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The Science Behind Exercise

By Victoria Saadat

Published on June 16, 2011

Exercise science is the study of how exercise and physical activity alter a person's physiology, defined as the body's structure and function. Early research in exercise science focused on the effects of different types of exercise and environments on the body. Today, the wealth of information about exercise science has motivated manufacturers and fitness specialists to formulate optimal nutrition and exercise regimens for their clients. Information about exercise routines and nutrition plans allow individuals to tailor their daily regimens to their specific goals. Certain types of exercise are best for weight loss while others promote muscle growth.

Courtesy of Essentials of Strength Training & Conditioning, National Strength & Conditioning Association

Figure 1: A single muscle fiber (a muscle cell) has a plasma membrane called the sarcolemma and cytoplasm called sarcoplasm, The muscle fiber also contains the myofibrils actin and myosin.

Muscle Growth

There are three types of muscle: smooth, cardiac, and skeletal. In order to build skeletal muscle (see Figure 1 for structure), it is helpful to know the basic science behind this seemingly daunting task. Such details are crucial in professional sports and competitive bodybuilding, leading both bodybuilders and professional athletes to plan their training routines around these uncovered scientific facts. The key is an exercise technique called progressive overload, wherein one overloads, or stresses, the muscles with each workout. By putting muscles through challenging tasks, such as sets with increased weights, or through new tasks, such as entirely new exercise routines, progressive overload can allow hypertrophy, or increase in muscle cell size, to take place (Hernandez and Kravitz).

There are two well-defined types of muscle hypertrophy: sarcomere hypertrophy and sacroplasmic hypertrophy. Sarcomere hypertrophy allows growth of the muscle contractile machinery, composed of actin and myosin filaments, which make up about 80% of each muscle cell. Sarcoplasmic hypertrophy, on the other hand, allows growth of the ATP-producing cell components that can provide energy to the muscle; these components make up the other 20% of muscle cells (Butt). While sarcoplasmic hypertrophy increases diameter and decreases density of muscle, sarcomere hypertrophy leads to a greater increase in density and will lead to greater muscle strength and athletic performance (Starr).

Sarcoplasmic hypertrophy occurs when muscle fiber cells experience an increase in their number of mitochondria, or ATP-producing cellular organelles, and in their concentrations of enzymes involved in cellular respiration. The muscle cells also have an increased volume of cytoplasm, known as sarcoplasm in muscle cells; glycogen, an energy storage carbohydrate; and intrastitial fluid, or fluid between cells. All of these changes allow the cells to produce a greater amount of ATP, an energy-rich molecule containing three phosphate groups. Muscle contraction involves removal of a phosphate molecule from ATP, releasing ADP and energy. Thus, since muscle contraction requires replenishment of ATP, sarcoplasmic hypertrophy increases the ATP supply. Even though this does not allow the muscles to improve their strength, it enhances their endurance by increasing the energy supply available to the muscle cells (Butt). People can achieve sarcoplasmic hypertrophy by lifting moderately heavy weights with a large number of repetitions (Strength Training).

David Richfield/Wikimedia Commons

Figure 2: Actin and myosin myofibrils are involved in muscle contraction.

Unlike sarcoplasmic hypertrophy, sarcomere hypertrophy increases muscle strength. Trainers and kinesiologists believe that in order for sarcomere hypertrophy to occur, muscle fibers must first be broken down (Starr). During exercise, the tension building up in muscle fibers causes damage to the cross-bridges between actin and myosin filaments; and ruptures the sarcolemma, or plasma membrane of muscle cells, leaking calcium into the intracellular space. This increase in the intracellular calcium ion levels activates enzymes called calpains to remove the damaged contractile tissue, triggering the immune system cells (including neutrophils, monocytes, and macrophages) to remove and break down the damaged fibers. The rupture of the sarcolemma also causes muscle cells near the site of damage to produce and release growth factors (Butt). Sarcomere hypertrophy can be accomplished by lifting heavy weights with a low number of repetitions (Strength Training).

