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Table of Contents Basic Anatomy and Physiology Anatomical Position and Planes of Movement Skeletal System Muscular System Energy System Nervous System Cardiovascular System Respiratory System Body System Responses to Physical Activity Conclusion Heart Rate, Stroke Volume and Blood Pressure Response...

Table of Contents Basic Anatomy and Physiology Anatomical Position and Planes of Movement Skeletal System Muscular System Energy System Nervous System Cardiovascular System Respiratory System Body System Responses to Physical Activity Conclusion Heart Rate, Stroke Volume and Blood Pressure Responses to Aerobic Activity References Canadian Society For Exercise Physiology Physical Activity Training For Health (CSEP-PATH ®) Copyright © 2013, 2019, 2021 Canadian Society for Exercise Physiology. CSEP-PATH® is a Registered Trademark of the Canadian Society for Exercise Physiology (CSEP). All rights reserved. Except for use in a review, the reproduction or utilization of this work in any form by any electronic, mechanical, or other means, now known or hereafter invented, including xerography, photocopying, and recording, and in any information storage and retrieval system, is forbidden without the written permission of the publisher. Notice: permission to print out and photocopy the CSEP-PATH® Tools (print and electronic formats) is permitted by users of the Canadian Society for Exercise Physiology – Physical Activity Training for Health (CSEP-PATH®) Resource Manual. Canadian Society for Exercise Physiology 101-495 Richmond Rd | Ottawa ON K2A 4B1 | Canada 1.877.651.3755 | [email protected] | csep.ca | @CSEPdotCA ISBN: 978-1-896900-60-5 Previous ISBNs CSEP-PATH® First Edition 978-1-896900-32-2 (2013), First Refreshed Edition: 978-1-896900-40-7 (2018), Second Edition: 978-1-896900-46-9 (2019) Printed in Canada BASIC ANATOMY AND PHYSIOLOGY A solid understanding of anatomy and the effects of physical activity on the body’s various systems is essential to prescribing safe and effective physical activity programs. This section provides an overview of key principles of anatomy and physiology that are relevant to quali ed exercise professionals. It is not intended to be comprehensive, but to provide a general refresher of subject matter that quali ed exercise professionals are expected to have studied extensively through other post-secondary coursework. As much as possible, it is written in practical terms to provide a usable script to draw upon when explaining the rationale and processes involved in program recommendations for clients. KEY CONCEPTS Anatomical position and planes of movement Skeletal system Muscular system Energy systems Nervous system Cardiovascular system Respiratory system Body system responses to physical activity Anatomical Position and Planes of Movement The anatomical position is the standard reference position used to describe the anatomy of the human body. In it, the body is assumed to be standing upright, with feet together, arms to the side, with the head, eyes and palms of the hands facing forward. Human movement is typically described using the planes of movement, which divide the body into dimensions that pass through the body (Figure 1.1). The sagittal plane (or medial plane) divides the body along the midline of the body into the left and right sides. The term medial refers to the inner side (toward or at the midline), while lateral refers to the outer side (away from the midline). The frontal plane (or coronal plane) separates the body into the anterior (front of the body) and posterior (back of the body) portions. The transverse plane (or horizontal plane) separates the body into the superior (toward the head or upper body) and inferior (away from the head or lower body). When describing relative positions of the body parts, the terms proximal or closer to the origin of reference (e.g., the elbow is proximal to the wrist) and distal or further away from the origin of reference (e.g., the foot is distal to the knee) are also commonly used terms (Figure 1.1). FIGURE 1.1 Anatomical Position Figure 1.2 provides images to assist with visualizing some of the following anatomical planes. FIGURE 1.2 Anatomical Planes Abduction Adduction External Internal rotation t ti Dorsi Plantar Eversion Inversion exion i Arm exion Circumdu Flexi Arm t i Extens Supination Pronation  Flexion and extension: Decreasing and increasing angle with the frontal plane. Abduction and adduction: Moving away from or toward the sagittal plane. Protraction and retraction: Moving forward or backward along a surface. Elevation and depression: Raising and lowering something. Medial and lateral rotation: Movement inwards and outwards around the midline of the body. Supination or pronation: Lying face up or face down. It is common to see movement described as happening in a particular dominant plane (e.g., walking might be described as happening in the sagittal plane) (Table 1.1). This is only a gross approximation, however, as movement at the individual joint level is most often happening in several planes simultaneously. During walking or running, for example, the hip will be exing and extending in the sagittal plane, abducting and adducting in the frontal plane, and rotating medially and laterally in the transverse plane.  