Physiology In Sport

Introduction

Biology is a branch of science that deals with living organisms and vital processes, both in animals and plants[1]. Physiology and Anatomy are two closely related branches of biology which provide the foundation for Exercise Physiology [2].

Anatomy investigates the basic structure of the body and the interrelationships between various body parts. Physiology derives from the Ancient Greek φύσις (Physis), meaning "Nature, Origin", and -λογία (-logia), meaning "Study of" [3]. It is the scientific discipline that deals with the processes or functions of living things, or the study of body functions. It allows us to understand and predict the body’s responses to stimuli; as well understand how the body maintains conditions within a narrow range of values in the presence of a continually changing environment [4].

Exercise Physiology has evolved from this study of anatomy and physiology, and examines how our bodies structures and functions are altered when we are exposed to acute and chronic bouts of exercise. It is primarily the study of how the body adapts physiologically to the acute or short term stress of exercise, and the chronic or long term stress of physical training.

Sport Physiology further applies these concepts from exercise physiology specifically to training the athlete and enhancing athlete performance within a specific sport. Exercise and sport physiology is about improving performance, by knowing how the body functions during exercise, and using scientific principles to allow your body to train better, perform better and recover quicker. Studies in exercise physiology help athletes achieve greatness e.g. it is now known that olympic weightlifting and plyometric training are two methods to increase vertical jump height [5].

The physiological response to exercise is dependent on the intensity, duration and frequency of the exercise as well as the environmental conditions[6]. During physical exercise, requirements for oxygen and substrate in skeletal muscle are increased, as are the removal of metabolites and carbon dioxide. Chemical, mechanical and thermal stimuli affect alterations in metabolic, cardiovascular and ventilatory function in order to meet these increased demands[6].


What Happens during Exercise

Musculoskeletal System

Exercise is about movement, and the muscular system is primarily responsible for creating movement. Therefore, the responses and adaptations of the muscular system to exercise are important parts of exercise physiology. During exercise, many changes take place in skeletal muscle, such as changes in temperature, acidity, and ion concentrations. These changes affect muscle performance and may lead to fatigue. Indeed, the mechanisms of muscle fatigue is an important area of inquiry in exercise physiology [7] [8]. In addition, the adaptations of the muscular system to exercise lead to long-term changes in exercise capability.

Depending on the type of exercise, changes in enzyme concentrations, contractile protein con- tent, and vascularization affect the ability of the muscle to perform work. For example, endurance exercise increases concentrations of enzymes in skeletal muscle that are involved in the aerobic production of energy [9] [10]. In contrast, strength training is associated with increases in the size of the muscle due to increased synthesis of contractile proteins, with little change in anaerobic enzyme content [11]. These types of adaptations are appropriate for a certain type of activity in that these adaptations will improve muscle performance in the types of activities that stimulated these adaptations.

If muscles are underloaded, it does not matter how much they are exercised, they will increase little in strength. On the other side, if they are trained with at least 50 percent of maximal force of contraction, they will develop strength rapidly even if the contractions are performed only a few times each day. Using this principle, experiments on muscle building have shown that six nearly maximal muscle contractions performed in three sets 3 days a week give approximately optimal increase in muscle strength, without producing chronic muscle fatigue.

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The musculoskeletal system is fundamental in exercise physiology because to exercise, there is the need of muscle functional contraction, of course. The strength of a muscle depend mostly by its cross sectional area[14], therefore size is key.

Mechanical Work performed by a muscle is the amount of force applied by the muscle multiplied by the distance over which the force is applied[15].

Muscle Strength is the maximal amount of tension or force that a muscle or a muscle group can voluntarily exert on a maximal effort[16] when type of muscle contraction, segment velocity and joint angle are specified[17].

The power of muscle contraction is different from muscle strength because power is a measure of the total amount of work that the muscle performs in a unit period of time and is generally measured in kilogram meters (kg-m) per minute[15].

Another important concept is endurance, defined as the ability to perform repeated contractions against a resistance or maintain a contraction for a period of time[16].

Types of Muscle Contraction

There are several types of muscle contraction[18], as follows:

  • Isometric; it occurs when tension is developed in the muscle without movement, therefore muscle origin and insertion do not move and there is not changes in muscle lenght.
  • Concentric; it occurs when tension is developed in the muscle with origin and insertion moving closer, therefore the muscle shortens.
  • Eccentric; it occurs when tension is developed in the muscle with origin and insertion moving apart, therefore the muscle lengthens.

Fast-Twitch and Slow-Twitch Muscle Fibers

Fast Twitch (FT) or type II fibers has two primary subdivisions, type IIa and type IIx. Both show rapid contraction speed, high capacity for anaerobic ATP production through glycolysis, a larger diameter; type IIa show also some aerobic capacity, therefore they are called also intermediate fibers.

Slow Twitch (ST) or type I fibers generates energy primarily through aerobic system. This type of fibers shows a relatively slow contraction speed, a higher number of larger mitochondria and larger amounts of myoglobin.

