Exercise physiology 2011

Exercise Physiology Muscles and Energy Systems

Introduction Exercise Physiology is the description and explanation of functional changes brought about by single (acute) or repeated exercise sessions (chronic exercise or training), often the object of improving the exercise response. In this definition the description of functional changes refers to what happens to the body and the explanation refers to understanding how the changes occur. For example, we know that repeated lifting of heavy weights usually results in greater ability to lift even heavier weights. This functional change brought about by repeated bouts of exercise can be explained partly by an increased growth of muscle tissue and partly by an improved ability of the nervous system to cause greater number of muscle fibres to contract simultaneously for the greatest possible force of contraction of the entire muscle. This understanding of how weight lifting ability develops has led to better training programs to improve the lifting performance.

Skeletal muscle Architecture Skeletal muscles can be classified according to the fibre arrangement around the tendon. The three classifications are Fusiform muscles Run the length of the muscle belly Designed for mobility because they produce contractions over a large range, yet produce low force Example – sartorius. Pennate muscles – designed for strength and power and run at angles to the tendon, they are further divided into: Unipennate Found on only one side of a central tendon Example – tibialis anterior bipennate run on either side of a central tendon example rectus femoris in the quadriceps multipennate branch out from several tendons enable great force to be generated example – deltoid Radiate muscles Radiate from the main tendon Compromise between fusiform and pennate Capable of producing strength and power yet retain their mobility Example – pectoralis major

Muscle Tissue Characteristics Excitability Nerve stimuli controls muscle action. Muscles can receive and respond to a stimulus from a nerve cell. Contractibility Muscle changes shape due to a stimulus. That is, can become shorter and thicker. Elasticity Muscle returns to normal length when force is removed. That is, after is has been contracted. Extensibility Muscles have the capacity to stretch when a force is applied. Atrophy – muscles can decrease in size as a result of injury, illness or lack of exercise. Hypertrophy – muscles can increase in size with an increase in physical activity

Muscle Fibre Types There are two different types of muscle fibres: Slow Twitch Muscle Fibres Red in colour Contract slowly but can contract repeatedly for prolonged periods of time. Therefore endurance fibres. Smaller than fast twitch fibres Fast Twitch Muscle Fibres White in colour Contract rapidly but easily exhausted Suited for speed and strength Larger than slow twitch fibres Your genetic inheritance of fibres determines your speed or endurance potential. Training will increase the efficiency of what you predominantly have – not increase the amounts of certain fibre type.

Muscle Action and Movements During a particular movement, a muscle performs one of the following roles: Agonist or Prime Mover – This muscle causes the major action. There is usually more than one prime mover in a joint action, and there are prime mover muscles for all moveable joints. Antagonist – This muscle must relax and lengthen to allow a movement to occur. It causes an opposite reaction to that caused by the agonist. Generally, muscle flexors and extensors work in an agonist – antagonist relationship. Synergist or assistant – This muscle assists the agonist to produce the required movement to reduce any excessive or unnecessary movements. Stabiliser or fixator – these muscles ensure that the joint remains stable while the agonist and antagonist are working. The muscle will shorten just slightly during contraction, causing only minimal movement to allow the action to be performed more effectively.

Types of Muscle contraction Muscle contractions are classified according to the movement they cause: Isotonic Contraction This is where the muscle length changes throughout the range of movement as force is being developed It is the most common form of contraction. Examples include push-ups, sit-ups, throwing, kicking and most sporting movements. There are two types of isotonic contractions: Concentric – the muscle length shortens during the contraction. Eccentric – the muscle lengthens while the force is developed. This occurs in activities that resist gravity, and it will slow the limb or trunk movement. Isometric Contraction This occurs when force is developed (tension), but there is no change in the length of the muscle. Is referred to as applying a force against an immoveable object. Examples include gripping a cricket bat – the forearm muscles perform an isometric contraction, and holding a weight in a stationary position. Isokinetic Contraction This develops maximal velocity throughout the entire range of motion. Highly specialised equipment, such as Cybex machines are required to perform these contractions. The amount of force applied by these machines always equals the amount of force applied by the muscle and this is done over the muscle full range of motion.