TABLE 1.1 Planes and Axis of Movement PLANE MOTION AXIS EXAMPLES Sagittal Flexion/extension Frontal Walking, squatting, overhead press Frontal Abduction/adduction Sagittal Jumping jack Side exion Lateral arm raise Transverse Internal/external Vertical Throwing rotation Baseball swing Horizontal Golf swing exion/extension Supination/pronation Skeletal System Part of the musculoskeletal system which provides for form, structure and movement of the body, the skeletal system (Figure 1.3) includes all of the bones and joints in the body that serve as a framework for tissues and organs, and acts as a protective structure for vital organs (e.g., the brain is protected by the skull and the lungs are protected by the rib cage). It also provides attachment points for muscles to allow movements at the joints. New blood cells are produced by the red bone marrow inside of our bones. Bones are complex living organs that act as the body’s warehouse for calcium, iron, and energy in the form of fat. FIGURE 1.3 Skeletal Structure – Anterior & Posterior Views (Copyright © 2019, Wolters Kluwer Health. All rights reserved.) Note: A full-sized version of this image is included in the online CSEP-PATH® Toolkit. Skeletal System Arrangement The skeletal system in an adult body is made up of 206 individual bones that are arranged into two major divisions called the axial and appendicular skeleton. The axial skeleton runs along the body’s midline axis and is made up of 80 bones in the skull, hyoid, auditory ossicles, ribs and sternum, vertebral column. The appendicular skeleton is made up of 126 bones in the upper limbs, lower limbs, pelvic girdle and shoulder girdle. Every bone in the body is classi ed as one of ve types (long, short, at, irregular, and sesamoid). Long bones are longer than they are wide and are the major bones of the limbs. They grow more than the other classes of bone throughout childhood and so are responsible for the bulk of our height as adults. A hollow medullary cavity is found in the center of long bones and serves as a storage area for bone marrow. Examples of long bones include the femur, tibia, bula, metatarsals, and phalanges. Short bones are about as long as they are wide and are often cubed or round in shape. The carpal bones of the wrist and the tarsal bones of the foot are examples of short bones. Flat bones vary greatly in size and shape, but have the common feature of being very thin in one direction. Because they are thin, at bones do not have a medullary cavity like the long bones. The frontal, parietal, and occipital bones of the cranium, as well as the rib and hip bones, are all examples of at bones. Irregular bones have a shape that does not t the pattern of the long, short, or at bones. The vertebrae, sacrum, and coccyx of the spine are all irregular bones. The sesamoid bones are formed after birth inside of tendons that run across joints and grow to protect tendons from stresses and strains at the joint and can help to give a mechanical advantage to muscles pulling on the tendon. The patella and the pisiform bones of the carpals are examples. Joints A joint (or articulation) is a point of contact between bones, between a bone and cartilage, or between a bone and a tooth. Synovial joints are the most common type of articulation and feature a small gap between the bones. This gap allows a free range of motion and space for synovial uid to lubricate the joint. Fibrous joints exist where bones are very tightly joined and o er little to no movement between the bones. Fibrous joints also hold teeth in their bony sockets. Finally, cartilaginous joints are formed where bone meets cartilage or where there is a layer of cartilage between two bones. These joints provide a small amount of exibility in the joint due to the gel-like consistency of cartilage. Muscular System The muscular system (Figure 1.4) is largely responsible for the movement of the human body. There are about 700 named muscles that make up roughly half of a person’s body weight. Each of these muscles is a discrete organ constructed of muscle tissue, blood vessels, tendons, and nerves. Muscle tissue is also found inside the heart, digestive organs, and blood vessels. The body has three major types of muscles – cardiac, smooth and skeletal. Cardiac muscles are responsible for contraction of the heart. Smooth muscles are responsible for many involuntary bodily functions, including the movement of food through the digestive system and the enlargement and contraction of blood vessels. Both cardiac and smooth muscles are involuntary