How to Make Your Workout More Effective: Metabolic Pathways the Secret to Better Performance
Let’s start at the beginning. This way you will understand how your amazing body does what is does without complaining or finding excuses. Then hopefully you will realize everything you need to reach your fitness goals is already inside you.
You need energy to perform work, whether it be getting up from a chair or performing a series of exercise. You get your energy from the foods you eat (carbohydrates, fats and protein). Your body then transforms the chemical energy found in food to into biological usable forms of energy. Metabolic pathways permit these transformations to occur.
Metabolism:
The production of energy from nutrients is known as metabolism. There are two different types of metabolic processes in the body and both need energy. They are catabolic (exergonic reactions) or anabolic (endergonic reactions).
If the energy is used to tissue build tissue, as when amino acids are combined to form proteins that make up muscle, the process is anabolism and the reaction is endergonic. Anabolic processes require energy to occur.
If the energy is produced from the breakdown of food and store for latter work the process is called catabolism. Catabolic processes release energy and are exergonic reactions. These two processes, anabolic and catabolic, are exact opposites of each other.
The sum of these catabolic or exergonic and anabolic or endergonic reactions makes up your metabolism.
Adenosine Triphosphate = Your Body's Energy $$$
Adenosine triphosphate (ATP) is the intermediate molecule that drives all metabolic pathways in your body. ATP allows for the transfer of energy form exergonic and endergonic reactions. You can think of it as your body’s energy currency. It is second to DNA as far importance to survival in your body. Without ATP no muscle contraction would occur and no muscle growth would happen.
The breakdown of one molecule of ATP to yield energy occurs by hydrolysis, simply because you need a molecule of water for the reaction to happen. Many process in the body work this way. It’s just a big work with a simple meaning.
The products of breaking down ATP are adenosine diphosphate (meaning 2 phosphates) inorganic phosphate, hydrogen and of course energy.
When you exercise, catabolic reactions dominate. This group of reactions is known as cellular respiration. The food you consume is potential energy (fuel). The chemical energy produced from food is stored as ATP (energy).
The ATP transfers or releases this energy so you can perform work, as in muscle contraction. ATP is used in most processes in your body, from your brain thinking to your heart beating.
Your muscles have very small amounts of ATP, enough to last about 2 seconds. As a result, your body is constantly synthesizing ATP. In order to do this your body is equipped with three different metabolic pathways or energy systems to generate ATP. These systems make energy available for physiological functions, such as exercise. They are the phosphagen system (ATP-CP), glycolysis and the oxidation system.
Understanding how energy systems work will allow your training program to be efficient and productive. Learning how to manipulate these systems with appropriate exercise intensity, duration and rest intervals will permit you to better reach your goals. You will train the system that matches your goals and you will see increase performance due to adaptation. This type of training is known as metabolic specificity.
Phosphagen System (ATP-CP):
As described above, when ATP is broken down it release one of its phosphates and becomes adenosine diphosphate (ADP). To replenish the ATP levels quickly, muscle cells contain a high-energy phosphate compound called creatine phosphate (CP).
Small amounts of CP are stored in cytosol or cytoplasm (the watery part) of your cells. When muscles are involved, the cytosol is called the sarcoplasm. This is important because it allows for immediate use of the ATP by the cell. No oxygen is needed for this reaction to occur.
CP can essentially give ADP its phosphate and turn ADP back to ATP, which can be used to supply energy for your muscles. This system provides energy quickly and can generate the greatest amount of force when compared to the other systems.
As ATP is broken down ADP accumulates. ADP can also assist in the regeneration of ATP. This reaction is important because it is a powerful stimulator for glycolysis. Both of these reactions are completed in a single step.
The ATP-CP system is designed to supply energy to your muscles very quickly. The system is most effective in the first 2-4 seconds after this it begins to decline-restricting its effectiveness. After about 10 seconds you have depleted your CP stores and will either have to rest or reply on the glycolic system for energy.
ATP is replenished with passive recovery in approximately 2.5 to 3 minutes. Complete repletion of CP can take up to 8 minutes to resynthesize. A golf swing, pitching a baseball, power lifting are all sporting activities that rely primarily on the ATP-CP system. However, getting out a chair, opening a jar or lifting a gallon of milk are also examples of utilizing the ATP-CP system.
The ATP-CP system is active at the start of exercise regardless of intensity. The system will dominate in the first 10 seconds of any activity or exercise. After this point the glycolytic system will begin to contribute (discussed next).
