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 as 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]


The Basic Principles in Exercise Physiology

The body's responses to a single bout of exercise are regulated by the principle of homeostasis. Homeostasis is defined as the ability of the body to maintain a stable internal environment for cells by closely regulating various critical variables such as pH or acid base balance, oxygen tension, blood glucose concentration and body temperature.[9]

The overload, specificity, reversibility and individuality principles influence training adaptations in the body, for health as well as performance.

The application of an specific and appropriate stressor can sometimes be referred to as overloading the system. The overload principle states that habitually overloading a system causes it to respond and adapt. The overload principle can be quantified according to load (intensity and duration), repetition, rest and frequency. Load refers to the intensity of the exercise stressor i.e in strength training it can refer to the amount of resistance or in swimming it can refer to speed. The greater the load, the greater the fatigue and recovery time needed. Repetition implies the number of times that a load is applied. Rest refers to the time interval between repetitions and frequency refers to the number of training sessions per week.[10]

The specificity principle states that only the system or body part repeatedly stressed will adapt to chronic overload. Therefore the overload principle will only apply to the system or body part used while exercising.[9]

Reversibility states that whereas training may enhance performance, inactivity will lead to a decrease in performance.[9]

The individuality principle states that while the physiological responses to a particular stressor can be mostly predictable, the precise responses and adaptations will still differ among individuals.[9]

[11]

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.[14][15] 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 content, and vascularisation 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.[16][17] 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.[18] 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 under loaded, 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.

The musculoskeletal system is fundamental in exercise physiology. The strength of a muscle is mostly determined by its cross sectional area.[19] 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.[20]

Muscle Strength is the maximal amount of tension or force that a muscle or a muscle group can voluntarily exert on a maximal effort[21] when the type of muscle contraction, segment velocity and joint angle is specified.[22].

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.[20]

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.[21]


Muscle Structure and Contraction


[23]
[24]

Types of Skeletal Muscle Actions

There are several types of skeletal muscle actions:[25][26]

  • Static (Isometric): it occurs when tension is developed in the muscle without movement, therefore the muscle origin and insertion does not move and there is no changes in muscle length. During a static muscle contraction, the myosin and actin myofilaments form cross-bridges and generate force, but the external force is greater than the muscle-produced force. No mechanical work (force x distance) is done, as there is no displacement, even though there is energy expenditure.[25]
  • Dynamic (Isotonic) Muscle Actions:
    • Concentric: The muscle produces enough force to overcome the external resistance. The muscle shortens and there is movement at the joint. The myosin and actin myofilaments form cross-bridges, and the filaments slide past each other causing muscle shortening. Energy expenditure results in positive mechanical work as force production and displacement occurs.[25]
    • Eccentric: The muscle lengthens while producing force. This happens because the external resistance moves in the direction opposite to the standard concentric (shortening) action.[25] A high force is produced by the contractile elements and this makes these type of muscle actions an important training stimulus. Eccentric muscle actions are also associated with muscle damage and soreness and it is advised that the eccentric component of exercise training should initially be limited.[25] Furthermore, eccentric contractions have clinical value during the rehabilitation of tendinopathies.[27][28]
  • Dynamic (Isokinetic) Muscle Actions: These muscle actions are characterised by constant velocity and can only be achieved in a laboratory or clinic setting. Specialised computerised equipment is necessary to maximise resistance at every angle of range of motion. The isokinetic contractions can be concentric or eccentric. These type of contractions and devices can help athletes perform exercises that simulates the actual speed and sport-specific activities.[25]

Skeletal Muscle Fibre Types

Human skeletal muscle fibres vary in terms of their mechanical, physiological and biochemical characteristics. Generally, human skeletal muscles have three types of fibres: Type I, Type IIa and Type IIx.

Fast Twitch (FT) or Type II fibers have two primary subdivisions, Type IIa and Type IIx. Type IIa fibres have intermediate properties - they are fast contracting fibres but also have a oxidative metabolic profile. Both Type IIa and Type IIx show rapid contraction speed, high capacity for anaerobic ATP production through glycolysis and a larger diameter. Type IIx fibres are fatigable fibres.[25]

Slow Twitch (ST) or type I fibers generate energy primarily through aerobic system. This type of fibre shows a relatively slow contraction speed, a higher number of larger mitochondria and larger amounts of myoglobin. These fibres are the slow, oxidative, fatigue-resistant fibres.[25]

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.