muscles because they are controlled by the body’s central nervous system. The skeletal muscles are the muscles that attach to bones (by tough connective tissue called tendons) and are voluntarily activated to produce movement. Skeletal Muscle Classi cation Skeletal muscles are named based on many di erent factors, including their location, origin and insertion, shape, size, direction, and function. Some muscles derive their names from their anatomical location, such as the rectus abdominis and transverse abdominis as each are located in the abdominal region. Others are named after the part of the bone to which they are attached (e.g., the tibialis anterior is attached to the anterior portion of the tibia). Still others are a hybrid of these two (e.g., the brachioradialis is named after its location in the brachial region and its attachment to the radius bone). Some muscles are named based on their connection to a stationary bone (origin) and a moving bone (insertion). Examples include the sternocleidomastoid (connecting the sternum and clavicle to the mastoid process of the skull) and the occipitofrontalis (connecting the occipital bone to the frontal bone). Where muscles connect to more than one bone or to more than one place on a bone it is often re ected in the name (e.g., the biceps have two origins, triceps have three, and quadriceps have four). Muscles can also be classi ed by shape. For example, the deltoids have a delta or triangular shape. The serratus muscles feature a serrated or saw-like shape. The rhomboid major is a rhombus or diamond shape. The size of the muscle can be used to distinguish between two muscles found in the same region. The gluteal region contains three muscles di erentiated by size – the gluteus maximus (large), gluteus medius (medium) and gluteus minimus (smallest). The direction in which the muscle bres run can also be used to identify a muscle. In the abdominal region, there are several sets of wide, at muscles. The muscles whose bres run straight up and down are the rectus abdominis, those running transversely (left to right) are the transverse abdominis, and the ones running at an angle are the obliques. Finally, muscles can also be classi ed by the type of function that they perform. Most of the muscles of the forearm are named based on their function because they are located in the same region and have similar shapes and sizes. For example, the exor group of the forearm exes the wrist and the ngers. The supinator is a muscle that supinates the wrist by rolling it over to face palm up. The adductors in the legs adduct or pull the legs together. FIGURE 1.4 Muscular System – Super cial Muscles (Copyright © 2019, Wolters Kluwer Health. All rights reserved.) Note: A full-sized version of this image is included in the online CSEP-PATH® Toolkit. Skeletal Muscle Cellular Composition and Action Skeletal muscle bres di er dramatically from other tissues of the body due to their highly specialized functions. The sarcolemma is the cell membrane of muscle bres and acts as a conductor for the electrochemical signals that stimulate muscle cells. Connected to the sarcolemma are transverse tubules that help carry these electrochemical signals into the middle of the muscle bre. The sarcoplasmic reticulum serves as a storage facility for calcium ions (Ca2+) that are vital to muscle contraction. Mitochondria are considered the ‘power houses’ of the cell as they produce ATP to fuel muscle contraction. Most of the muscle bre’s structure is made up of myo brils, which are the contractile structures of the cell, and are made up of many protein bres arranged into repeating subunits called sarcomeres (the functional unit of muscle bres). Sarcomeres are made of two types of protein bres. Thick laments are made of bonded units of the protein myosin. Thin laments are made of three proteins: actin (which contains myosin-binding sites that allow myosin to connect to and move actin during muscle contraction); tropomyosin (which wrap around actin, covering the myosin binding sites); and troponin (which move tropomyosin away from myosin binding sites during muscle contraction). Skeletal muscles contract through a process outlined in the sliding lament model, where actin laments interact with myosin laments, appearing to ‘slide over’ each other, resulting in a shortening of the length of the sarcomeres and hence contracting or shortening of the muscle bre. As a number of muscle bres shorten at the same time, the whole muscle contracts and causes the tendon to pull on the bone causing movement. For the muscle to return to normal length, cessation of the electrochemical signal needs to occur, stopping the activation and contraction process and allowing the muscle to resume its natural resting length, or another force must be applied (e.g., contraction of the opposing muscle group). The stimulus to contract is initiated in response to nerve impulses (e.g., action