Physiological Effects of Endurance Training

Physiological Effects of Endurance Training

Endurance training produces many physiological changes, both during training and after the training period is complete. These changes are biochemical and also involve changes in the cardio-pulmonary system. The correct way to perform endurance training has been a subject of controversy in recent years. There are many differences in training methods. These differences and the effects of endurance training will be the subject of this research. The jury is still out as to what constitutes the perfect duration and intensity of training program.

Studies have shown that a focused training program can increase maximum oxygen intake by 15-30% over a three-month period (7) and that can increase to 50% if the training is sustained for over 2 years. The body makes many metabolic adaptations as well. These adaptations drop rapidly in the first few weeks after training is stopped (1).

Duration and Intensity of Different Training Programs

There are many theories and methods for endurance training. Some say that a moderate workout sustained for a longer period of time has the most benefits for the body. Others feel that the workout should be of a moderate time period, but should be more high intensity. Some feel that a high intensity, but short duration work out is the best way to train the cardio-vascular system. The Fartlek training system holds the philosophy the training should be guided by how the athlete feels. All of these methods have advantages and disadvantages.

Each type of training program has a different effect on the body. The training program should be fitted to the person's goals and final desired result. A training program for a person who intended to participate in a sprinting event would be considerably different than that of a person who wanted to participate in a long-distance cycling event. Physiological adaptations to resistance and endurance training can even produce opposite results (8). For instance, resistance training decreases capillary density and mitochondrial volume density in muscles, whereas endurance training causes an increase (8). There is no one-size-fits-all training program and the opposite physiological effects of these different types of training can make cross training more difficult.

Sale (6) concluded that endurance training has been known to result in decreased muscle-fiber size and a loss in strength, while resistance training causes an increase in both. These two training techniques can be integrated into a training system designed to meet the athlete's specific goals. The intensity, duration and frequency can be modified to meet the athlete's specific goals (6).

One of the more popular training methods is designed to improve resistance to fatigue and improve endurance performance. The training method is intended for endurance athletes. The training method was suggested in Hawley et al. (4) and involves a three phased approach. In the first phase, the training is moderate and of long duration. Long duration is defined as a session lasting more and 60 minutes or more. Phase two involves two interval-training sessions per week. These interval sessions can replace two of the moderate intensity workouts of phase one. The third phase involves working out at an intensity that is consistent with race pace. This phased approach is designed to create the physiological changes needed for endurance in phases that will maximize efficiency (4).

Finn (3) suggests that high-intensity intermittent training is best for producing the greatest gains in power and endurance. This goes against conventional ideals regarding endurance training that suggest the lower intensity, longer periods of training are better for endurance training. Finn (3) stresses that athletes should gradually phase in high intensity intermittent training and build up over a time. These are just to examples of trainers who have combined training techniques to produce the desired effect that the athlete wants. There are many programs that combine techniques successfully to achieve the goals of the program.

Biochemical changes

Any type of training program involves a wide variety of biochemical changes in the body. These biochemical changes include changes in the muscle fibre itself, changes in the way the enzymes remove by-products such as lactate, and the way the body utilizes and distributes oxygen. The following research will explain the known biochemical changes in the body that result from endurance training.

Endurance training involves improving the efficiency with which the body converts the energy in foods to a form of energy that it can use to perform work. All foods are composed of carbohydrates, fats, and proteins. Carbohydrates are the most important food source for short interval, high intensity events, such as sprints. Fat is converted more slowly and releases its energy over a longer period of time. This is the preferred food source for endurance events. Proteins are the most complex energy forms and break down the most slowly. These are usually used to repair and maintain body tissues.

The three basic foods are all converted to a single chemical compound called adenosine triphosphate (ATP). An oxidation process releases the energy from this food. The food supply must be continually renewed in order for the process to continue. This process is commonly known as metabolism. There are different mechanisms for achieving this. The different types of training discussed in the previous section all promote different mechanisms for achieving efficient metabolism of energy from foods..

Endurance training makes the muscle fibers synthesize more myoglobin. Myoglobin binds to oxygen in the body and aids in overall body metabolism. Endurance training results in more efficient oxidation of carbohydrates and results in an increase in the number size and membrane surface area of skeletal muscle mitochondria. This mitochondrial increase is associated with increases in the level of activity and concentration of the enzymes involved in the Krebs cycle and in ETC. The Krebs cycle is where fatty acids are used for moving NADH from the cytoplasm of the cell to the mitochondria where it is used in ETC to produce ATP.

NADH is a product of glycolysis.

Endurance training also produces changes in the body's oxidation of fat. Endurance training results in an increase of muscular triglyceride stores. This gives the body ready supply of energy already in the vicinity where it is needed. The availability of fats to the skeletal muscles is increased. The enzymes required for the breakdown of the fat stored in the muscle is increased as well. There is another effect as well, fatty deposits are cleared from the blood vessels where they can cause heart disease.