Muscle hypertrophy

Muscle sizes are determined mainly by genetic and anabolic hormone secretion. Training can add another 30 to 60 percent of muscle hypertrophy, mostly from increased muscle fibers diameter, but in a small part also from increased number of fibers (hyperplasia).

Hypertrophied muscle are characterized by:

  • an increased number of myofibrils;
  • increased number of mitochondrial enzymes;
  • increase in ATP and phosphocreatine amounts available;
  • increased stored glycogen and triglyceride;

thus enhancing both aerobic and anaerobic systems.

Metabolic Systems

With respect to exercise, the area of metabolism involves the study of how the body generates energy for muscular work. The energy for exercise, in the form of adenosine triphosphate (ATP), is derived from the breakdown of food from the diet. Originally in the form of protein, fat, and carbohydrate, the energy is made available by different enzymatic pathways that break down food and ultimately lead to ATP formation. The specific metabolic pathway used and the associated food broken down for energy are affected by the type of exercise and length of time that a person is performing and have implications for the ability of the person to perform that exercise. These are important issues in exercise physiology because they affect decisions regarding the type, intensity, and duration of exercise to be prescribed to an athlete.

In order to meet the increased demands for ATP when exercising, there is an increase in the chemical reactions in the body providing ATP. In aerobic metabolism, the chemical reactions use oxygen to completely break down carbohydrates e.g., glycogen, glucose and fats for energy. With moderate levels of exercise, the muscles can use aerobic metabolism to meet the increased energy requirements. Aerobic metabolism does not allow for maximum power output of the muscles but aerobic activity can be sustained for long periods of time. The body first uses the stored oxygen available in the body and then the exercise level is limited by the capacities of the respiratory and cardiovascular systems to provide more oxygen to the active cells.

These systems do not work on a on-off base but rather in a conveniently mixed mode with considerable overlap between them.

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Phosphocreatine-Creatine System

Phosphocreatine is another chemical compound that has a high-energy phosphate bond that can be hydrolysed to provide energy and resynthesize ATP. This occurs within a small fraction of a second. Therefore, all the energy stored in the muscle phosphocreatineis almost instantaneously available for muscle contraction, just as is the energy stored in ATP.

Thus, the energy from the phosphagen system (ATP and Phosphocreatine stored in the muscle) is used for maximal short bursts of muscle power.

Lactic Acid System

The stored glycogen in muscle can be split into glucose and the glucose then used for energy. Glycolysis is the first part of this process, which occurs without use of oxygen and, therefore, is said to be anaerobic metabolism. During glycolysis, each glucose molecule is split into two pyruvic acid molecules, and energy is released to form four ATP molecules for each original glucose molecule.

These pyruvic acid molecules can then be used by mitochondria in muscle cells, reacting with oxygen and providing more ATP molecules (oxidative stage), but if the exercise is too intense then it is likely the oxygen is insufficient for this second stage to occur, therefore pyruvic acid is converted into lactic acid. By doing so considerable amount of ATP are formed without oxygen, but also of lactic acid which will diffuse into interstitial fluid and bloodstream.

Another characteristic of the glycogen-lactic acid system is that it can form ATP molecules about 2.5 times as rapidly as can the oxidative mechanism of the mitochondria. Therefore, when large amounts of ATP are required for short to moderate periods of muscle contraction, this anaerobic glycolysis mechanism can be used as a rapid source of energy. It is, however, only about one half as rapid as the phosphagen system. Under optimal conditions, the glycogen-lactic acid system can provide 1.3 to 1.6 minutes of maximal muscle activity in addition to the 8 to 10 seconds provided by the phosphagen system, although at somewhat reduced muscle power.

Aerobic System

The aerobic system is the oxidation of glucose, fatty acids and amino acids. Combined with oxygen these compounds are able to release great amounts of energy used to provide ATP.

This system provides less ATP per minute than the phosphagen system and the lactic acid system, but can last as long as there are nutrients to provide substrates.

The aerobic system is thus useful for less powerful but prolonged activities.

Cardiovascular System

The cardiovascular system is responsible for the transport of blood, and therefore oxygen and nutrients, to the tissues of the body. Similarly, the cardiovascular system facilitates removal of waste products such as carbon dioxide from the body. In addition, the cardiovascular system is centrally involved in the dissipation of heat, which is critical during prolonged exercise.

The primary components of the cardiovascular system are the heart, which pumps the blood, and the arteries and veins, which carry the blood to and from the tissues. Clearly, the cardiovascular system’s functions are critical during exercise; therefore, a large proportion of study and research in exercise physiology focuses on the responses and adaptations of the cardiovascular system to exercise.

Important beneficial effects of exercise on the cardiovascular system include a decrease in resting blood pressure (an important risk factor in cardiovascular disease) and a decrease in blood cholesterol levels (reducing the risk for developing atherosclerosis). Furthermore, exercise is an important component of the cardiac rehabilitation process following a cardiac event such as a heart attack [19].

Individuals with training in exercise physiology are playing important roles in the research and implementation of exercise programs for the prevention of cardiovascular dis- ease and the rehabilitation of individuals with cardiovascular disease [20].