potentials) transmitted from the brain and spinal cord of the Central Nervous System along the nerve cells and bres that make up the Peripheral Nervous System. Essentially, an action potential travels along a nervous pathway to a neuromuscular junction where it triggers the release of a neurotransmitter that signals the muscle bres to contract. The muscle bres are activated by motor neurons (a motor unit is made up of a single motor neuron, as well as all of the skeletal muscle bres that it activates). A single motor neuron can control several hundred muscle bres at a time, depending on the size and function of the muscle. For example, motor neurons for eye muscles may control 10–100 bres, while motor neurons for large leg muscles may control thousands of muscle bres. Groups of motor units often work together to coordinate the contractions of a single muscle. The number of muscle bres recruited regulates the force generated, according to the size principle of recruitment. For example, when one lifts a lightweight object, fewer muscle bres will be recruited. Lifting a heavier weight will require recruitment of many more muscle bres. Skeletal muscles are made of bundles of muscle bres that are within a continuum from slow- to fast-twitch and a continuum from oxidative to glycolytic energy supply. Based on their contractile (i.e., twitch) and metabolic (i.e., energy supply) properties and patterns of use, muscle bres are also characterized along a continuum of fatigue characteristics. Slow- twitch oxidative bres contract more slowly because they express contractile proteins (i.e., myosin ATPase) with slower kinetics. The contraction process in these bres is more e cient and they have a high level of mitochondria, oxidative enzymes and myoglobin to supply vast amounts of ATP through aerobic metabolism, and are therefore resistant to fatigue. This makes them better-suited to continuous work over time, such as distance running or maintaining posture. Fast-twitch glycolytic bres have contractile proteins that develop more force more quickly. This makes them better suited to provide substantial contributions to powerful activities like sprinting and jumping. They have little aerobic energy supply, make greater use of anaerobic metabolism, and fatigue more quickly. Intermediate fast twitch oxidative bres have fast contraction characteristics, but also a high level of aerobic and anaerobic energy supply. Hence, they have a greater balance in their contractile and energy supply characteristics than pure fast-twitch bres and therefore have an intermediate fatigue pro le. Skeletal muscles contain a genetically determined mixture of both slow and fast bre types and the ratio may in uence the kind of activities an individual will excel at. For example, an individual with a high percentage of fast twitch bres may excel at sprinting whereas those with predominantly more slow twitch bres may prefer endurance activities. The pattern of training can further in uence the metabolic characteristics of these bres. Skeletal Muscle Biomechanics Skeletal muscles work as a biomechanical system in which the bones and joints form levers and the muscle acts as the e ort force. The joint acts as the fulcrum and the bone that the muscle moves acts as the lever. The object being moved acts as the load. There are three classes of levers, but the vast majority of the levers in the body are third class levers in which the fulcrum is at the end of the lever and the e ort is between the fulcrum and the load at the other end of the lever. Skeletal muscles are arranged in antagonistic pairs (or opposing muscle groups) around joints (where two or more bones meet and are held together by ligaments). To move the bone, the agonist muscles – which include prime movers and synergists (or helpers) – will contract and generate the force required for the movement. As the prime and synergist muscles generate the movement, the antagonist muscles relax or lengthen. Joint stabilizers are muscles that prevent unwanted movement at the joints (e.g., rotator cu at the shoulder or piriformis muscle at the hip). There are also global stabilizers that stabilize the trunk (or ‘core’) to create a more solid foundation for movement. Energy System Energy is required to fuel the working muscles during physical activity. To release energy, a cellular respiration process metabolizes nutrients (i.e., protein, carbohydrate and fat from food intake) to yield high-energy molecules called adenosine triphosphate (ATP), which are then available in muscle cells to provide an immediate source of energy for muscle action. The storage capacity is limited to a few seconds of energy supply however. As ATP is a very reactive molecule, it is maintained in very low concentration in tissues, and requires a very responsive system of energy support to re-supply once it is used. During activity, the body manufactures ATP through three primary pathways depending on the