The removal of the damaged fiber cells activates myogenic stem cells known as satellite cells, allowing them to repair the damaged muscle fibers and promote muscular growth (Hernandez and Kravitz). The released growth factors can also stimulate the satellite cells to proliferate (Butt). Located on the outer surface of muscle fibers, satellite cells contain only one nucleus (Hernandez and Kravitz). Once muscle damage activates satellite cells, these stem cells differentiate into myoblasts, which further differentiate into skeletal muscle cells that can then fuse to existing muscle fibers in the damaged site (McNally and MacLeod). This fusion adds the satellite cells' nuclei to the skeletal cells. The increase in the number of nuclei in the skeletal muscle will allow the muscle fibers to produce larger quantities of actin and myosin proteins (Hernandez and Kravitz). Since actin and myosin play a key role in muscle contraction, the increase in the number of this myofibril filaments enhances the muscles' strength and ability to contract during exercise (Hoyles Fitness).

Natural Ergogenic Supplements

Bodybuilders and others trying to gain muscle mass commonly take certain natural, ergogenic (muscle building) supplements that aid in muscle growth, although some health care professionals warn about the unknown long-term effects of these supplements. Two of the most important supplements are creatine monohydrate and glutamine.

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Figure 3: Phosphocreatine, or phosphorylated creatine, donates its phosphate group to ADP to create ATP, a high-energy molecule needed for muscle contraction.

Creatine is a nutrient that is found naturally in our bodies and is composed of the three amino acids arginine, glycine, and methionine. About 95% of our body's creatine is in skeletal muscle, and about 60 to 70% of this muscular supply is phosphorylated, or bound to phosphate groups. As muscle contraction releases ADP molecules, phosphorylated creatine, or phosphocreatine, donates phosphate groups to ADP to replenish the ATP supply (see Figure 3). While short-term use of creatine supplements is considered safe, medical professionals are unaware of their long-term effects and warn against long-term use of these supplements (Hogans; Nickels). Furthermore, people already taking or considering to take creatine supplements should take them under the supervision of a health care provider to ensure that this supplement will not interfere with other medical treatments or procedures or aggravate any existing medical conditions (Nickels).

In times of protein deficiency, the body may actually break down muscle tissue to provide amino acids to other protein-building biochemical processes in the body. In order to maintain their supply of protein, many bodybuilders and athletes take the amino acid glutamine as a daily supplement. The most abundant amino acid in the body, Glutamine is also found naturally in most protein and dairy products. By preventing breakdown of muscle over time, glutamine acts as an anti-catabolic agent that increases the effectiveness of workouts. As with creatine, those considering to take glutamine should do so under the supervision of a health care provider (University of Maryland Medical Center).

Weight Loss

Exercise not only plays a role in building muscle strength and endurance but also is a key component of controlled weight loss and/or overall health. Essentially, any net loss of calories contributes to weight loss. Conversely, net gain of calories leads to weight gain.

Swimming is a healthy sport that is not only enjoyable, but also capable of relieving joint pain, making it easy to continue for a lifetime. However, it has been noted that swimming actually contributes less to weight loss than was once expected. Those aiming to lose weight may be better off engaging in other forms of aerobic exercises. Swimming has been shown to induce changes in the hormones involved in suppressing or triggering hunger and appetite, making the swimmer feel extremely hungry after a water workout. After swimming, there are low levels of leptin (an appetite-suppressing hormone) and high levels of ghrelin (an appetite-enhancing hormone) circulating throughout the body; this hormonal imbalance causes the sensation of hunger. Swimming works all the muscles of the body and develops the swimmer's cardiovascular fitness, endurance, and muscular strength. Swimming also burns approximately 3 calories per pound of bodyweight per mile. That is, a 150-pound person burns almost 500 calories per mile swum! Even though a person burns a high amount of calories while swimming, the post-workout calorie burn that is typical of some sports, such as running and biking, is absent (Finn, 2006). During land sports, the body temperature increases; as a result, for 18 hours after the workout, the body usually burns calories at a higher rate than the normal resting metabolic rate in an attempt to cool down to normal body temperature. However, since swimming does not cause an increase in body temperature, this after-burn does not occur (Luebbers, 2011).

Conclusion

No matter your age, exercise can promote strength, flexibility, and heart health. Staying fit gives you more energy, allowing you to do more with life. It maintains bone health, controls blood-pressure, aids circulation, and promotes better sleep quality. During exercise, mood-lifting chemicals called endorphins are released from the brain, thereby fighting depression and promoting a more positive mood about life. Exercise is one of the most important elements of a healthy lifestyle and can reduce the risk of diseases including coronary heart disease, diabetes, hypertension (high blood pressure), and obesity (Bourhenne). By getting started with an exercise routine, finding an exercise buddy, and gradually increasing workout intensity, exercise can become a key component and health-endorsing part of your life.

Works Cited

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