Concepts of Excercise

Introduction

Now we take care of the ca
lories-in part of our equation. What about calories-out?

We all know that exercise is the way to get our calories out. But is there a way that we can get most of our calories from our fat store?

I am going to describe how to pick the right exercises to efficiently burn fat. It will begin with an introduction to the Basic Concepts of Exercise Physiology. Using the concepts we learned in this page, I will guide you to design an exercise regime to maximize fat loss in the subsequent Cardiovascular Exercises page and Weight Training page.

Introduction to Human Energy Systems

Similar to the way we digest and absorb energy, the way we expend our energy is also highly complex. Our Primary Energy System is the ATP System which provides immediate energy at all time. There are three Secondary Energy Systems: Phosphocreatine System, Lactic Acid System and Aerobic System. They are used by our body to replenish the ATPs broken down for energy in the Primary Energy System. At any given time, all three Secondary Energy Systems are employed. The proportion in which each system is employed depends on the energy substrates availability and the intensity of the exercise.

ATP System (Anaerobic)

For immediate energy, our body just grab the ATPs (Adenosine Triphosphate, ie the energy currency used by humans) floating around in our body. Unfortunately, humans only store about 80g to 100g of ATPs at any given time. To replenish used ATPs, we have three Secondary Energy Systems that can resynthesis them: Phosphocreatine System, Lactic Acid System and Aerobic System.

Phosphocreatine System (Anaerobic)

Phosphocreatine (PCr) System is the fastest system to generate ATPs. It doesn't require oxygen. When our body senses that we are beginning to exert maximal effort, it breaks down PCr for energy to resynthesis ATPs that are used up. While PCr is being broken down, energy from other sources helps to resynthesis PCr such that PCr can be used when all-out effort is needed later. Since resysnthesis of PCr is much slower than its break down, PCr System can only provide energy for 5-8 seconds of maximal effort. Beyond that, other fuel source must be used. As a result, PCr System is the primary fuel source of sprinting, intermittent all-out efforts in ball games, weight lifting and other exercises that requires quick burst of energy.

Lactic Acid System (Anaerobic)

Lactic Acid System is the second fastest. It also doesn't require oxygen. It releases the glucose in muscle glycogen. Then glucose is broken down into lactic acid to release ATPs. The lactic acid generated can be transported to cells to be oxidated for energy or it can be transported to the liver to re-generate glucose when we have energy from other sources. Similar to the PCr System, the Lactic Acid System can only lasts about 60 to 180 seconds of all-out effort. It is the primary fuel source for short distance runs like 200m and 400m or 100m swim.

Aerobic System

Aerobic System is the slowest way to generate energy. Unlike the other two, it requires oxygen. But the good thing is that it can use many different kinds of molecules for energy: glucose, lactic acid, fatty acid, keto acids from the breakdown of proteins and many others. Since most people do have a big reserve of energy in body fat, energy supply from the Aerobic Sytem can lasts very long. Basically, any exercises that lasts more than a couple minutes use this as the primary fuel source. One thing we can learn from these energy systems is that, Aerobic Energy System is the only way to burn fat.

Muscles

To perform any exercise, we need to use our muscles. There are three types of muscle in our body: skeletal, cardiac and smooth. Cardiac muscle is the muscle of heart. Smooth muscle is the muscle of internal organs. Skeletal muscle is the muscle in our limbs and torso that allows us to move. In this section, we will concentrate our discussion on Skeletal Muscle.

Each piece of human skeletal muscle is like a bulk of fiber-like cells. We call those cells Muscle Fibers. There are three major types of muscle fibers: 1) Type I muscle fiber; 2) Type IIa muscle fiber; and 3) Type IIx muscle fiber.

Comparison of the different muscle fiber types:

It is obvious from the table above that Type IIx is the most powerful fiber but it doesn't last long (because it mostly relies on anaerobic energy system). In contrast, Type I is the least prone to fatigue but it is not as strong (because it mostly relies on aerobic energy system). Type IIa is somewhere in between. When we exert power with our muscle, the slow Type I muscle fibers are recruited first. If that's not enough, then Type IIa fibers are recruited and finally Type IIx.

It turns out that every piece of muscles in our body contains all three types of muscle fiber. However, the muscle fiber type composition and number of muscle fibers are basically determined by genetics. It is possible to weight train to stimulate the muscle fibers (Type IIa and IIx) to grow bigger and more powerful. It is also possible to do enough cardiovascular exercises to improve the oxidization power of a muscle (to improve the oxidization efficiency of Type I muscle). [EEP00]

Three Phases of Energy Expenditure

• For each exercise session, energy is expended in three different times. Obviously, energy is expended during the exercise. Let's call this Exercise Energy Expenditure. Most cardio machines give you an approximation of energy you expended during exercise. You can check the number of calories burned in most cardio machines. For other type of exercises, you can estimate your energy expenditure using some online calories counters like Calories Per Hour.com.