rate of energy supply needed and how much oxygen is available to the muscles. Although one of the three energy systems will be the dominant source of energy during particular types of physical activity, all of the exercise energy systems are active at all times. It is simply the relative amount of energy that each system is providing that will change with activities of varying intensity levels and duration. The Anaerobic Alactic Energy System (high energy phosphate system) is the dominant source of energy for the high rate of energy supply that is typically required at the onset of any movement. It can provide energy immediately, it does not require any oxygen (i.e., is ‘anaerobic’), and it does not directly produce lactic acid (i.e., is ‘alactic’). This system provides ATP energy through a combination of ATP already stored in the muscles and by converting phosphocreatine (PCr) into usable ATP. The PCr substrate is also used as an energy capacitor or shuttle for energy from other sources (e.g., aerobic respiration) when oxygen becomes available from the mitochondria in the muscle (Table 1.2).  TABLE 1.2 Energy System Characteristics TIME ENERGY FRAME BY- ACTIVITY SYSTEM FUELS (Seconds) PRODUCTS EXAMPLES Anaerobic ATP 0–15 ADP 100m sprint (Alactic) CP Cr + Pi Jumping, (creatine + agility, inorganic weight lifting phosphate) Anaerobic CHO 15–120 Lactic Acid 200m (Lactic) incomplete 2 ATP / mol (power) breakdown CHO 800m (endurance) Resistance training Aerobic CHO 120 – CO2 Distance FATS several Water running hours 1500m (PROTEIN Heat 30 minutes), there is continuous and a gradual reliance on fat as a fuel source. In only the more severe states of energy reliance (i.e., severe calorie restriction or at the end of a prolonged endurance event like a marathon), protein is oxidized to supply energy for the working muscle. Nervous System The nervous system (Figure 1.5) consists of the brain, spinal cord, sensory organs, and all of the nerves that connect these organs with the rest of the body. Together, these organs are responsible for the control of the body and communication among its parts. FIGURE 1.5 Anatomy of the Nervous System (Copyright © 2019, Wolters Kluwer Health. All rights reserved.) Note: A full-sized version of this image is included in the online CSEP-PATH® Toolkit. Central Nervous System The brain and spinal cord together form the central nervous system (CNS), where information is processed and responses originate. The brain is located inside the cranial cavity, where the bones of the skull surround and protect it. The approximately 100 billion neurons of the brain form the main control center of the body. The brain is the seat of higher mental functions such as consciousness, memory, planning, and voluntary actions. It also controls lower body functions such as the maintenance of respiration, heart rate, blood pressure, and digestion. The spinal cord is a long, thin mass of bundled neurons that carries information through the vertebral cavity of the spine beginning at the medulla oblongata of the brain on its superior end and continuing downwards to the lumbar region of the spine. In the lumbar region, the spinal cord separates into a bundle of individual nerves called the cauda equina (due to its resemblance to a horse’s tail) that continues downward to the sacrum and coccyx. The white matter of the brain and spinal cord functions as the main conduit of nerve signals to the body from the brain. The grey matter of the brain and spinal cord integrates responses to stimuli. Peripheral Nervous System The peripheral nervous system (PNS) consists of the nerves and collection of nerve bodies (i.e., ganglia) outside of the brain and spinal cord. Its main function is to connect the CNS to the limbs and organs. Unlike the CNS, the PNS is not protected by the bones of the skull and spine. The PNS is divided into the somatic nervous system (SNS) and the autonomic nervous system (ANS). The SNS is the only consciously controlled part of the PNS and is responsible for stimulating skeletal muscles in the body. The ANS controls subconscious e ectors such as visceral muscle tissue, cardiac muscle tissue, and glandular tissue. The sympathetic division of the ANS forms the body’s “ ght or ight” response to stress, danger, excitement, exercise, emotions, and embarrassment. It increases respiration and heart rate, releases adrenaline and other stress hormones, and decreases digestion to cope with these situations. The parasympathetic division of the ANS forms the body’s “rest and digest” response when the body is relaxed, resting, or feeding. It works to undo the work of the sympathetic division after a stressful situation. Among other functions, the parasympathetic division works to decrease respiration and heart rate, increase digestion, and permit the elimination of wastes. The enteric nervous system (ENS) is the division of the ANS that is responsible for regulating digestion and