Glycolysis:
Glycolysis literally means the breakdown of glucose, a one molecule carbohydrate. You can get glucose from glycogen (the stored form of glucose) or from your blood. Carbohydrate classification is spoken further in this article. You need carbohydrates for this system to operate.
Like the ATP-CP system, glycolysis occurs in the sarcoplasm in muscles cells. Glycolysis supplements the ATP-CP system, but then eventually becomes the predominant source of ATP for activities that lasts between 2-3 minutes.
Glycolysis is a series of 9 or 10 complex steps, depending if you start with glucose from the blood or glycogen stored in the muscle or liver. Glycolysis will produce a total of 4 ATP, but 2 ATP are used in the first two steps (known as the investment phase), so the net ATP produced from glycolysis are two if starting with glucose. If you started with glycogen, then you end up with a net of three molecules of ATP opposed to two, because glycogen can skip the first step in glycolysis that uses one ATP.
As you can see from the diagram below, glycolysis in much more involved than the one step reaction from the ATP-CP system.
Glycolysis can occur in two ways. It is can either occur by fast glycolysis or slow glycolysis. In both processes the end-products of importance are two pyruvate molecules and 2 NADH+ molecules (coenzyme).
If energy demands are high, as in heavy resistance training, pyruvate takes a hydrogen (proton) molecule from NADH+ to create lactate. NADH+ is now turned back to NAD and helps maintain glycolysis. No oxygen is required for this reaction and it is sometimes called anaerobic glycolysis. Fast Glycolysis replenishes ATP faster than slow glycolysis, but is still limited by duration (2-3 minutes).
If energy demand is low enough and oxygen is present, then pyruvate is transported to the mitochondria (an organelle found in large numbers in most cells) and enters the Krebs cycle or citric acid cycle (same thing two different names). This is known as slow glycolysis and is sometimes called aerobic glycolysis. This is also considered the first step in oxidative metabolism.
Lactic Acid Is NOT the Cause of Fatigue!
The difference between lactate and lactic acid is something you should understand. Most sources are outdated or just use the terms incorrectly. The end result fast glycolysis is lactate plus hydrogen not lactic acid. Lactic acid cannot even exist in the blood because the blood maintains a neutral pH of 7.
Also, lactate is not why you feel that burning sensation when you run up hills or lift heavy weights
The accumulation of hydrogen will slightly decrease muscle pH and that leads to fatigue. This is what is causing the burning sensation you feel when you exercise. The hydrolysis of ATP contributes significantly to the accumulation of Hydrogen in your muscles and is thought to be the major cause of peripheral fatigue during exercise or “the burn" (Robergs and colleges, 2004)
Lactate actually helps your body recover from exercise. Lactate can either be oxidized in the muscle fiber it was produced or transported through the blood to other muscle fibers for oxidation. Your body can also convert lactate back to glucose to be used again. In this case lactate is transported in the blood to the liver and converted back to glucose. This process is known as The Cori Cycle.
If muscle did not produce lactate, acidosis and muscle fatigue would occur more quickly and exercise performance would be severely impaired.
However, lactate concentrations do correlate (not the cause) with muscle fatigue and remains a good indirect marker for metabolic acidosis.
At intensities of 50% to 60% of maximum oxygen uptake in untrained individuals and at 70% to 80% in trained individuals lactate clearance cannot keep up with lactate accumulation. This point is known as your lactate threshold.
At this point there is an exponential increase in lactate relative to exercise intensity. This is known as onset blood lactate accumulation (OBLA). You can extend OBLA by training at or near your lactate threshold. This will allow you to build up a tolerance to the fatigue and your body will adapt to it. Then you will be able to train at high intensities because your lactate threshold has shifted to right (displayed in the graph below).
At relative intensities of exercise above 60% of maximal oxygen uptake, muscle glycogen becomes an increasingly important energy substrate. Your entire muscle glycogen stores can become depleted during exercise, it all depends on the intensity and duration of the event. High intensity exercise with repeated bouts, like resistance training and very long duration exercises, like a marathon would, deplete glycogen stores the most.
Your muscles store about 300 to 400g and glycogen and accounts for about 80% of all glycogen stores in your body. Your liver stores account for the other 20%.
You can replenish your glycogen by consuming carbohydrates after exercise. The NSCA guide is to consume 0.7 to 3.0 g of carbohydrate per kg (= pounds/2.2) of body weight every 2 hours following exercise.
