In This Chapter: The Chemistry of Energy Production

Transcription

1 In This Chapter: The Chemistry of Energy Production The Three Energy Systems Immediate Energy: The Phosphagen System Short-term Energy: The Glycolytic System Long-term Energy: The Oxidative System Putting It All Together

2 ENERGY FOR MUSCULAR ACTIVITY 7 After completing this chapter you should be able to: use and understand the basic terminology of human metabolism related to exercise; describe the basic chemical processes the body uses to produce energy in the muscles; demonstrate an understanding of the body s three energy systems and their contribution to muscular contraction and activity; discuss the effects of training and exercise on the energy systems. 133

3 7 Energy for Muscular Activity I am building a fire, and every day I train, I add more fuel. At just the right moment, I light the match. MIA HAMM Humans are capable of performing amazing physical feats. Sprinters run down the track with astonishing speed and power; power lifters hoist hundreds of kilograms of weight, making it look effortless; swimmers traverse an entire lake or channel against the elements; hurdlers gracefully clear all obstacles in their way; and basketball players seem to defy the laws of gravity in their flight to the basket. While various combinations of physical ability, skill, and training are required to accomplish these feats, the common denominators in each case are the muscle activation patterns described previously and the development of energy at rates and in sufficient quantity to meet the needs of the activity. The energy needs for endurance events performed at relatively low intensity levels significantly differ from events requiring immediate power output performed at highest intensity levels. For effective planning of training programs coaches need to know the energy demands of their sport. The production of a movement during contraction occurs as the muscle pulls on the bones through the tendinous attachments to the bones. Even a single contraction requires a significant input of energy. Just as a car requires the appropriate fuel to run efficiently, so too do our muscles require energy for maximal performance. However, depending on the activity in which you are engaged, the body will make use of different energy systems that have been adapted for supplying energy at the required rate and in the necessary amount for that particular activity. In other words, the body will produce energy at a higher rate (but for a shorter duration) during an activity demanding power than one requiring endurance, where energy is required in greater quantities, but at a low rate. What are the body s primary sources of energy? What other fuels do we use? Why do our muscles produce energy differently under varying circumstances? These are some of the many questions that will be answered in this chapter. We will also explore some methods of testing and assessing energy production as well as the way the body adapts to exercise. The Chemistry of Energy Production Since adenosine triphosphate (ATP) fuels all biochemical processes, it is often referred to as the energy currency of the body. 134 Foundations of Kinesiology All energy in the human body is derived from the breakdown of three complex nutrients: carbohydrates, fats, and proteins. The end result of the breakdown of these substances is the production of various amounts of the molecule adenosine triphosphate (ATP), the energy currency of the body. ATP provides the energy for fuelling all biochemical processes of the body like muscular work or digesting food. The capacity to perform muscular work depends on sufficient energy supply at the required rate for the duration of the activity. I am strong to the finish because I eat all my spinach. No matter what the source of fuel is, we all need energy for maximal muscular performance.

4 Energy for Muscular Activity 7 Energy is liberated for muscular work when the chemical bond between ATP and its phosphate subgroup is broken through hydrolysis (Figure 7.1). The breakdown of ATP into adenosine diphosphate (ADP) and a free phosphate (P) provides the fuel for contractile activity in working muscles the formation of cross bridges between the actin and myosin filaments (see Chapter 5, Muscle Structure and Function). When the body performs physical work, it needs a continuous supply of ATP. Although muscle has a small supply of ATP stored within it that satisfies the initial requirements of the body, these initial stores are used up very quickly. Therefore, if activity is to continue, more ATP is needed. Energy Terminology A!ert Hydrolysis of ATP to ADP and P releases energy. ATP ADP P Resynthesis of ATP from ADP and P requires energy. Energy Luckily, ATP is a renewable resource that can be regenerated provided sufficient chemical energy is supplied via metabolic pathways. Muscle cells are able to continually resynthesize ATP by the recombination of ADP with a free phosphate (P) (Figure 7.1). This metabolic process is termed ATP resynthesis, and it can occur at a very fast pace in the body during exercise and recovery. The regeneration of ATP, however, requires the addition of energy, which is supplied through the breakdown of complex food molecules, such as carbohydrates, fats, and proteins. The degradation of these nutrients within the body releases energy from their chemical bonds, which is then used to synthesize ATP. The Three Energy Systems Figure 7.1 The breakdown of ATP into ADP and P releases energy to fuel muscular activity, while the resysnthesis of ATP from ADP and P requires energy from the food we eat. The production of ATP involves three energy systems, each of which produces ATP at a distinct rate and for a given maximal duration: (1) the immediate or phosphagen system; (2) the short-term or glycolytic system; and (3) the long-term or oxidative system. The main features of Studying Human Movement and Health 135