the function of the digestive organs. Nerves are bundles of axons in the PNS that act as information highways to carry signals between the brain and spinal cord and the rest of the body. Neurons that carry information one-way from sensory receptors to the central nervous system are called a erent neurons. E erent neurons carry signals one-way from the central nervous system to e ectors such as muscles and glands. Mixed nerves (which contain both a erent and e erent axons) function like 2-way streets. Extending from the inferior side of the brain are 12 pairs of cranial nerves, each of which is identi ed by a Roman Numeral (I to XII) based upon its location along the anterior-posterior axis of the brain. Each nerve also has a descriptive name (e.g., olfactory, optic) that identi es its function or location. The cranial nerves provide a direct connection to the brain for the special sense organs, muscles of the head, neck, and shoulders, the heart and the gastrointestinal tract. Extending from the left and right sides of the spinal cord are 31 pairs of spinal nerves, which carry both sensory and motor signals between the spinal cord and speci c regions of the body. The 31 spinal nerves are split into groups named for the ve regions of the vertebral column: eight pairs of cervical nerves, 12 pairs of thoracic nerves, ve pairs of lumbar nerves, ve pairs of sacral nerves and one pair of coccygeal nerves. Each spinal nerve exits from the spinal cord through the intervertebral foramen between a pair of vertebrae or between the C1 vertebra and the occipital bone of the skull. All of the body’s sense organs are components of the nervous system. Vision, taste, smell, hearing, and balance are all detected by specialized organs such as the eyes, taste buds, and olfactory epithelium. Sensory receptors for the general senses like touch, temperature, and pain are found throughout most of the body. All of the sensory receptors of the body are connected to a erent neurons that carry their sensory information to the CNS to be processed and integrated. Cardiovascular System The cardiovascular system includes the heart and the circulatory network of blood vessels that includes arteries, veins and capillaries. Its function is to deliver oxygen and nutrients to the organs of the body. Given muscle is the primary organ responsible for moving the body, the cardiovascular system has an important role to play in supplying oxygen and nutrients to the muscle during physical activity and exercise. The heart (Figure 1.6) is a four-chambered muscular pump and is responsible for pumping blood around the body. It consists of two collecting chambers (the right and left atria) and two pumping chambers (the right and left ventricles). The heart also includes four valves that keep the blood moving in the right direction (i.e., preventing back ow). The atrioventricular (AV) valves separate the atria from the ventricles and the semilunar valves separate the ventricles from the aorta and pulmonary artery. The right side of the heart receives de-oxygenated blood from the periphery (right atria) and pumps it to the lungs (right ventricle). This is called the pulmonary circuit. The blood is oxygenated in the lungs and carried back to the left atrium via the pulmonary vein. The blood then moves into the left ventricle where it is pumped into the aorta and carried throughout the body. This is called the systemic circuit. When the blood ows from the heart it enters the vascular system, a vast system of blood vessels that carry oxygen and nutrients to the tissues of the body (e.g., digestive system, liver, kidneys, muscles, skin). Starting with the aorta, the blood vessels that carry blood away from the heart are arteries. Arteries continue to subdivide until they reach their smallest size (called arterioles) and then lead into the tiniest blood vessels (called capillaries). Capillaries have very thin walls across which oxygen and carbon dioxide, nutrients, minerals, vitamins and hormones easily di use to and from the tissues. The blood vessels that bring blood back to the heart are veins. The smallest veins (called venules) collect deoxygenated blood from the capillaries and transport it along the veins leading back to the heart. FIGURE 1.6 Anatomy of the Heart (Copyright © 2019, Wolters Kluwer Health. All rights reserved.) Respiratory System The respiratory system consists of the nose, nasal cavity, pharynx, larynx, trachea, bronchial tree, and the lungs. Its primary function is to lter air that enters the body and allow for oxygen-carbon dioxide gas exchange in the alveolar sacs in the lungs. Air is sucked into the lungs (Figure 1.7) through the process of inhalation in which the diaphragm and intercostal muscles contract, pushing the rib cage out and up to draw air into the lungs. In exhalation, the diaphragm and intercostal muscles relax to pull the rib cage in and down and forcing air out of the lungs. Upon inhalation, air