Muscle Structure Muscles are composed of bundles of fibres Each fibre is composed of a covering or membrane called sarcolemma and a gelatin-like substance called sarcoplasm that contains hundreds of contractile myofibrils, mitochondria and the sarcoplasmic reticulum Each myofibril contains many protein threads called myofilaments (thick and thin filaments) Within this structure A bands are dark as it contains thin and thick filaments. I bands appear light as they contain mostly thin filaments. When a muscle is relaxed, the central portion of each A band appears lighter than the outer portions because the thin filaments do not meet the centre of the A band. The central lighter area of the A band is the H-zone and it disappears when muscles contract. Each I band is bisected by a Z line or disc, which anchors the thin filaments. The distance between the two Z lines is known as a sarcomere. When a muscle fibre contracts, the Z lines move closer together and therefore shorten the sarcomere.

Microscopic structure of a skeletal muscle

Thick and thin filamentes Actin: is a protein molecule (thin filament) that helps muscles to contract. Myosin: Actin attaches to the myosin (thick filament) during contraction Actin and myosin cannot become attached without the cooperation of the two other proteins within the thin filaments. Tropomyosin : blocks attachment sites on actin where myosin attaches during contraction Troponin : pulls tropomyosin away from the attachment sites so that the contraction proceeds

Muscle contraction After contraction – The Z line: are pulled closer together The A band: length does not change The I band: length are shorter The H zone: disappeared The muscles contract by the: sliding of the thin filaments towards each other (across the myosin filaments). As the filaments move together, the Z lines are pulled together and the A band shortens At the same time, the H zones of the A bands disappear as it becomes occupied by actin. This notation that the thin filaments slide towards each other during contraction is called the Sliding Filament Theory.

Nervous control of muscular contraction Nervous control facilitates the contraction of a muscle. Receptors: The body receives info from these sense organs (proprioceptors) and provide info about the body Effectors: Are the muscles that act (or effect) on info from the brain

Neuromuscular junction Messages must travel long distances throughout the body. Neurons are linked in neural chains to ensure all muscle fibres receive messages from the brain. A synapse is the junction between the dendrite of one neuron and the axon of the next neuron in the chain. A neuromuscular synapse is the junction between the axon and the muscle where the nerve impulse stimulates the muscle.

Motor units A motor unit consist of: the motor nerve cell (neuron) that originates in the spinal cord and all of the muscle fibres that are supplied by that neuron The number of fibres within each motor unit varies according to the precision of the movement required. Precise movements: Muscles such as those in the hand (writing and threading a needle) have small motor units, where one motor neuron may be responsible for a few fibres Gross motor movements: Muscles such as the quadriceps (running and kicking) have large motor units, where one motor neuron may be responsible for thousands of fibres.

The diagram shows two motor units. Motor unit 1 innervates two muscle fibres whereas Motor unit 2 innervates three muscle fibres.

Neural control of muscular strength The body has an amazing ability to call forth just the right amount of muscular force to perform an endless variety of tasks – from drawing a line on a piece of paper to lifting a 50kg weight. If it were not for this ability to control contractile force, it would be common to see ice cream cones plastered to people’s faces. There are two ways by which we can vary the strength of a muscle contraction: vary the total number of units recruited in a muscle vary the frequency with which a given number of motor units are recruited

The All or none law It is not possible for some fibres to contract while others relax – that is all of the muscle fibres within a motor unit must all contract or not contract. Also if the muscle fibre is not activated sufficiently – it will not contract. This is called the All or None Law – a motor unit and its fibres either contract maximally or do not contract at all. For sub maximal contractions some motor units are at rest , whilst others produce the required force.

The Contractile process When a motor nerve fibre delivers a stimulus (action potential) to a skeletal muscle fibre at the motor endplate, it spreads rapidly over the entire sarcolemma. It then arrives at the neuromuscular junction, causing a release of a chemical substance called acetylcholine from the nerve ending. Acetylcholine travels across the synaptic cleft between the nerve and muscle, stimulating another impulse. The action potential is transmitted down the T-Tubules toward the interior of the fibre. Transmission of the action potential down the T-tubules causes calcium to be released from the sarcoplasmic reticulum. These calcium ions are then bound to troponin molecules. In the absence of calcium, tropomyosin blocks the attachment sites for myosin.