5 7 Energy for Muscular Activity Table 7.1 A summary of the main features of the body s three energy systems. Energy Pathway Anaerobic (without oxygen) Aerobic (with oxygen) Energy System Phosphagen (anaerobic alactic) Glycolytic (anaerobic lactic) Oxidative (aerobic) Energy Process Hydrolysis + ATP resynthesis Glycolysis Oxidative phosphorylation Fuel Source ATP, creatine phosphate (CP) Glucose Glucose, amino acids, fatty acids ATP Output 1 molecule 2 molecules per molecule of glucose 36 molecules per molecule of glucose By-products None Lactic acid Water, carbon dioxide Activities Supports high-power-output activities Supports high-intensity activity Supports long-duration activities Advantages Large amount of energy in a short time, fast recovery Not limited by oxygen delivery High ATP yield, able to remove lactic acid Disadvantages Limited amount of ATP produced Buildup of lactic acid contributes to fatigue Requires oxygen and is slower to meet energy demands Immediate energy Short-term energy Long-term energy Terminology A!ert these energy systems are presented and summarized in Table 7.1. The three energy systems are designated as aerobic or anaerobic, depending on whether or not oxygen is needed to produce energy. While oxygen is not required (anaerobic) by either the phosphagen or glycolytic systems, the oxidative system depends on oxygen (aerobic) to produce energy. Similarly, the two anaerobic systems can be separated on the basis of whether or not lactic acid is produced during energy production: lactic acid is produced with the glycolytic system, but no lactic acid is produced by the phosphagen system. Immediate Energy: The Phosphagen System 136 Foundations of Kinesiology Many sporting activities, such as weightlifting, sprinting and field events in track and field, and short-course swimming (25 metres), sometimes

6 Energy for Muscular Activity 7 described as high-power-output activities, require an immediate high rate of energy production because intensive muscle activity is done over a short time interval. The primary fuel source for these activities is the phosphagen system. Under these conditions, creatine phosphate (CP), another highenergy compound found in the muscle cell, can be broken down to produce phosphate and creatine. The free phosphate then bonds with ADP to reform ATP (Figure 7.2). Since there is only a small amount of ATP and CP stored within each muscle fibre, and because this system produces energy at a very high rate, this system can provide immediate energy for muscle contractions only in the initial 7 to 12 seconds of high-intensity activity. The phosphagen system does not produce any by-products such as lactic acid. The phosphagen system is sometimes referred to as the anaerobic alactic system. Creatine phosphate (CP) ATP C P ADP Characteristics of the Phosphagen System The utility of the phosphagen system is that (1) it can produce very large amounts of energy in a really short amount of time and (2) its rate of recovery is relatively rapid. The system can supply energy only until the intramuscular stores of ATP are exhausted, and thereafter, for as long as there is a sufficient local supply of creatine phosphate to resynthesize ATP from ADP. However, the total muscle stores of ATP are very small and are depleted after only a few seconds of high-intensity work. Since the store of creatine phosphate in muscle is also small, it too is depleted rapidly during high-intensity work. The initial concentrations of high-energy phosphates in the muscle are limiting factors in an individual s ability to perform short-term highintensity work. If the athlete must continue the activity for a period longer than 7 to 12 seconds of very highly vigorous work, or longer than 15 to 30 seconds of moderately intensive work, the phosphagen system cannot provide all the energy for the activity. It is for this reason that a 100-metre runner often loses speed after only 80 metres as the store of high-energy phosphates is exhausted and the body begins using another energy source, the short-term or glycolytic energy source (Table 7.1). Similarly, in weight training, short-term sets (e.g., three of 20-second duration) during maximal strength and power training are dependent on stored ATP and CP as the primary energy source. Figure 7.2 Energy production by the phosphagen system through ATP resynthesis from CP. DID YOU KNOW The use of creatine as a supplement among elite athletes is based on the notion that it can increase intracellular concentrations of CP prior to exercise. In theory, this would allow short-term, high-intensity activity to continue for a longer period of time. There is evidence to suggest ingested creatine has a small ergogenic effect, but this result, when present, is seen only in elite athletes. Studying Human Movement and Health 137

