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To move around is a fundamental part of human existence. Humans move to gather and prepare food, protect themselves, and to reproduce. This type of functional movement is called physical activity.

Meanwhile, exercise is a planned act of moving at specific speeds and for a given duration and/or against a resistance. In this chapter we will answer questions about exercise and how to plan an exercise program to achieve a desired outcome such as muscle development, better performance, and body sculpting.

Why Do or Should We Exercise?
To the public, the terms exercise and workout are synonymous. Regular exercise can provide numerous benefits. Depending upon the type of exercise, these benefits can include:

• improved cardiovascular health,
• a tool for weight management,
• improved body composition,
• a positive impact on bone density,
• a vehicle for relaxation and social interaction, and
• improved self-image.


What Is Exercise Training?
When we exercise regularly the muscles that are involved can adapt to be more efficient in performing the exercise task. This is a “trainingexercise-basicseffect” or “adaptation” that is visually obvious for weight trainers as the targeted muscles enlarge to provide more strength and power. Their exercises will involve near maximal or maximal intensity for very short durations. Meanwhile, during regular exercise, consisting of lower intensity tasks performed for longer durations, muscle will adapt to become more inclined to aerobic energy metabolism, as will be explained shortly.
What Are the Most Important Concepts in Exercise Training?
The most important aspects of training are the intensity and duration of the exercise. The relationship between these factors is what determines the nature of the associated adaptation. Aspects of genetic predisposition also will influence the degree of adaptation as well as the inclination toward a certain type of training. More on the genetics of training and achievement in sports soon enough.
What Does Exercise Intensity Mean?
Exercise intensity refers to the level of exertion. For instance, lifting a weight that results in muscular fatigue after just a few repetitions or “reps” of an exercise is pretty high with respect to intensity. So too would be an all-out running or cycling sprint where fatigue occurs in a minute or so. Basically, the higher the intensity, the shorter the possible duration of the exercise. To reach such a high level of intensity, exercise often includes resistance against an otherwise simple movement of a muscle group or related groups. Examples of resistance training include weight training or running on an incline (for example, running on hills or a graded treadmill) or cycling (for example, cycling uphill or an exercise bike with variable resistance). It is the level of the resistance that dictates the level of intensity. Higher intensity and muscular fatigue will be associated with muscle adaptations that will allow for greater strength and power. In this case, muscles can enlarge or “hypertrophy.”

What Is the Difference Between Work, Strength, and Power?
Work relates the amount of force necessary to move something (for example, a weight) a certain distance—hence the term “workout.” Strength then refers to the amount of force that can produced by someone to perform work. Further still, power is concerned with how long it takes to perform the work. The faster the work can be performed the more powerful the effort. Mathematically:
Work = Force × Distance
and
Power = Work × Time

What Does Exercise Duration Mean?
Duration refers to how long an exercise is performed continuously. Activities like running and cycling are performed at a lower or moderate intensity and tend to last for a half to one hour or longer. Sustained exercise for longer durations is often called endurance training. It is also referred to as cardiovascular training as adaptations can include the development of a more powerful heart and more blood vessels in our heart and skeletal muscle. Intensity and duration are the most important factors in determining if an exercise is resistance or endurance or both.
How Does Exercise Change Our Body?
It is the intensity level of an exercise that will be the primary determinant of the range of adaptation. This means that although some sports are associated with a certain type of adaptation, it is not an absolute. For instance, weight training can be more aerobic and cardiovascular if the weights (resistance) are not heavy enough and the number of reps is very high. Running and biking are often associated with more aerobic and cardiovascular adaptations but it is easy for runners or cyclists to train for greater strength and power by including more resistance in their training.

Muscle Is Fueled Primarily by Carbohydrates and Fat
What Fuels Muscle Activity?

Muscle contraction is fueled by ATP, which is generated by both anaerobic and aerobic energy metabolism. Because ATP is found in low concentrations in all cells of the body, these ATP-generating mechanisms must be increased with the onset of activity in an attempt to meet ATP demands of working muscle cells. This means that muscle cells need to stoke up those chemical reaction pathways that break down carbohydrate and fat for ATP generation. Muscle cells have a little stored carbohydrate (glycogen) and fat and also receive glucose and fatty acids from the blood. So increased blood delivery to the exercising muscle delivers not only needed oxygen but also fuel.
What Is Creatine?
Another power source for working muscle is creatine phosphate. Creatine is a substance found mostly in skeletal muscle cells, but it is also found in heart muscle cells and brain. When ATP is abundant in these cells, such as when muscle is not active (at rest), phosphate is transferred to creatine. This forms creatine phosphate, which is a rapid ATP-regenerating source. When ATP is used to power muscle contractions, the phosphate of creatine phosphate can be transferred to ADP to regenerate ATP. This involves only one chemical reaction and can happen very rapidly.

Are There Different Types of Muscle Fibers?
Researchers refer to skeletal muscle cells as fibers because they are thin and long. In fact, some muscle fibers can extend the entire length of a muscle, such as in the biceps. That is several inches! In addition to their unique design, skeletal muscle cells are not all the same. In fact, humans are blessed with more than one type of skeletal muscle cell, which vary in performance and metabolic properties. This allows our body to efficiently perform a broad range of activities or sports that vary in nature. This includes sports that are longer duration/lower intensity and short duration/higher intensity.
What Are the Different Classes of Muscle Cells?
Muscle cells are grouped into two general categories or “types” (Type I and II). Type II muscle fibers are often subclassified as IIa, IIb, and IIc. For this book, it is enough to only distinguish between the two main types. Skeletal muscle is actually bundles of a mixture of Type I and II muscle fibers. In fact, the average person will tend to have about a 50/50 mixture of Types I and II muscle fibers. Meanwhile, highly successful athletes tend to have a significant imbalance one way or the other which, as will soon be discussed, will allow them to excel at a particular sport.