• Immediately after the exercise, the body needs energy to recover. The recovery is performed to return our body back to the pre-exercise state. It includes oxidation of lactic acid accumulated in blood, repair torn muscle tissues, etc. Let's call it Post-exercise Energy Expenditure (PEEE). PEEE is found to be an insignificant source of energy expenditure. It is about 22kcal for one hour of low intensity exercise or 41kcal for one hour of high intensity exercise. [PRHM97]

• For muscle tissues gained as a result of exercise, you also need energy to maintain them. It costs 22 calories per day to maintain one kg of muscle whereas only 3 calories per day is needed to maintain one kg of fat. [BPAHM00]

Energy Storage

• Energy substrates like carbohydrate, protein and fat are used to provide energy for our body to function. But what happens to them when we consume them in excess of our current needs? Where are they being stored? And how are they going to be utilized when our body is running out of energy from food? In this section, I will explain that to you.

• First, we will start with Carbohydrate. Our body stores excess carbohydrate as glycogen. Glycogen is mostly stored in liver and muscle (skeletal, cardiac, smooth). There is also a small amount stored in kidney and small intestine.

• Note that glycogen stored in muscle is only used locally for the muscle that stores it. In contrast, the glycogen in liver can be released to the blood as glucose and then pick up by tissues for energy.

• If you have exercised continuously for over two hours like taking a long hike or near the end of an intense basketball match, most likely you might have experienced muscle cramp. This is because your muscle exhausted all its glycogen and your liver couldn't make glucose fast enough to replenish the loss. To combat this problem, you are recommended to drink sports drink like Gatorade every hour during prolong exercises. Sports drinks is basically sugar water with salt. The sugar inside will help you replenish the muscle glycogen. The salt inside will help you replenish the salt you lost while sweating. Obviously, sports drinks are very bad for fat loss if you drink it in other cirsumstances because the sugar inside will make you fat as discussed in the diet page.

• Next, we will continue with fat. Excess Fat are stored in various places under our skin or wrapped around our internal organs. We call this subcutaneous fat. There is also a small amount of fat stored in skeletal muscle that we call intramuscular fat.

• Subcutaneous fat is what makes us look fat. So reduction of the fat there is the goal of these pages. Subcutaneous fat is released from the fat tissue throughout the day to the blood and is transported to various places in the body to be oxidized for energy.

• Intramuscular fat is similar to muscle glycogen. They can only be used locally for the muscle. During exercise, majority of energy from fat comes from intramuscular fat.

• Finally, we turn our eyes to protein. There is technically no natural storage for protein. However, if you do weight training, protein is used to build muscle. In a sense, this stores protein for future use. Without weight training or any physical activity, there is no way to "store" protein.

• During exercises, a very small percentage (about 3-6%) of muscle protein is broken down for energy. [N02] Muscle protein is also broken down during starvation. That's why you see severely starved people are nothing but bones wrapped with skin.

• As we can see, while muscle protein is little used as an energy source and protein is not stored naturally. We can consider muscle protein as protein storage in all practicality. During muscle protein breakdown into amino acids, amino acids are released to the blood stream. Therefore, the energy from amino acids can be taken up by body tissues

Energy Storage breakdown of a 70kg non-obese man:

Source: Everything except Protein is from [FM04]. Muscle Protein content is suggested by [BPAHN00]

Why do I need exercises in addition to diet?

• You may wonder why you need to do exercises to lose fat. You might argue that if we can reduce fat using the diet I described in the Diet page, why do we need to do exercises? There are two major reasons.

• Let me start with a not so obvious one. Since our stomach always stay the same size, doing exercises allow us to eat more calories dense food than living a sedantary life. For small stomach people, that means you can eat more energy dense food like your favorite ice-cream or candies. For people who has a big stomach, exercise is the way to increase food choices such that it becomes easier to maintain a diet. Secondly, exercise is good for general physical health. It makes you feel better about yourself and keep you in a upbeat mood. Both are pluses for maintaining a diet.

• Another natural question you might have is that if I do exercises but change nothing about my diet, can I lose fat? Apparently, you can. However, this won't easy, if you know that to lose 100g of fat, you need to burn 900 calories. This is equivalent to a 70kg person running at 8km/h for 1 hour 36 minutes! To do the roughtly the same, you need to skip a McDonald's Meal of Big Mac + Medium Fries + Diet Coke (940kcal). For most people, the latter is easier. Obviously, a combination of both diet and exercise can make the process even easier and effective.

Out Take

• I hope what I described in this page helps you understand the basics of Exercise Physiology. In the next two sections, I will use concepts developed here to explore the effective means to lose fat using Cardiovascular Exercises and/or Weight Training. Good luck and happy fat loss!

Academic Papers

• [PRHM97] Postexercise energy expenditure and substrate oxidation in young women resulting from exercise bouts of different intensity. , JF Phelain, E Reinke, MA Harris, CL Melby, Journal of the American College of Nutrition 1997 Apr;16(2):140-6.

---

Textbook References

• [FM04] Functional Metabolism, Kenneth B. Storey, 2004
• [N02] Nutrition, 2002 Update, Pual Insel, R. Elaine Turner, and Don Ross, 2002
• [EEP00] Essentials of Exercise Physiology, Third Edition, William D. McArdle, Frank I. Katch and Victor L. Katch, 2000
• [BPAHN00] Biochemical and Physiological Aspects of Human Nutrition, Martha H. Stipanuk, 2000