7 7 Energy for Muscular Activity THE CREATINE HYPE AND MUSCLE TYPE Considering the role creatine phosphate (CP) plays in explosive activities, it is important to note that Type II (fast twitch) muscle fbres contain higher concentrations of CP than Type I (slow twitch) muscle fbres. Therefore, individuals with higher percentages of Type II fbres (sprinters, athletes in jumping and throwing events in track and feld, weightlifters) may be able to replenish ATP faster through the phosphagen system during explosive anaerobic exercise or training. A higher percentage of Type II muscle fibres may allow some athletes to replenish ATP faster. Short-term Energy: The Glycolytic System The glycolytic system is sometimes referred to as the anaerobic lactic system. A second energy system called the glycolytic system uses a complex biochemical process called glycolysis, which involves a stepwise breakdown of carbohydrates (either glycogen stored in the muscle or glucose delivered in the blood) to resynthesize ATP (Figure 7.3). If glycolysis occurs at a high enough rate, a metabolic by-product called lactic acid is produced. Glycogen 2 ATP Lactic acid Glucose Glycolysis Pyruvic acid Figure 7.3 Energy production by the glycolytic system through glycolysis. Terminology A!ert 138 Foundations of Kinesiology During each step of glycolysis, a specific enzyme breaks down the chemical bonds of stored glycogen or blood glucose in the absence of oxygen. The final product in the complex series of breakdowns is termed pyruvic acid, which proceeds in one of two directions. It can be converted into lactic acid if the rate of pyruvic acid production is high or it can be converted to pyruvate (the buffered salt form of pyruvic acid) and shuttled into the mitochondria (singular mitochondrion), the specialized cellular organelles where the reactions of aerobic metabolism occur. More specifically, if the rate of pyruvic acid production exceeds the rate at which pyruvate can be processed in the mitochondria, excess pyruvic acid is converted into lactic acid. Lactic acid is then either transferred to other areas of the body for processing (e.g., the Cori cycle in the liver; see page 142) or is stored in the muscle until the lactic acid can be processed in the local mitochondria. For example, if an athlete is running at a high speed and burning sugars for energy, pyruvic acid will accumulate more quickly

8 Energy for Muscular Activity 7 than can be processed in the mitochondria, and the excess pyruvic acid will be converted into lactic acid. Once the athlete slows down and is no longer using glycolysis to produce energy, the accumulated lactic acid can be processed in the mitochondria, which allows the athlete to recover. The source of fuel for the glycolytic system is carbohydrate. Glycogen (the stored form of carbohydrate in the muscles and the liver) and blood glucose (the circulating form of carbohydrate) are derived from the carbohydrates that make up the human diet. Carbohydrates (pasta, rice, bread, fruits and vegetables, starchy foods, sweets) are the primary dietary sources of glucose and serve as the primary energy fuels for the brain, muscles, heart, liver, and various other organs. Once ingested, these foods are broken down into glucose by the digestive system. Glucose then enters the bloodstream and is circulated around the body. Some glucose stays in the blood, but most is stored in the liver and the muscles as glycogen, which consists of hundreds of glucose molecules linked together. The process of forming glycogen from glucose is called glycogenesis. But before stored glycogen can be used to produce energy it must first be converted back to glucose through a process called glycogenolysis. Carbohydrates are the primary dietary sources of glucose and serve as the primary energy fuels for the human body. Characteristics of the Glycolytic System Glycolysis provides ATP for muscular work at a rapid rate, which supports high-intensity activity. However, the by-product of anaerobic metabolism is lactic acid, which rapidly dissociates into a salt compound called lactate (a substance usually measured under laboratory conditions) and hydrogen ions (H + ). Hydrogen ions are known to induce muscle fatigue (think muscle burn ) and also hamper the transmission of electrical signals at the neuromuscular junction, which can limit fibre activation. The hydrogen ions compete with calcium for cross bridge binding sites (see Chapter 5), thereby limiting the strength of muscle fibre contraction. Thus, a high production of lactic acid ultimately limits continued performance of intense activities. Effects of Training on the Glycolytic System At any given level of work, the rate of lactic acid accumulation is decreased in the endurance trained individual, which means the anaerobic threshold is higher and the individual can work at a higher rate of activity before the accumulation of lactic acid begins. The opposite is true for sprinters, who benefit from rapid production of ATP and compete in short events where lactic acid buildup is not a concern. These sprint athletes actually benefit from a high rate of lactic acid production and train to produce as much lactic acid as possible. During high-intensity exercise, the rate of lactic acid buildup can be decreased by decreasing the intensity of the activity or by increasing the ability to handle the lactic acid (i.e., remove it from the muscles). Decreasing lactic acid production at any given rate of work can be achieved by increasing the effectiveness of the oxidative system (described in the next section of this chapter), which reduces the requirements for energy from anaerobic sources. Note, however, that this is not a strategy that will work for sprint- or power-oriented performances as the rate of energy demand for this DID YOU KNOW The exercise intensity at which lactic acid begins to accumulate within the blood is known as the anaerobic threshold. The anaerobic threshold can be thought of as the point during exercise when you begin to feel discomfort and a burning sensation in the muscles. Studying Human Movement and Health 139