Performance and Metabolic Properties of Muscle Fibers

Type I Muscle Fibers Type II Muscle Fibers
Develop force more slowly than Type II
muscle fibers
Develop force more quickly
(more powerful)
Have more mitochondria and
capillaries and thus are more aerobic
Have fewer mitochondria and
capillaries and thus are more anaerobic

Generate very little lactic acid (lactate)

Do not fatigue quickly

Generate more lactate

Fatigue quickly

 

 

 

Type I muscle fibers are more aerobic and can perform longer than
Type II muscle fibers.


What Are Type I Muscle Cells?
Type I fibers (sometimes called slow-twitch or slow-oxidative fibers) are better designed for prolonged exercise performed at a lowerexercise_basicsintensity. In comparison to Type II fibers, Type I fibers will have more mitochondria and rely more heavily on the aerobic generation of ATP. The primary energy molecules used to generate ATP in these muscle cells will be fatty acids and glucose. Since ATP production in mitochondria requires oxygen, proper function of these muscle fibers is very dependent upon oxygen supply via the blood. Luckily, Type I muscle cells always seem to have many capillaries around them to deliver oxygen-endowed blood. In addition, Type I fibers contain a substance called myoglobin. As mentioned in Chapter 10, myoglobin is an iron-containing protein that binds oxygen and serves as an oxygen reserve for these cells during exercise.
What Are Type II Muscle Cells?
Type II muscle fibers (sometimes called fast-twitch or fast-glycolytic fibers) can execute a much faster speed of contraction than Type I muscle fibers. This is to say that Type II muscle fibers are designed to generate force more rapidly, thereby allowing them to be more powerful. This will allow a job to be performed in a shorter amount of time. Meanwhile, Type II muscle fibers are relatively limited in their ability to generate ATP by aerobic means. So, when these cells break down glucose to pyruvate and generate a couple ATP in the process, much of the pyruvate that is formed will then be converted to lactic acid (lactate). This is because these muscle cells have less mitochondria and receive less oxygen as they are served by fewer blood vessels.

How Does the Brain Know Which Type of Muscle Cells to Use for Different Sports?
This is a no-brainer for the brain! This is because the brain will always call upon Type I muscle fibers first and then Type II. The major factor will be the required force to perform the exercise. For instance, when an exercise requires less force (for example, jogging, fast walking, casual cycling) the brain will for the most part call upon Type I muscle fibers. However, as the necessary force to perform an exercise increases (such as running, cycling fast, weightlifting), the brain will also call upon Type II muscle fibers to generate force to support the force generated by Type I fibers.

How Does Recruiting Different Muscle Fibers Relate to Performance?
Calling upon Type II fibers is sort of a win/lose situation for performance. It is a winner in that it will allow us to generate a lot more force to perform an exercise. However, it is a loser in that the exercise will become fatiguing as more lactic acid is generated in Type II fibers. This is why 5K runners cannot sprint the entire race. What they will do instead is run at the highest level they are able to, but that also keeps them from fatiguing before the end of the race. Their brains will call upon enough Type II muscle fibers to generate the force that allows them to run faster but not, however, enough Type II muscle fibers to generate critical levels of lactic acid and other factors that would result in fatigue before they cross the finish line.
Do Successful Athletes Have an Imbalance of Muscle Fiber Type?
Successful athletes seem to have an imbalance in muscle fiber types that favors excelling in a sport. For instance, successful sprinters often have a higher percentage of Type II fibers, allowing them to generate more force in a very brief period of time. This then allows them to be more powerful, generate more speed, and complete a sprint distance more quickly. Con- versely, successful endurance athletes tend to have a greater percentage of Type I muscle fibers. This allows them to generate more force through aerobic energy systems in muscle cells. They can perform at a higher intensity before they generate critical amounts of lactic acid. People who excel at certain sports tend to have a genetic predisposition based on predominance of muscle fiber type.
Are Athletes Born or Developed?
Often the question is asked whether top athletes are born or bred. The answer is both, but probably more of the former than the latter. Most very successful athletes are born (genetics) with the propensity to excel physically at a particular sport. Training can then improve that potential. This is mostly true for sports that are endurance based or involve extreme power, as mentioned. An athlete’s genes direct the formation of more Type I or Type muscle cells and body design and potential for skill development to excel at one or more sports. Then, to truly excel at a sport, the athlete must train and practice to optimize that performance. Can Training Allow Muscle Fibers to Change Type?
We do know that training results in changes in muscle metabolism, which may make us think that it is possible for Type I fibers to change into Type II fibers and vice versa. However, this probably is not the case. For instance, endurance training can lead to changes associated with Type II muscle fibers that will make them more aerobic. The fibers will adapt to have an increased ability to generate ATP by using oxygen. However, they don’t adapt to the point where we would classify them as Type I. Oppositely, we all know that resistance training (for example, weight lifting) improves the strength and power of a muscle group. Although it would be logical to think that half of this effect might be related to adaptations in Type I muscle fibers—as though they are being transformed into Type II muscle fibers—surprisingly this is not the case either. In fact, as the muscle group grows in size, most of the growth is related to enlargement (hypertrophy) of Type II fibers.

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