Chapter 7: Physiology of Exercise

By William Armstrong, PhD

Learning Objectives

• Describe and analyze the fundamental principles of physiology and exercise physiology, including how bodily systems interact to maintain homeostasis and adapt to physical exercise.

• Explain the concept of pathophysiology and how it differs from normal physiological processes.

• Describe key factors involved in the development and implementation of exercise programs.

Chapter Content

Please watch the following video that provides an introduction to the field of Exercise Physiology.

Below is the transcript from the video if you’d like to be able to read along.

Introduction to Exercise Physiology

Physiology is the scientific study of the normal functions and mechanisms that occur within living organisms. It is a branch of biology that focuses on understanding how various bodily systems, organs, tissues, and cells work together to maintain homeostasis, which is the stable internal environment required for an organism’s survival.

Physiologists seek to uncover the underlying principles that govern the processes of life, including processes such as metabolism, growth, reproduction, movement, and response to stimuli. They investigate the interactions between different systems and how they contribute to the overall functioning of an organism. This field is closely related to anatomy, which deals with the structure of organisms, as the two disciplines often intersect in studying how form and function are interconnected.

Physiology covers a wide range of topics, from the molecular and cellular level to the level of whole organisms. It can be applied to understanding human health and disease, as well as the functioning of other animals and even plants. This knowledge helps medical professionals diagnose and treat various health conditions and allows scientists to develop a deeper understanding of life processes and potential interventions.

Exercise physiology, or what is sometimes referred to as “sport physiology” is a specialized branch of physiology that focuses on how the body responds and adapts to physical exercise and physical activity. It explores the intricate physiological processes that occur in the body during exercise, ranging from the molecular and cellular level to the level of organ systems. Exercise physiologists study how various systems, such as the cardiovascular, respiratory, muscular, and metabolic systems, interact and adapt to different types and intensities of exercise. Both “Exercise” and “Sport” are somewhat misnomers because, in reality, exercise physiology is “work physiology” that is the physiological responses to the human organism under a variety of loads greater than the resting physiology.

Key areas of study within exercise physiology include the cardiorespiratory responses, muscular adaptations, metabolic responses, thermoregulation, endocrinology, immune system responses, training adaptations, performance enhancement, and a variety of clinical adaptations.

The cardiovascular response involves understanding how the heart and blood vessels respond to exercise. It includes examining factors like heart rate, stroke volume, cardiac output, and blood pressure during exercise.

Exercise impacts breathing patterns and oxygen exchange in the lungs. Exercise physiologists, then, study how the respiratory system adjusts to meet the increased oxygen demand during physical activity.

Collectively, we often refer to these as the cardiorespiratory response of cardiorespiratory exercise physiology.

Exercise triggers various changes in skeletal muscles, such as increased blood flow, energy production, and muscle fiber recruitment. Researchers explore how muscles adapt to different types of exercise, including resistance training and endurance activities.

Exercise alters energy metabolism to provide the body with the energy needed for physical activity. This includes studying processes like glycolysis, aerobic and anaerobic metabolism, and the utilization of nutrients for energy production.

Exercise generates heat, and the body needs to regulate its temperature to prevent overheating. Exercise physiologists study how the body maintains its temperature during exercise and how environmental factors impact this regulation.

Exercise influences the release of hormones, such as adrenaline, insulin, and cortisol. Researchers in this field investigate how hormonal responses affect energy utilization, muscle growth, and overall health.

Exercise has a profound effect on the health and function of the immune system. In Physiology of Exercise, we discuss topics like Neiman’s ‘J’ and the “open window” theory.

Exercise physiology explores how the body adapts to different training programs over time. This includes studying concepts like aerobic capacity, strength gains, and muscle hypertrophy.

Exercise physiologists often work with athletes to optimize their training regimens and performance. They may use their knowledge to design personalized training plans and strategies for improving athletic performance.

Exercise physiology is also relevant to clinical settings. It can be used to design exercise interventions for individuals with chronic diseases, such as diabetes, cardiovascular disease, and obesity, to improve their overall health and well-being. Many exercise science students go on to apply the principles of exercise physiology in clinical careers such as cardiac and pulmonary rehabilitation, gerontology, physical and occupational therapy, and medicine.

