the level of lethality, the body metabolism increases to about 100 per cent above normal. By compari-son, the metabolism of the body during a marathon race may increase to 2000 per cent above normal.   Female and Male Athletes  Most of the quantitative data that are given for the young male athlete, not because it is desirable to know only these values but because it is only in male athletes that relatively complete measurements have been made. However, for those measurements that have been made in th...

  Muscles in Exercise Strength, Power, and Endurance of Muscles The final common determinant of success in athletic events is what the muscles can do for you—what strength they can give when it is needed, what power they can achieve in the performance of work, and how long they can continue their activity. The strength of a muscle is determined mainly by its size, with a  maximal contractile force between 3 and4 kg/cm 2 of muscle cross-sectional area. Thus, a man whois well supplied with testosterone or who has enlarged his muscles through an exercise training program will have correspondingly increased muscle strength. To give an example of muscle strength, a world-class weight lifter might have a quadriceps muscle with a cross-sectional area as great as 150 square centimeters. This would translate into a maximal contractile strength of 525 kilograms (or 1155 pounds), with all this force applied to the patellar tendon. Therefore, one can readily understand how it is possible ...

  Muscle Metabolic Systems in Exercise The same basic metabolic systems are present in muscle as in other parts of the body;. However, special quantitative measures of the activities of three meta-bolic systems are exceedingly important in understand-ing the limits of physical activity. These systems are (1)  the phosphocreatine-creatine system , (2)  the glycogen-lactic acid system,  and (3)  the aerobic system. Adenosine Triphosphate.  The source of energy actuallyused to cause muscle contraction is adenosine triphos-phate (ATP), which has the following basic formula: Adenosine-PO 3  ~ PO 3  ~ PO 3 - The bonds attaching the last two phosphate radicals to the molecule, designated by the symbol ~, are  high-energy phosphate bonds. Each of these bonds stores7300 calories of energy per mole of ATP under standard conditions (and even slightly more than this under the physical conditions in the body). Therefore, when one phosphate radical is remo...

  Phosphocreatine-Creatine System Phosphocreatine (also called  creatine phosphate ) is another chemical compound that has a high-energy phosphate bond, with the following formula: Creatine ~ PO 3 - This can decompose to  creatine  and  phosphate ion,  as shown to the left in Figure 84–1, and in doing so release large amounts of energy. In fact, the high-energy phos-phate bond of phosphocreatine has more energy than the bond of ATP, 10,300 calories per mole in compari-son with 7300. Therefore, phosphocreatine can easily provide enough energy to reconstitute the high-energy bond of ATP. Furthermore, most muscle cells have two to four times as much phosphocreatine as ATP. A special characteristic of energy transfer from phos-phocreatine to ATP is that it occurs within a small frac-tion of a second. Therefore, all the energy stored in the muscle phosphocreatine is almost instantaneously avail-able for muscle contraction, just as is the energy stored in ATP. Th...

  Nutrients Used During Muscle Activity In addition to the large usage of carbohydrates by the muscles during exercise, especially during the early stages of exercise, muscles use large amounts of fat for energy in the form of  fatty acids  and  acetoacetic acid , and they use to a much less extent proteins in the form of  amino acids.  In fact, even under the best conditions, in those endurance athletic events that last  longer than 4 to 5 hours, the glycogen stores of the muscle become almost totally depleted and are of little further use for energizing muscle contraction. Instead, the muscle now depends on energy from other sources, mainly from fats. Figure 84–4 shows the approximate relative usage of carbohydrates and fat for energy during prolonged exhaustive exercise under three dietary conditions:high-carbohydrate diet, mixed diet, and high-fat diet. Note that most of the energy is derived from carbohy-drates during the first few seconds or minu...

  Effect of Athletic Training on Muscles and Muscle Performance Importance of Maximal Resistance Training.  One of the car-dinal principles of muscle development during athletic training is the following: Muscles that function under no load, even if they are exercised for hours on end, increase little in strength. At the other extreme, muscles that contract at more than 50 per cent maximal force of contraction will develop strength rapidly even if the contractions are performed only a few times each day. Using this principle, experiments on muscle building have shown that six nearly maximal muscle contractions performed in three sets 3 days a week give approxi-mately optimal increase in muscle strength, without producing chronic muscle fatigue. The upper curve in Figure 84–5 shows the approxi-mate percentage increase in strength that can be achieved in a previously untrained young person by this resistive training program, demonstrating that the  muscle strength increases...

  Respiration in Exercise Although one’s respiratory ability is of relatively little concern in the performance of sprint types of athletics, it is critical for maximal performance in endurance athletics. Oxygen Consumptionand Pulmonary Ventilationin   Exercise. Normal oxygen consumption for a young man at rest is about 250 ml/min. However, under maximal conditions, this can be increased to approximately the following average levels: Figure 84–6 shows the relation between  oxygen con-sumption  and  total pulmonary ventilation  at differentlevels of exercise. It is clear from this figure, as would be expected, that there is a linear relation. Both oxygen consumption and total pulmonary ventilation increase about 20-fold between the resting state and maximal intensity of exercise  in the well-trained athlete. Limits of Pulmonary Ventilation.  How severely do we stressour respiratory systems during exercise? This can be answered by the following comp...

  Cardiovascular System in Exercise   Muscle Blood Flow.  A key requirement of cardiovascular function in exercise is to deliver the required oxygen  and other nutrients to the exercising muscles.    For this purpose, the muscle blood flow increases drastically during exercise. Figure 84–8 shows a recording of muscle blood flow in the calf of a person for a period of 6 minutes during moderately strong intermittent con-tractions. Note not only the great increase in flow— about 13-fold—but also the flow decrease during each muscle contraction. Two points can be made from this study: (1) The actual contractile process itself temporarily decreases muscle blood flow because the contracting skeletal muscle compresses the intramuscu-lar blood vessels; therefore, strong  tonic  muscle con-tractions can cause rapid muscle fatigue because of lack of delivery of enough oxygen and other nutrients during the continuous contraction. (2) The blood flow to muscle...