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Exercises increase some components of the cardiovascular system, such as:

  • stroke volume (SV),
  • cardiac output,
  • systolic blood pressure (BP),
  • mean arterial pressure.

To meet the metabolic demands of skeletal muscle during exercise, 2 major adjustments to blood flow must occur. First, cardiac output from the heart must increase. Second, blood flow from inactive organs and tissues must be redistributed to active skeletal muscle. At rest, muscles receive approximately 20% of the total blood flow, but during exercise, the blood flow to muscles increases to 80-85%.

Generally, the longer the duration of exercise, the greater the role the cardiovascular system plays in metabolism and performance during the exercise bout. An example would be the 100-meter sprint (little or no cardiovascular involvement) versus a marathon (maximal cardiovascular involvement).

Pulmonary System

The pulmonary system is important for the exchange of oxygen and carbon dioxide between the air and blood. The primary component of the pulmonary system is the lungs, which vary in volume from 4-6L and if laid out flat would cover a huge surface area from 60 - 80m2 [20]. Exercise places a great deal of stress on the pulmonary system as oxygen consumption and carbon dioxide production are increased during exercise, thus increasing the pulmonary ventilation rate. The control and regulation of the pulmonary system during exercise are areas of much research. As with the cardiovascular system, the interplay of exercise and the neurological control of breathing is not completely understood. Surprisingly, most evidence indicates that there are few, if any, adaptations to exercise in the pulmonary system itself in healthy individuals [21]. However, adaptations in the musculature structures that controls breathing are apparent [22].

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Oxygen consumption in sport activities

Oxygen consumption rises exponentially during the firsts minutes of exercise, reaching a steady rate around the third minute and then remaining relatively stable. In such condition, energy required by the working muscles and ATP production in aerobic metabolism are balanced, without lactate accumulation in blood.

VO2max or Maximal Oxygen Consumption

Maximal oxygen consumption (VO2max) is the region where oxygen consumption reach a steady state or increases only slightly with additional increases in exercise intensity. It provides a quantitative measures of a person's capacity for aerobic ATP resynthesis.

VO2max in short-term studies is found to increase only 10% with the effect of training. However, that of a person who runs in marathons is 45% greater than that of an untrained person. This is believed to be partly genetically determined (eg, stronger respiratory muscles, larger chest size in relation to body size) and partly due to long-term training.

Oxygen Diffusing Capacity

Oxygen diffusing capacity is a measure of the rate at which oxygen can diffuse from the alveoli into the blood. An increase in diffusing capacity is observed in a state of maximal exercise.

During exercise, increased blood flow through the lungs causes all of the pulmonary capillaries to be perfused at their maximal level, providing a greater surface area through which oxygen can diffuse into the pulmonary capillary blood. Athletes who require greater amounts of oxygen per minute have been found to have higher diffusing capacities.

Both arterial blood oxygen pressure and carbon dioxide pressure remains almost at normal level even during strenuous exercise, as they are well compensated.

Nervous System

Among the many functions of the nervous system is the control of movement by way of the skeletal muscles, which are under voluntary (and reflex) control. Most of the study of the neural control of movement is considered the domain of motor control and learning. However, certain areas of inquiry are also of interest to exercise physiologists. Two notable areas are neuromuscular fatigue and neurological adaptations to strength training. With respect to neuromuscular fatigue, research suggests that under certain conditions the Central Nervous System may play an important role in the development of fatigue [8]. For example, changes in brain levels of serotonin and dopamine may influence fatigue [23] [24]. In addition, the firing rate of motor units can change during fatigue [25], which may be due to an elegant interplay between peripheral receptors and the CNS.

Similarly, strength training may influence the CNS control of muscle activation by changing the number of motor units that the CNS will activate during a contraction and the firing rate of the active muscle [26]. Much of the data regarding neurological adaptations to strength training are contradictory, but this remains an important area of study. These areas of study are important to basic researchers in exercise physiology, and new information in these areas may also have implications in the rehabilitation of individuals with neuromuscular disorders.

The Autonomic Nervous System (ANS) is involved in the involuntary control of body functions. The ANS has two divisions. The Sympathetic Nervous System becomes active during situations of increased stress, such as during exercise. The Parasympathetic Nervous System is more active during resting conditions. Most notable in exercise physiology is the autonomic control of the cardiovascular system. For example, during exercise an increase in Sympathetic Activity and a decrease in Parasympathetic Activity result in an increase in activity of the heart and an increase in blood pressure. In addition, the ANS is involved in the redistribution of blood flow away from inactive tissues, such as the gastrointestinal tract, and toward the active tissues during exercise.

Endocrine System

The endocrine system is the system of hormones, which are chemicals released into the blood by certain types of glands called endocrine glands. Many hormones are important during exercise and may affect performance. For example, during exercise the hormone called growth hormone increases in concentration in the blood. This hormone is important in regulating blood glucose concentrations. Similarly, other hormones, such as cortisol, epinephrine, and testosterone, increase during exercise. Their effects may be short term in that they affect the body during the exercise bout. Other effects are prolonged and may be important in the long-term adaptation to regular exercise.

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