Overall, exercise physiology plays a crucial role in advancing our understanding of how the human body responds to physical activity, contributing to the development of effective exercise strategies for both health promotion and performance enhancement.

While physiology and exercise physiology examine the normal responses to the stresses placed on the body systems, pathophysiology is the study of how normal physiological processes are disrupted or altered in disease or injury. It explores the mechanisms and underlying causes of various diseases and conditions, focusing on the functional changes that occur at the cellular, tissue, and organ levels. Pathophysiology aims to understand how these disruptions lead to the signs, symptoms, and clinical manifestations of different diseases.

In essence, pathophysiology bridges the gap between basic physiology (the study of normal bodily functions) and clinical medicine. By understanding the underlying mechanisms of disease, healthcare professionals can make informed decisions about diagnosis, treatment, and management of patients.

The terms, pathology and pathophysiology, both refer to the study of disease. Pathology is a broad term dealing with ALL aspects of disease. Pathophysiology, on the other hand, focuses on the ABNORMAL functioning of diseased organs and organ systems with applications to diagnosis and patient care.

Key aspects of pathophysiology include the etiology and pathogenesis of disease. In Pathophysiology and Exercise, the emphasis is on understanding the etiology and pathogenesis of a range of diseases and on examining the role of exercise in the prevention and treatment of these diseases.

Etiology involves studying the causes of diseases and conditions. Etiological factors can include genetic mutations, infections, environmental factors, lifestyle choices, and more.

Pathogenesis refers to the sequence of events that occur from the initial cause of a disease to the development of its clinical manifestations. It explores the mechanisms by which a disease progresses and the cellular and molecular changes that occur.

Pathophysiology delves into the cellular and molecular alterations that occur in diseased states. This includes changes in cell structure, function, and signaling pathways.

Diseases often affect specific organ systems or multiple systems. Pathophysiology explains how dysfunction in various organ systems contributes to the overall disease process.

Pathophysiology helps explain the signs and symptoms observed in patients with specific diseases. It connects the underlying changes at the cellular and molecular levels to the observable effects on the patient.

A solid understanding of pathophysiology is essential for accurate diagnosis and effective treatment. Medical professionals use this knowledge to develop targeted therapies and interventions that address the underlying causes of disease.

Pathophysiological insights drive medical research and the development of new treatments. Researchers may uncover novel therapeutic targets based on their understanding of the disease mechanisms.

Pathophysiology is a multidisciplinary field that draws from various branches of science, including physiology, biochemistry, genetics, immunology, and pharmacology. It’s a crucial foundation for healthcare professionals, including doctors, nurses, pharmacists, and researchers, as they work to prevent, diagnose, and treat a wide range of diseases and conditions. Moreover, the exercise scientist—even those working with the athletic population—is likely to encounter pathologies that will impact the exercise prescription.

As Exercise Scientists—Exercise Physiologists—we are most interested in increasing performance, whether for sport, health, or just our recreational activities and activities of daily living.

The flowchart presented here summarizes the Factors Affecting Performance:

All forms of energy are interchangeable. For example, exercise requires that chemical energy be converted to mechanical energy, for example, muscle contraction.

What chemical compound is considered the energy currency for muscle contraction??

Hopefully, you remember from Anatomy & Physiology that this is adenosine triphosphate or ATP.

The required energy for muscle contraction comes from the food we eat in the form of carbohydrates, fats, and proteins. Carbohydrates are the preferred sources of energy for synthesis of ATP. The processes by which ATP is produced from carbohydrates, fats, and, to a lesser degree, proteins are discussed in Physiology of Exercise and Sports Nutrition.

In exercise physiology, the human body relies on three primary bioenergetic systems to provide the energy needed for various levels of physical activity. These systems involve different metabolic pathways and energy sources. The three bioenergy systems are the Phosphagen or ATP-PC System, the Glycolytic System, and the Oxidative System.

The ATP-PC system is the immediate source of energy for short bursts of intense activity. It relies on stored high-energy phosphate compounds, primarily adenosine triphosphate and creatine phosphate in the muscles. When muscles contract, ATP is broken down into adenosine diphosphate and inorganic phosphate, releasing energy. The phosphagen system provides rapid and powerful energy but is limited in duration, lasting only a few seconds. It is heavily used in activities like sprinting, weightlifting, and jumping.