The rate of glycogen repletion is highest in the first 2 hours and then begins to fall off. However, there is a maximum rate of repletion in the range of 1.5 grams of carbohydrates per kg body weight during the first 2 hours post exercise. Thus a strategy of taking all your carbohydrates replacement in the first few hours is not going to work.
Oxidation System:
The oxidative system is your primary source of ATP at rest and during low-intensity activities, like walking. This system uses carbohydrates, but can use fat and protein as substrates too. Your body does not like to use protein for energy. Rather it prefers to build muscle or make hormones with protein. However, under long-term starvation or long bouts of exercise (>90 minutes) protein can be used as an energy source.
The oxidative system is slower at replenishing ATP, but can sustain output for a longer duration if intensity is low enough, because of fat stores. At rest about 70% of your ATP is derived from fat and 30% is derived by carbohydrates. The trade-off is decreased intensity output with oxidative metabolism.
All oxidation occurs in the mitochondria of the cell. The oxidative system begins at glycolysis. If there is enough oxygen present in the cell the 2 pyruvate molecules will enter the Krebs cycle. The krebs cycle is a series of complex reactions that produces 2 ATPs.
The main molecules produced in the Krebs cycle are NADH+ and FADH (flavin adenine dinucleotide). These molecules are transport to the electron transport chain (ETC) to produce ATP from ADP. You will get 3 molecules of ATP from NADH and 2 molecules from FADH for a total of 32 ATP molecules from the ETC.
The oxidative system, being with glycolysis, results in a net of 38 ATP from one molecule of glucose (39 net if glucose comes from glycogen).
Fats can also be used in the oxidative system. Triglycerides stored in fat cells are broken down to free fatty acids. Fat oxidation occurs also occurs in the mitochondria of the cell, where it undergoes beta oxidation and then enters the krebs cycle and ETC. The amount of ATP produced depends on the fatty acid that is oxidized. It takes longer to breakdown triglycerides to ATP than it does glucose this leads to further decreases in your ability to maintain intensity.
Protein can be used in the oxidative system too. The contribution of protein as an energy is minimal in short-term exercise, but may contribute 3% to 18% during prolong activity (>90 minutes). Proteins are broken down to their amino acids. This degradation produces waste that are eliminated though the formation of urea and ammonia. Ammonia is a concern because it can lead to fatigue. The oxidization of amino acids involves the Krebs cycle and ETC, similar to other substrate oxidation.
In general, there is an inverse relationship between a given energy system’s maximum rate of ATP production (i.e., ATP produced per unit of time) and the total amount of ATP it is capable of producing over a long period.
As a result, the phosphagen energy system primarily supplies ATP for high-intensity activities of short duration, the glycolytic system for moderate- to high-intensity activities of short to medium duration, and the oxidative system for low-intensity activities of long duration.
The extent to which each of the three energy systems contributes to ATP production depends primarily on the intensity of muscular activity and secondarily on the duration.
At no time, during either exercise or rest, does any single energy system provide the complete supply of energy.
Metabolic Specificity
Metabolic specificity is training a specific energy system that best matches your sport or goal. You can work specific energy systems by selecting appropriate exercise intensities and rest intervals. In this way, physiological adaptations can begin to occur in your body. This will improve your efficiency or power and increase your performance within the energy system you select to train.
Interval Training
Interval training consists of three elements: a selected work interval, a target time for completing the work and a predetermined recovery or rest period before the next work interval.
The length or your work interval will depend on the energy system you want to train. Therefore, work times up to 10 seconds would stress the ATP-CP system and 30 seconds to 3 minutes would stress the glycolic system. Anything over 3-5 minutes would emphasize the oxidative system.
Your target time for completing the interval would depend on your ability. The length and type of recover depends on the energy system stressed. Work intervals with greater intensity would need longer rest periods to replenish the system. Also, as the duration of the interval increases intensity decrease and so does the recovery period.
The National Strength and Conditioning Associate (NSCA) provides guidelines for work–to-rest ratios that are meant to develop specific energy systems based on the duration of the metabolic pathway and the recovery of that pathway. These are not hard rules. Your program needs to address your level of fitness, abilities and goals.
The theory behind interval training is with the right work to rest ratios you can exercise at high intensities with the same or less fatigue that continuous exercise at the same work load. More training is accomplished and metabolic adaptations occur.
Before attempting an interval training program or any exercise program it is best to talk to a medical professional.