Muscle Strength Determinants

The amount of force that a muscle can generate varies individually. Genetics play a big role in this force generation, but there are other determinants as well:[29]

  • Nerve supply: The number of motor units recruited determine the amount of force. Slow twitch (Type I fibre) motor units are easily recruited, whereas Fast twitch (Type IIx) motor units hold more muscle fibres and can therefore generate more force.[29]
  • Muscle length: Most force is produced when muscles are working in mid-range. Mid-range is the position where there is optimal overlap of the thin and thick filaments at sarcomere level.[29]
  • Speed of shortening: More force is generated with slower movement. A dynamic (isotonic) muscle action produces more force than a static (isometric) contraction.[29]
  • Mechanical advantage: Most muscles work at a mechanical disadvantage due to the position of the muscle insertion point in relation to the portion of the limb being moved.[29] (Think of knee extension, where the quadriceps acts across the bone levers of the femur and tibia, the knee joint is the fulcrum and the quadriceps inserting onto the upper end of the tibia).
  • Muscle fibre pennation: The fascicles in muscle are arranged according to the shape of the muscle. More force will be produced by muscles where the fascicles are parallel with the longitudinal axis of the muscle.[29]
  • Connective tissue: Connective tissue in and around the muscle provides support and also increases the muscle's ability to produce force.[29]

    Energy 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.[30] 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.[25] 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.[25][29][30]

    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.[30] With moderate levels of exercise, the muscles can use aerobic metabolism to meet the increased energy requirements.[25] Aerobic metabolism does not allow for maximum power output of the muscles but aerobic activity can be sustained for long periods of time.[30] 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.

    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 phosphocreatine is almost instantaneously available for muscle contraction, just as is the energy stored in ATP.

    At the start of exercise ATP is broken down into ADP + Pi, resulting in ATP being reformed by the creatine phosphate (CP) reaction. A phosphate is donated to ADP from CP to reform ATP. This method is the fastest and simplest way to produce energy for a muscle contraction. This energy source lasts for about 5 seconds as muscle cells only store a small amount of ATP and CP. This reaction provides energy for the start of the exercise and short-term high intensity exercise. This energy production is done without oxygen, thus an anaerobic method of energy production.[29]

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

    Anaerobic Glycolysis (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.[25][29]

    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.[25]

    Oxidative Phosphorylation (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 occurs in the mitochondria of the cell. Two metabolic pathways, the Krebs cycle and the electron transport chain, work together. These pathways remove hydrogen from from carbohydrates, fats and proteins so that the potential energy in the hydrogen can be implemented to produce ATP.[29]

    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 longer-term aerobic exercise activities.[30]

    [31]

    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. Although all systems (i.e. pulmonary, respiratory, skeletal muscle and cardiovascular system) are involved in constructing an appropriate response to exercise training, the cardiovascular system can be seen as the central hub.[32] 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.[33]

    Individuals with training in exercise physiology are playing important roles in the research and implementation of exercise programs for the prevention of cardiovascular disease and the rehabilitation of individuals with cardiovascular disease.[34]

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

    • stroke volume (SV)[32]
    • cardiac output[32]
    • systolic blood pressure (BP)[32]
    • mean arterial pressure[32]

    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).

    [35]


    Heart Rate

    During exercise heart rate (HR) increases. This happens alongside oxygen uptake during exercise in order to reach steady-state heart rate during constant workload, sub-maximal exercise. In incremental maximal exercise heart rate increases up to maximal heart rate (HRmax). Initially cardiac output during exercise increases as a result of an increase in stroke volume. With a further increase in exercise workload, further increases in cardiac output becomes dependent on heart rate. In healthy people maximal exercise is limited by maximal heart rate (HRmax). Maximal heart rate can be estimated using the equation of 220 - age. In trained athletes an increase in stroke volume is noticed. This therefore allows for a greater cardiac output in trained individuals for a given heart rate.[29]


    Blood Flow and Pressure

    During exercise various factors in the exercising muscle cause vasodilation and opening of dormant capillaries. These factors are:

    • increase in temperature
    • decrease in oxygenation
    • decrease in metabolic products

    As a result an significant increase in blood flow to the muscle is observed.[29]

    To maintain a sufficient blood pressure, systemically vasoconstriction causes blood to move from the periphery to the central circulation. This kept balance between vasoconstriction vasodilation, ensures that there is little change in blood pressure during steady-state exercise.[29] During incremental exercise, the large increase in cardiac output required during high levels of exercise, can cause an increase in the systolic pressure up to around 200mmHg, but the diastolic pressure remains stable.[29]


    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.[34] 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.[38] However, adaptations in the musculature structures that controls breathing are apparent.[39]

    Oxygen uptake.