The glycolytic system generates energy through the breakdown of glucose or glycogen in the absence of oxygen. This system provides energy for activities that are of moderate intensity and duration, such as high-intensity weightlifting sets, shorter sprints, and other activities lasting up to a couple of minutes. Glycolysis involves the conversion of glucose or glycogen into pyruvate, which can further be converted to lactate if oxygen is limited. While this system produces energy relatively quickly, it also produces lactate, which can lead to muscle fatigue and discomfort.

The oxidative system produces energy through aerobic metabolism, which relies on oxygen to generate energy from carbohydrates, fats, and, to a lesser extent, proteins. This system is used for activities of lower intensity but longer duration, such as jogging, cycling, and endurance events. Aerobic metabolism takes place in the mitochondria of cells and provides a steady supply of energy over an extended period. While it is slower to produce energy compared to the other systems, it is highly efficient and can support activities for hours, even days.

These three bioenergy systems work together to provide the energy required for various types of physical activities. The systems utilized during exercise depend on factors such as the intensity, duration, and type of activity being performed. For example, a sprinter relies heavily on the phosphagen system, while a marathon runner primarily uses the oxidative system. During most activities, a combination of these systems contributes to meeting the energy demands of the body.

The ATP-PC system provides the immediate source of ATP for short-term, near maximal, muscle contractions.

Adenosine diphosphate receives an inorganic phosphate from phosphocreatine in a reaction that requires the enzyme creatine kinase.

We will begin with carbohydrate metabolism, because glucose is the primary source of energy for ATP synthesis in skeletal muscle contractions.

CHOs break down to CO₂ and water in the process of producing ATP.

One glucose molecule will produce a net gain of 32 ATP.

Glycolysis occurs in the cytosol of the muscle cell.
We will walk through the process in class, but, in short, glycolysis converts a glucose molecule to 2 molecules of pyruvate or 2 molecules of lactate.

As we will see, there are two phases to glycolysis:

The energy investment phase requires 2 ATP, and

the energy generation phase produces 4 ATP, 2 NADH, and 2 pyruvate or 2 lactate.

The net reaction of glycolysis is:

C₆H₁₂O₆ + 2 NAD + 2 ADP + 2 Pi → 2 pyruvate + 2 NADH + 2 ATP

Aerobic production of ATP occurs inside the mitochondria and involves the interaction of two cooperative metabolic pathways:

Tricarboxcylic Acid Cycle (or the TCA cycle; Note: the text refers to the Krebs Cycle, I much prefer TCA, which is the preferred biochemistry term), and the

Electron transport chain

The primary function of the TCA cycle is the oxidation of CHO, fats and proteins using NADH and FADH as hydrogen carriers.

In the electron transport chain H⁺ from NADH and FADH are accepted by O₂ to form water via oxidative phosphorylation

In the Electron Transport Chain, ATP is formed during the phosphorylation of NADH and FADH. Pairs of electrons are passed down a series of compounds that undergo oxidation and reduction.

Two or three ATP are formed per electron pair, depending at what point the coenzyme enters the electron transport chain—NADH ⇒ 2.5; FADH ⇒ 1.5.

The electron transport chain results in pumping of H⁺ ions across inner mitochondrial membrane, which results in H⁺ gradient across membrane. Energy released to form ATP as H⁺ diffuse back across the membrane.

Adipose triglycerides are broken down and released as free fatty acids and glycerol.

Free fatty acids are taken up by the cells and broken down into acetyl CoA through a process known as “β-Oxidation.”

In β–Oxidation, free fatty acid molecules are broken down into 2-C fragments; each step generates molecules of acetyl CoA, NADH, and FADH₂

The interaction between aerobic and anaerobic energy metabolism during exercise is not discrete. Rather, it acts as more of a continuum.

These figures are helpful to visualize how the aerobic/anaerobic continuum apply to sports and the exercise prescription.

The exercise continuum extends from low-intensity physical activity to low-intensity cardiorespiratory exercise to maximal effort.

The foundation of this is developed in courses such as Physiology of Exercise.

The following slides will give you a taste of what is learned in Physiology of Exercise.

The muscle sarcomere is the basic functional unit of a muscle fiber or muscle cell. It is responsible for muscle contraction and is composed of highly organized structures that allow muscle fibers to contract efficiently. Sarcomeres are found in both skeletal and cardiac muscle tissues.