Rodas et al., (5) showed that high-intensity intermittent training increases the oxidative enzyme activity in the muscle. These increases included increases in citrate synthase, 3-hydroxyacyl-CoAdehydrogenase (5). These are the changes responsible for the rate of fat oxidation and that result in the reduction of carbohydrate oxidation in endurance athletes. These enzymes are decreased in short duration, high intensity athletes.

Cardio-respiratory changes

All of these metabolic activities require a large amount of oxygen in order to convert the energy of food into muscular work. This will produce an oxygen deficit if allowed to occur without a mechanism to supply additional oxygen. In order to supply the extra needed oxygen, the body increases the mitochondria, oxidative enzymes and the number of capillaries per muscle fiber.

The effects of endurance training have several beneficial effects, even when the body is at rest. These include cardiac hypertrophy (an enlarged heart muscle), a decreased resting heart rate, an increase in heart stroke volume, little or no change in resting respiratory rates, and an increased capillary density. These changes are due to the body's need to increase the oxygen to the muscles. Every chemical reaction must be balanced. In order to maintain balance, it must have a sufficient supply of each element. If on element needed for the reaction is limited, then the reaction itself is limited. The body needs to convert more food to energy and therefore needs more oxygen. The heart, lungs, and blood vessels are the mechanism that supplies the needed oxygen.

As the body demands more oxygen, it must also improve the delivery system. Endurance training has known effects on the heart and lungs including cardiac hypertrophy, a decreased resting heart rate, increased stroke volume, and increased skeletal capillary density and hypertrophy (3). These improvements allow the body to have the oxygen needed to oxidize glycogen and fat. The increased stroke volume means that the heart pumps a larger volume of blood with each beat. This allows the heart to be able to beat less times per minute and pump more blood per beat. The increased capillaries create a larger transport system to move the oxygen to where it is needed.

The heart only has two ways to increase the oxygen supply to the muscles. It can either increase its rate and beat more times per minute, or increase its volume. At the beginning of endurance training, the mechanism is not in place to supply the extra needed oxygen. Therefore the respiratory system increases its rate and the heart also increases its rate. As endurance training progresses, the capillaries increase, the heart begins to gain size and power, allowing it to pump more blood per beat. When this happens, the lungs are able to slow and down and the heart does not need to work as hard to supply the extra needed oxygen. It is these physical changes that allow endurance training to decrease a person's risk of heart disease, stroke, and diabetes (3). These are the benefits of endurance training that have a long-term lasting effect.

Submaximal exercise is exercise that is maintained at 60-70% of a person's maximum heart rate for the prescribed duration. The maximum heart rate is defined as the point where the heart is not able to keep up with oxygen demand and cannot produce at any faster rate. Exercise maintained at the submaximal heart rate is considered to be a moderate exercise level. At this level of exercise there may be a slight increase in oxygen consumption. Muscles use less glycogen and there is a decrease in lactate accumulation. Lactate is the chemical responsible for limiting the usage of glycogen for energy (1). It prevents the entire system from running at an unsustainable speed. At these levels there is an increase in performance velocity. There is no change or even a slight decrease in cardiac output. There is an increase in stroke volume and a decrease in heart rate.

At submaximal exercise, the body gains its extra needed energy by decreasing the inhibitor, lactate production. However, the other system has to do little to compensate for the increased activity. The same is not true, however, if one exercises at maximal exercise rate. This rate is at approximately 95% of maximum heart rate. At this rate one begins to see massive increases in oxygen demand. This equates to the body actually doing work, which requires more energy.

A maximal exercise rate, the body goes through many changes. It has an increased demand for oxygen. The means that the body must find a way to increase to supply of oxygen needed to convert ATP to energy. There is an increased cardiac output, either by way of increased heart rate, in an untrained person, or by way of increased stroke volume in a trained person. Cardiac output = stroke volume x heart rate. There may actually be a slight decrease in heart rate when the exercise intensity increases from sub-maximal to maximal. There is a greater pulmonary diffusion capacity, that is a greater amount of oxygen entering the blood in the lungs. There is also an increased blood flow per kilogram.

All of these changes add up to work. If one will recall the first law of thermodynamics, energy can neither be created nor destroyed. The second law of thermodynamics states that a system will seek to achieve stasis, where input and output stops. The body at rest, in a healthy person is in stasis. It produces the exact amount of energy that it needs to carry out automatic functions. The body at maximum exercise level is a perfect example of this principle at work. At sub-maximal exercise rate, the body does not show signs of an increased oxygen demand. This would indicate that it had all of the oxygen that it needs. There is little change from the resting state. The little amount that it does need, it can attain from reducing the limiting factor to increasing energy output on the cellular level.