    Oxygen uptake (VO2) is the amount of oxygen that the body takes up and utilises.[29] Oxygen consumption rises exponentially during the first minutes of exercise, reaching a steady rate around the third minute and then remaining relatively stable. In such conditions, energy required by the working muscles and ATP production in aerobic metabolism are balanced, without lactate accumulation in blood.

    VO2max or Maximal Oxygen Uptake

    Maximal oxygen consumption (VO2max) is the region where oxygen consumption reach a steady state or increases only slightly with additional increases in exercise intensity.[29] It provides a quantitative measures of a person's capacity for aerobic ATP resynthesis. Maximal oxygen uptake is dependent on a person's:

    • gender
    • height
    • weight
    • lung function
    • fitness level
    • type of activity being performed

    VO2max is exercise specific and is greater in activities involving large muscle groups.[29] Trained athletes can have a higher Maximal Oxygen Uptake than sedentary individuals because of their enhanced stroke volume, improved myocardial function and a higher capacity for oxidative metabolism in active muscles.[25] 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 (e.g, 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.

    Ventilation

    During light to moderate intensity exercise/sports activities, ventilation increases linearly with oxygen uptake and carbon dioxide production. This is necessary to meet the body's oxygen requirement and to expel the additional produced carbon dioxide. Initially the increase in ventilation is achieved by an increase in the tidal volume, and with increasing demand by increasing the respiratory rate.[29] During heavy to maximal exercise a rise in ventilation is observed in response to the lactate threshold. This is known as the ventilatory threshold. Should a person continue exercising a further rise in ventilation is observed at the onset of blood lactate accumulation (OBLA). This happens in order to expel more carbon dioxide in an effort to reduce the acidity in the blood. This rise in ventilation at the OBLA is known as respiratory compensation.[29]

    Arteriovenous Oxygen Difference

    The arteriovenous oxygen difference is a measure of the amount of oxygen taken up from the blood by the tissues. Cardiac output and arteriovenous oxygen difference are the determinants of overall oxygen uptake. During exercise blood flow increases to the tissues; haemoglobin dissociates quicker and easier. This results in a greater arteriovenous oxygen difference during exercise. In trained athletes, the arteriovenous oxygen difference is greater as a result of the tissues becoming more efficient in oxygen uptake with aerobic training.[29]


    Thermoregulation during Exercise

    The heat produced by the increase in metabolism during exercise must be dissipated in order to prevent a dangerous increase in core temperature.[29] This is best accomplished by vasodilation of the blood vessels in the skin. This allows for the heated blood to pass close to the body surface and losing heat through radiation and conduction. The sweat glands is stimulated by the heated blood resulting in an increase in sweat production and thus losing more heat through evaporation. This evaporation of sweat leads to fluid and electrolyte loss, which can result in dehydration.[29]

    Dehydration may lead to impaired cognitive and exercise performance and heat stroke. Vasoconstriction happens at the viscera to maintain blood pressure in response to the fluid loss and redirection of blood to the skin.[29]

    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.[15] For example, changes in brain levels of serotonin and dopamine may influence fatigue.[40][41] In addition, the firing rate of motor units can change during fatigue [42], 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 [43]. 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.