A sarcomere is delimited by two Z-discs (also called Z-lines), which are dense protein structures that anchor the thin filaments. Within the sarcomere, there are two main types of protein filaments:

Thin, Actin Filaments are composed primarily of the protein actin. Actin filaments extend from the Z-disc toward the center of the sarcomere. They are attached to the Z-disc and are interspersed between the thicker myosin filaments.

Thick, Myosin Filaments are composed of the protein myosin. Myosin filaments are located in the center of the sarcomere, overlapping with the actin filaments. The myosin heads protrude from the thick filaments and interact with the actin filaments during muscle contraction.

The interaction between these thin and thick filaments during muscle contraction follows the sliding filament theory. When a muscle contracts, the myosin heads bind to the actin filaments and pull them toward the center of the sarcomere, causing the sarcomere to shorten. This overall shortening of sarcomeres across a muscle fiber leads to muscle contraction.

The sarcomere structure also includes other proteins such as troponin and tropomyosin, which play crucial roles in regulating the interaction between actin and myosin. Calcium ions play a pivotal role in triggering the interaction by binding to troponin and allowing the myosin heads to attach to the actin filaments.

Understanding the sarcomere’s structure and the interactions between its components is essential for comprehending the mechanics of muscle contraction, as well as various muscular diseases and conditions.

A motor unit is a functional unit of the neuromuscular system that consists of a motor neuron and all the muscle fibers it innervates (stimulates). Motor units play a crucial role in enabling voluntary muscle movements.

 

The excitation-contraction coupling refers to the series of events that occur in skeletal muscle cells when an action potential is transmitted along the motor neuron and leads to the contraction of the muscle fiber. This process involves the coordinated interaction between the nervous system and the muscle cells, enabling the conversion of an electrical signal into a mechanical contraction.

The process begins when a motor neuron releases the neurotransmitter acetylcholine at the neuromuscular junction, which is the point of contact between the motor neuron and the muscle fiber. Acetylcholine binds to receptors on the muscle fiber’s membrane (sarcolemma) and triggers an action potential, an electrical signal that travels along the muscle cell’s membrane.

The action potential propagates deep into the muscle fiber through structures called transverse tubules or T-tubules. T-tubules are invaginations of the sarcolemma that allow the electrical signal to reach the interior of the muscle fiber.

Surrounding the myofibrils within the muscle fiber is the sarcoplasmic reticulum, a specialized network of membrane-bound sacs that stores calcium ions. When the action potential reaches the T-tubules, it triggers the release of calcium from the sarcoplasmic reticulum into the cytoplasm of the muscle fiber.

The released calcium binds to troponin, causing a conformational change in troponin, which in turn causes tropomyosin to shift its position. Tropomyosin normally covers the binding sites on the actin filaments where myosin heads would attach.

With the binding sites on actin exposed, myosin heads bind to actin, forming cross-bridges. These cross-bridges are the structural basis for muscle contraction. The energy for this interaction comes from the hydrolysis of ATP to ADP and inorganic phosphate by the myosin heads.

Upon binding, the myosin heads undergo a conformational change that results in the myosin heads pulling the actin filaments toward the center of the sarcomere. This is known as the power stroke.

The myosin heads need to detach from actin to relax the muscle. ATP binds to the myosin heads, causing them to detach from actin. The ATP is then hydrolyzed to provide energy for the myosin heads to return to their initial position.

Muscle relaxation occurs when the action potential ceases and calcium is actively pumped back into the sarcoplasmic reticulum by calcium pumps. As calcium is removed from troponin, tropomyosin returns to its blocking position, preventing further cross-bridge formation.

The excitation-contraction coupling ensures that muscle contraction occurs in response to neural signaling, and the process is finely regulated to provide the necessary force and control for various movements.

The Sliding Filament Theory is a widely accepted explanation for how muscles contract at the molecular level. Proposed by Andrew Huxley and Hugh Huxley in the mid-20th century, this theory provides a comprehensive explanation of how the interactions between actin and myosin filaments lead to muscle contraction.

The Sliding Filament Theory explains how muscle contraction occurs on a molecular level. It accounts for the coordinated interaction between actin and myosin filaments, driven by the binding and hydrolysis of ATP, as well as the regulation of calcium ions and troponin-tropomyosin complexes.