enters the lungs through the bronchi, which divide into the secondary bronchi (two in the left and three in the right), and continue to sub-divide into the tertiary bronchi, bronchioles, terminal bronchioles and nally into small air lled sacs called alveoli. Each alveoli is covered by capillaries through which oxygen passes from the alveoli into the bloodstream to be returned to the heart and pumped throughout the body. It is here also that carbon dioxide passes from the bloodstream into the alveoli where it is eliminated by the lungs through exhalation. FIGURE 1.7 Anatomy of the Lungs (Copyright © 2019, Wolters Kluwer Health. All rights reserved.) Body System Responses to Physical Activity The nervous, cardiovascular, respiratory, energy, and musculoskeletal systems all work closely together to get the body moving, to provide the oxygen and nutrients required for such movement and to remove metabolic waste (i.e., carbon dioxide, heat and waste products such as lactic acid) that are produced as a result of the movement. Aerobic Responses When an individual engages in aerobic activity, a number of short- term physiological adaptations occur. The heart gets messages from the body that tell it when to pump more or less blood to meet the body’s needs. When the body is at rest (e.g., sitting), breathing slows and the heart pumps more slowly to provide for the lower amounts of oxygen needed. During physical activity, the body’s oxygen demand and production of waste products increases and the heart responds by increasing its cardiac output (the volume of blood ejected from the heart per minute). Cardiac output will typically increase from 5.0 L · min–1 at rest to about 20– 30 L · min–1 during more intense physical activity. Blood ow to the working muscles increases and a larger fraction of the available oxygen is extracted from the circulating blood. If the increase in ow and enhanced oxygen extraction match the demands of the e ort, the activity remains aerobic. The activity is anaerobic when oxygen delivery is inadequate and lactic acid accumulates in the blood and working muscles. Over a period of time, an individual’s engagement in regular physical activity will produce a number of important longer-term physiological adaptations (Table 1.3). With regular aerobic physical activity, for example, the cardiovascular and respiratory systems will become more e cient at delivering oxygen to working muscles. More red blood cells will be produced to carry more oxygen in the blood. Arteries will widen and become more elastic, reducing blood pressure. The heart will grow stronger so it can pump more blood with each beat. The resting heart rate may decrease. Post-activity heart rate recovery times will drop. More capillaries will grow within the muscles so they can more e ciently extract oxygen. More capillaries also grow around the alveoli in the lungs, increasing the e ciency of the exchange of oxygen and carbon dioxide. The diaphragm and intercostal muscles may become stronger and better able to move the chest cavity for breathing. Regular engagement in weight bearing aerobic activities (e.g., walking or jogging) can help make the muscles, tendons, ligaments and bones stronger and more resistant to injury and age-related decline. TABLE 1.3 Physiological Adaptations to Physical Activity Training PHYSIOLOGICAL ADAPTATION RAISES LOWERS Aerobic Training cardiac output peripheral blood volume resistance hematocrit heart volume blood ow to lungs size/number of mitochondria mitochondral enzymatic activity capillarization in fat oxidation enzyme activity blood supply/vascularization to heart muscle stroke volume left ventricular volume ventricular wall thickness Resistance Training muscle strength % body fat PHYSIOLOGICAL muscle power ADAPTATION low back pain  balance and sarcopenia and coordination osteoporosis basal metabolic rate insulin lean tissue mass concentration/ response to glucose muscle endurance challenge motor performance insulin sensitivity Source: Adapted from the ACSM Resource Manual for Guidelines for Exercise Testing and Prescription, 6th. Resistance Training Responses Resistance training improves musculoskeletal tness by increasing strength and muscular endurance. Along with adaptations to skeletal muscles and bone, neural adaptations also contribute signi cantly to strength gains (particularly at the outset) by increasing the recruitment and activation of motor units, and by decreasing co-contraction of antagonistic muscle groups (Sale, 1988). Changes in force production can occur rapidly during the initial 2– 6 weeks of resistance training, with no changes in muscle size. If untrained, the brain may activate required motor units at slightly di erent times, causing ine cient movement. Resistance training aids coordination of motor unit recruitment (i.e., the ability to activate needed motor units at the exact time) for maximal strength production and movement e