    References

    1. Definition of BIOLOGY [Internet]. [cited 2016 May 31]. Available from: http://www.merriam-webster.com/dictionary/biology
    2. Branches of biology - Biology-Online Dictionary [Internet]. [cited 2016 May 31]. Available from: http://www.biology-online.org/dictionary/Branches_of_biology
    3. Physiology on Online Etymology Dictionary [Internet]. [cited 2016 May 31]. Available from: http://www.etymonline.com/index.php?term=physiology
    4. Tate P. Seeley’s Principles of Anatomy and Physiology. 2 edition. New York: McGraw-Hill Education; 2011. p 960
    5. Hackett D, Davies T, Soomro N, Halaki M. Olympic weightlifting training improves vertical jump height in sportspeople: a systematic review with meta-analysis. Br J Sports Med. 2016 Jul;50(14):865-72
    6. 6.0 6.1 Burton DA, Stokes K, Hall GM. Physiological effects of exercise. Contin Educ Anaesth Crit Care Pain. 2004 Jan 12;4(6):185–8.
    7. QuickTeach PE. Exercise Physiology Introduction and Overview. Published on Nov 8, 2015. Available from https://www.youtube.com/watch?v=7b7BiUguwsk
    8. iHealthTube.com. Defining Exercise Physiology. Published on Jul 9, 2010. Available from https://www.youtube.com/watch?v=gybE_nqVHUI&t=45s
    9. 9.0 9.1 9.2 9.3 Powers SK and Howley ET. Exercise Physiology: Theory and Application to Fitness and Performance.10th edition. North Ride, NSW, Australia. McGraw Hill. 2014
    10. Brooks G, Fahey TD, Baldwin KM.Exercise Physiology: Human Bioenergetics and its Applications. 4th Edition. Boston. McGraw Hill. 2005.
    11. Osiris Salazar.Published on Feb 7, 2018 The Energetics of Exercise. Available from https://www.youtube.com/watch?v=q-NWIDlUCuY&t=341s (last accessed 28 March 2019)
    12. Smarak Mishra.Exercise Physiology Lecture Part 1 . Available from: https://www.youtube.com/watch?v=V2snzIxnPJI [last accessed 3/4/2019]
    13. Smarak Mishra. Exercise Physiology Lecture Part 2. Available from: https://www.youtube.com/watch?v=BMzKVrwivhQ&t=11s [last accessed 2/4/2019]
    14. MacLaren DPM, Gibson H, Parry-Billings M & Edwards RHT. A review of metabolic and physiological factors in fatigue. Exercise and Sport Sciences Review, 1989;17:29–66.
    15. 15.0 15.1 Weir JP, Beck TW, Cramer, JT & Housh TJ. Is fatigue all in your head? A critical review of the central governor model. British Journal of Sports Medicine. 2006;40(7): 573–586.
    16. Gollnick PD & King D. Effects of exercise and training on mitochondria of rat skeletal muscle. American Journal of Physiology. 1969;216:1502–1509.
    17. Holloszy, J.. Effects of exercise on mitochondrial oxygen uptake and respiratory enzyme activity in skeletal muscle. Journal of Biological Chemistry. 1967;242:2278–2282.
    18. Tesch PA, Komi PV & Hakkinen K. Enzymatic adaptations consequent to long-term strength training. International Journal of Sports Medicine. 1987;8:66–69.
    19. Maughan RJ, Watson JS, Weir J. Strength and cross-sectional area of human skeletal muscle. J Physiol. 1983;338:37–49.
    20. 20.0 20.1 Hall JE. Guyton and Hall Textbook of Medical Physiology, 12e. 12th edition. Philadelphia, Pa: Saunders; 2010. p1120.
    21. 21.0 21.1 Foss ML, Keteyian SJ, Fox EL. Fox’s Physiological Basis for Exercise and Sport. 6th edition. Boston, Mass: William C Brown Pub; 1998. p 620
    22. Knuttgen HG. Neuromuscular mechanisms for therapeutic and conditioning exercise. University Park Press; 1976.
    23. Armando Hasudungan. "Myology - Skeletal Muscle Contraction". Available from: https://www.youtube.com/watch?v=Vs0tZV35_pw [last accessed 01/06/2016]
    24. Graham Johnson. "Muscle Contraction Overview Animation". Available from: https://www.youtube.com/watch?v=GneonFlcZG8 [last accessed 01/06/2016]