Historically, there have been two classes of muscle fiber: Fast v. slow or white v. red

Physiologically, we classify fibers as fast or slow twitch, as fast-fatiguing or slow-fatiguing, or as glycolytic or oxidative. As such there are three classification of fiber types in human skeletal muscle.

Type IIx fibers as considered fast-twitch fibers or fast-glycolytic fibers.

Type IIa fibers are the intermediate fibers or fast-oxidative glycolytic fibers.

Type I fibers are slow-twitch fibers and labeled as slow-oxidative fibers.

Non-athletes have about 50% slow and 50% fast fibers; power athletes (e.g., sprinters) have a Higher percentage of fast fibers; and endurance athletes (e.g., distance runners) have higher percentage of slow fibers.

The Force-Velocity Relationship will be important as we consider strength and power training for sport. The maximum velocity of shortening is greatest at the lowest force. Velocity decreases as the required force increase. Notice that the curve crosses the velocity line at what is referred to as the “maximal isometric contraction.” Any greater resistance load on the muscle will result in an eccentric contraction—where the muscle lengthens under load.

Note that at any absolute force the speed of movement is greater in muscle with higher percent of fast-twitch fibers.

Purposes of the cardiorespiratory system include the transport O₂ and nutrients to tissues, the removal of CO₂ wastes from tissues, and the regulation of body temperature.

Understanding the events of the cardiac cycle and the electrical activities that control heart function are important to understanding the electrocardiograph (ECG) and how it may be interpreted to diagnose dysrhythmias. The normal ECG represents the normal conduction of the heart during the cardiac cycle.

The sinoatrial (SA) node, located in the anterolateral wall or the right atrium initiates the cardiac cycle. The atrial depolarization (represented by the P-wave) spreads through the atria to the atrioventricular (AV) node where it is delayed momentarily before passing to the bundle of His (P-R interval). The depolarization passes from the bundle of His to the right and left bundle branches depolarizing the septal wall and the walls of the ventricles. The bundle branches extend into subendocardial branches (Purkinje fibers). This depolarization of the ventricles (and subsequent repolarization of the atria) produces the QRS complex. The period of iso-electrical activity following ventricular contraction generates the ST segment, and the T-wave represents the ventricular repolarization. Deviations from the normal ECG may indicate disease or dysfunction of the heart muscle.

There are two ways for the cardiovascular system to adjust blood flow during exercise. This includes increasing cardiac output and redistribution of blood flow. Recall that cardiac output is the volume of blood pumped in a minute or the product of heart rate and stroke volume. Blood redistribution is controlled by vasoconstriction or vasodilation of blood vessels (specifically of the arterioles) to the tissues and organs. Disruption of these functions affects performance and, hence, is pathological. The purpose of cardiorespiratory exercise is to improve function.

The Fick equation explains VO_2max which the measure of cardiorespiratory performance.

According to the Fick equation, maximal oxygen uptake is defined by the body’s ability to deliver oxygen to the tissue (that is, the muscle) and the ability of the tissue to utilize oxygen. Delivery is determined by the cardiac output—the amount of blood that is circulated in a minute’s time. Cardiac output is the product of the heart rate and the stroke volume, or the amount of blood pumped with each beat of the heart. Oxygen uptake is demonstrated by the arteriovenous oxygen uptake—the difference in blood oxygen content as the blood passes through the tissue from the arterial side of the capillary to the venous side. Thus, VO_2max is the product of the heart rate times the stroke volume times the a-vO₂ difference.

Heart rate is controlled by the parasympathetic and sympathetic nervous systems, and exercise training will generally lead to a decreased heart rate at any given exercise intensity—thereby making the heart muscle more efficient. The increase in blood flow or cardiac output to the muscle then is the result of an increase in stroke volume. Exercise also increases the capacity of the muscle for utilizing oxygen to provide the ATP necessary for muscle contraction. Thus, increases in maximal stroke volume and a-vO₂ difference max are the necessary adaptations that improve cardiorespiratory performance.

Here we see a summary of the regulation of cardiovascular adjustments to exercise…

and how they are controlled.

The term respiration can have two definitions in physiology. These are divided into two separate but related subdivisions: pulmonary respiration and cellular respiration. Pulmonary respiration, or ventilation, refers to the exchange of gasses in the lungs. Cellular respiration refers to the exchange of gasses at the cellular level.