ciency. Coordinated motor unit activation works much like rowers rowing a boat in sync versus the rowers rowing at di erent times. Over time, resistance training can also lengthen the time a motor unit can stay activated, delivering improvements in muscular endurance (e.g., the ability to perform a greater number of push- ups or pull-ups in succession) (Table 1.3). After 4–6 weeks of resistance training, increases in muscle size (called muscle hypertrophy) contribute more to strength gains than neural adaptation. Hypertrophy results from an increase in total amount of contractile protein, number and size of myo brils per bre, and the amount of connective tissue surrounding the muscle bres (Goldberg et al., 1975). Since long-term resistance training can yield continued strength increases without hypertrophy, a secondary phase of neural adaptation is likely responsible for strength gains occurring during 6–12 months of training (Deschenes and Kraemer 2002). Evidence also suggests that resistance training induced hypertrophy is an important mechanism underlying strength gains in older women and men. This implies that older adults can e ectively counter age-related loss of muscle mass (or sarcopenia) with resistance training. Resistance training also has bene cial e ects on bone health that may decrease risk of osteoporosis and bone fractures, particularly in women. Improvements in bone mineral density appear to be site-speci c (i.e., greater changes occur in bones to which the exercising muscles attach). Resistance training also improves the size and strength of ligaments and tendons (Edgerton, 1973; Fleck et al., 1986; Tipton et al., 1975), which may increase joint stability, thereby reducing risk of sprains and dislocations. As muscles grow stronger, the ratio of lean body mass to fat can also improve, further reducing strain on muscles, bones and joints. Furthermore, regular exibility training (i.e., stretching) can increase exibility and limit the tendency of the ligaments and tendons to shorten and restrict movement as a person ages. It can also help in preventing injuries, improving posture, reducing lower back pain, and improve balance during movement. Heart Rate, Stroke Volume and Blood Pressure Responses to Aerobic Activity Heart Rate (HR) is the number of heart beats per minute (bpm). An average normal resting HR is about 60–80 bpm, although it varies by gender (higher for women), age (lower with age) and aerobic tness level (lower for more t individuals). When the HR increases in response to physical activity, the magnitude of  c eases espo se to p ys ca act ty, t e ag tude o the response will also vary by the person’s age, health status, tness level, type of activity, intensity of activity and external conditions such as temperature. Stroke volume (SV) refers to the volume of blood that is pumped by the heart with each contraction. During physical activity, the muscles require more oxygen and the stroke volume increases until the intensity reaches about 50% of maximal oxygen uptake (VO2max). After that, the heart rate will increase to produce the cardiac output required to meet the required oxygen demand. After a period of regular physical activity training, an individual’s stroke volume is likely to be higher and resting heart rate lower. Blood Pressure (BP) refers to the pressure that the blood exerts on the walls of the blood vessels and re ects the e ciency of blood ow through the cardiovascular system. The lowest pressure (just before the heart contracts) is the diastolic pressure and the highest pressure (just after the heart contracts) is the systolic pressure. An average healthy adult would have a resting BP of 120 mm Hg (systolic) over 80 mm Hg (diastolic), i.e., the typical blood pressure one would see in an inactive, but otherwise healthy adult. Normal blood pressure readings for an active, healthy adult would typically read 110/70. During physical activity, the systolic blood pressure increases from its normal resting value as the body attempts to force blood through the vigorously contracting muscles. In contrast, the diastolic pressure shows little change during aerobic physical activity but may elevate in resistance activity. Increases in blood pressure during any type of activity are greater if resting values are already high, and in such circumstances, may reach dangerous levels. Conclusion Our bodies were built to move, meaning that these positive physiological processes are a means for humans to be active (historically to seek food) and stay healthy. Because maintaining physiological adaptations is a metabolically expensive situation for most tissues and especially muscle, an absence of physical activity or prolonged periods of sedentary behaviour reverse the positive processes and typically result in decline or decrease in adaptations or system function (i.e., a simple matching of supply and demand). 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