    25. 25.00 25.01 25.02 25.03 25.04 25.05 25.06 25.07 25.08 25.09 25.10 25.11 25.12 25.13 Rivera-Brown AM, Frontera MD. Principles of exercise physiology: Responses to acute exercise and long-term adaptations to training. PM&R. 2012; 4: 797-804.
    26. Clarkson HM. Musculoskeletal Assessment: Joint Range of Motion and Manual Muscle Strength. Lippincott Williams & Wilkins; 2000. p 455
    27. Murtaugh B, Ihm JM. Eccentric training for the treatment of tendinopathies. Curr Sports Med Rep. 2013;12(3):175-82
    28. Frizziero A, Vittadini F, Fusco A, Giombini A, Masiero S. Efficacy of eccentric exercise in lower limb tendinopathies in athletes. J Sports Med Phys Fitness. 2016;56(11):1352-1358.
    29. 29.00 29.01 29.02 29.03 29.04 29.05 29.06 29.07 29.08 29.09 29.10 29.11 29.12 29.13 29.14 29.15 29.16 29.17 29.18 29.19 29.20 29.21 29.22 29.23 Glynn, AJ, Fiddler H. Chapter 1: Introduction to Exercise Physiology in The physiotherapist’s pocket guide to exercise: assessment, prescription and training. Elsevier Health Sciences, 2009. p1 - 11
    30. 30.0 30.1 30.2 30.3 30.4 Frontera WR, Ochala J. Skeletal Muscle: A Brief Review of Structure and Function. Calcif Tissue Int. 2015;96: 183.
    31. Whats Up Dude.Energy Systems - ATP Energy In The Body - Adenosine Triphosphate - Glycolysis.Published on Jan 18, 2017. Available from https://www.youtube.com/watch?v=dWe8vtztW-4 (last accessed 13 June 2019)
    32. 32.0 32.1 32.2 32.3 32.4 Lavie CJ , Arena R , Swift DL , Johannsen NM , Sui X, Lee D, Earnest CP, Church TS, O’Keefe JS , Milani RV, and Blair SN. Exercise and the Cardiovascular System: Clinical Science and Cardiovascular Outcomes. Circulation Research. 2015;117:207–219.
    33. Leon, A. S., Franklin, B. A., Costa, F., Balady, G. J., Berra, K. A., Stewart, K. J., et al. (2005). Cardiac rehabilitation and secondary prevention of coronary heart disease: An American Heart Association scientific statement from the Council on Clinical Cardiology (subcommittee on exercise, cardiac rehabilitation, and prevention) and the Council on Nutrition, Physical Activity, and Metabolism (subcommittee on physical activity), in collaboration with the American Association of Cardiovascular and Pulmonary Rehabilitation. Circulation, 111(3), 369–376.
    34. 34.0 34.1 McArdle WD, Katch FI, Katch VL. Essentials of exercise physiology. Lippincott Williams & Wilkins; 2006.
    35. Mike Tyler.Anatomy & Physiology | Adaptations to Exercise: The Cardiovascular System. Published 16 January 2018. Available from: https://www.youtube.com/watch?v=2JyropRjCkU (last accessed 2 May 2019)
    36. Mike Tyler.A&P Respiratory System 06 - Responses to Exercise. Published 9 January 2018.Available from: https://www.youtube.com/watch?v=dH9K738oOGI (last accessed 2 May 2019)
    37. Mike Tyler.A&P Respiratory System 07 - Adaptations to Exercise. Published 16 January 2018.Available from: https://www.youtube.com/watch?v=2JyropRjCkU&t=276s (last accessed 2 May 2019)
    38. McArdle, WD., Katch FL, & Katch VL. Exercise physiology: Energy, nutrition and human performance (5th ed.). Philadelphia: Lippincott Williams & Wilkins.2007.
    39. Housh TJ, Housh DJ & de Vries HA. Applied exercise and sport physiology (3rd ed.). Scottsdale, AZ: Holcomb Hathaway. 2012.
    40. Blomstrand E. A role for branched-chain amino acids in reducing central fatigue. Journal of Nutrition, 2006;136(2):544S–547S.
    41. Davis JM., Alderson NL & Welsh RS. Serotonin and central nervous system fatigue: Nutritional considerations. American Journal of Clinical Nutrition. 2000;72(2 Suppl):573S–578S.
    42. Rubinstein S & Kamen G. Decreases in motor unit firing rate during sustained maximal-effort contractions in young and older adults. Journal of Electromyography and Kinesiology. 2005;15(6):536–543.
    43. Gabriel DA, Kamen G & Frost G. Neural adaptations to resistive exercise: Mechanisms and recommendations for training practices. Sports Medicine. 2006;36(2):133–149.