The respiratory system serves two purposes during exercise: the exchange of gasses between the environment and the body and the regulation of acid-base balance during exercise. The lungs are a means of gas exchange between the external environment and the body. Ventilation is the mechanical process of moving air into and out of the lungs. The movement of air occurs via bulk flow and the movement of molecules due to pressure differences. Inspiration and expiration follow Boyle’s Law, which states that pressure and volume are inversely related.

Like blood flow, airflow depends on the pressure difference between two ends of airway and the resistance of the airways. Airway resistance depends on the diameter and is thus a factor in chronic obstructive lung disease, asthma, and exercise-induced asthma.

Pulmonary ventilation refers to the amount of air moved in or out of the lungs per minute. Ventilation is the product of tidal volume and breathing frequency. It can also be expressed as the sum of the alveolar ventilation and dead space ventilation. Alveolar ventilation refers to the volume of gas that arrives at the alveoli and areas of gas exchange with the pulmonary capillaries. Dead space ventilation refers to the air that remains in the lungs and does not affect the blood oxygenation. Disruptions of these thus affect pulmonary function.

Cardiorespiratory endurance refers to the ability of the heart, lungs, and blood vessels to supply oxygen and nutrients to the muscles to perform physical activity without undue fatigue. It is measured as

and can be assessed by the exercise scientist using a variety of field tests or laboratory equipment. Our understanding of the Fick equation can provide insight into how

is improved and how this affects performance. In response to endurance training, parasympathetic innervation to the heart increase, thereby decreasing the heart rate at any given work intensity and increasing the work that can be performed at the maximum heart rate. The effect of exercise is greatest on stroke volume and avO₂diff—roughly split between the two. Stroke volume increases owing to increased contractility and reduced peripheral resistance. It is increased because of an increased end-diastolic volume resulting from an increased venous return and an increased filling time. Increased plasma volume also facilitates an increase in end-diastolic volume and stroke volume. Arterio-venous oxygen difference increases because of increased capillarization and the mitochondrial density of the muscle, as well as other positive metabolic changes within the tissue cells.

Strength training also benefits heart function by improving posture and the strength of the respiratory muscles and accessory respiratory muscles. Strength training also permits a greater physical capacity to do work which greatly affects one’s capacity to perform activities that will improve and/or maintain cardiorespiratory endurance.

Exercise also benefits vascular health and body composition to reduce the load on the cardiovascular and respiratory systems. Regular exercise can significantly affect the risk and prognosis of many pathologies that will be discussed in Pathophysiology.

The purpose of exercise training is to improve one’s health, appearance, and physical performance.

The understanding of physiology and how the body responds and adapts to the stresses placed on its systems is the purpose of courses such as Physiology of Exercise.

When considering exercise programming one must consider the goals of the individual. What are we trying to accomplish. As such, there are several underlying principles to consider. The so-called “Principles of Adaptation” include specificity, overload, progression, reversibility, and individuality.

Specificity is sometimes called the “SAID Principle” because its definitions is stated as: “the body makes Specific Adaptations to Imposed Demands.”

For a body system to adapt, it must be stressed to a level greater than that to which it is accustomed. This is the “overload principle.”

Overload must be systematic. In other words, increases must be made in the training load to stimulate continued progress. This is the principle of progression. Often, we refer to the principle of “Progressive Overload.”

Unfortunately, if the exercise stress is removed, progress will be lost in a short period of time. This is the “principle of reversibility”—use it or lose it.

Often neglected in the exercise prescription is the principle of individuality. It is important to understand that everyone is different and will have different responses to the exercise stimulus. People have different goals and preferences. In addition, as is discussed in Physiology of Exercise, there are responders and non-responders when it comes to exercise.

When considering exercise, we divide the components of physical fitness into two broad categories: health-related and neuromotor skill-related. The health-related components of physical fitness include cardiorespiratory endurance, muscle endurance, muscle strength, flexibility, and body composition. The neuromotor skill-related components of physical fitness are speed, power, agility, balance, coordination, and reaction time.

In general, these apply to everyone regardless of age, gender, and physical ability. Through an understanding of exercise physiology, we refine the exercise prescription to suit the individual. Being able to adapt the exercise to meet the needs of the individual is where exercise is both a science and an art.