The Contractile process continued In the presence of calcium ions, troponin changes its shape and pulls tropomyosin away from the myosin attachment sites on actin. Once the myosin attachment sites on actin are blocked, myosin molecules can form ‘cross bridges’ with actin. Myosin molecules consist of globular heads and long thin backbones. Myosin heads are moveable and are activated in the presence of ATP so they can attach themselves to the unblocked sites of actin. Once the myosin – ATP complex is attached to actin, myosin can serve as an enzyme to split ATP into ADP plus phosphate (P) with the simultaneous release of chemical energy to make the myosin heads swivel toward the centre of the A band and move the thin filaments in that direction. ATP ADP + P + Energy for contraction (Myosin ATPase activity)

Muscle Twitch, Wave Summation and Tetanuws An action potential (electrical current) that flows along a motor neuron is not capable of directly exciting muscle fibres. Instead it excites the fibre(s) that it innervates by chemical transmission. Each action potential travelling down a motor neuron results in a short period of activation of the muscle fibres within the motor unit. This is referred to as a twitch. Although calcium release during a twitch is sufficient to allow optimal activation, calcium is removed before reaching its maximum and the muscle relaxes (Figure 2.1 a). If a second twitch occurs before the fibre completely relaxes, force from the two twitches summates (wave summation) and the resulting force is greater than that produced by a single twitch (Figure 2.1 b) Decreasing the time interval between the twitches and increasing the frequency results in greater summation of force. The stimuli may be delivered at so high a frequency so that the twitches begin to merge and even completely fuse – a condition called Tetanus (Figure 2.1 c and d). This is the maximal amount of force the motor unit can develop.

Summary of muscle contraction and relaxation Conditions in Relaxed Muscles: Few or no impulses reach muscles Calcium ions bound to sarcoplasmic reticulum Tropomyosin – Troponin block attachment sites for myosin on actin ATP bound to myosin heads Muscle Contraction: Nerve impulse arrives at neuromuscular junction , causing stimulus to spread across the sarcolemma and down the T-Tubules Depolarisation of the T-Tubules trigger the release of calcium, which spreads to the myofibrils (especially at the A band) Calcium is bound by Troponin, which changes shape and pulls tropomyosin away from the attachment sites for the myosin heads on actin Myosin-ATP attaches to actin and forms actomyosin-ATP Actomyosin-ATP is broken down to actomyosin + ADP + P + energy to make myosin heads and thin filaments move toward the centre of the A band

Summary of muscle contraction and relaxation Continuation of Contraction Cross bride cycling New ATP molecules bind to myosin heads, causing the release of myosin and actin. In the meantime, other myosin heads have bound to other attachment sites to maintain the contraction If other actin sites are unblocked, previously released myosin heads may be reattached, ATP is split and myosin heads plus thin filaments move towards the centre of the A band New ATP molecules bind to myosin heads and cause their release from actin. Cross bridge cycling occurs as long as the nerve impulse continues and ATP is available Relaxation: Nerve impulse stops – no longer a depolarisation or the release of calcium No nerve stimulus therefore calcium is withdrawn from Troponin and stored for further contractile activity Without calcium, Troponin changes shape and allows tropomyosin to block myosin sites Muscle relaxes Revision B

Energy Energy is: the capacity to generate activity, to produce or do work The Human body produces and uses energy in order to perform a variety of tasks including: Maintenance of vital functions such as brain activity, breathing and circulation, body temp and hormonal activity Digestive process Physical activity Energy comes originally from: The food we eat Through digestion, food is: Broken down and stored in the cells in the form of chemical energy waiting to be used Energy is measured in units of: Kilojoules or calories

Fuel for exercise Our body’s energy needs come from the breakdown of the nutrients carbohydrates, fats and proteins, in the food we eat. Carbohydrates – preferred source Protein – normally for growth and repair Fats - acts as a concentrated fuel source. The main source for sub maximal activity and at rest Water, minerals and vitamins (although not used directly for fuel However, the energy released during the breakdown of carbohydrates, fats and proteins is not immediately used. Instead an important energy rich chemical compound known as ATP (adenosine triphosphate) is formed. Energy is released when ATP is broken down. The ATP stored in the muscles is used up within seconds, so it is vital to keep making ATP so the muscles can keep contracting. Energy for muscular activity comes from the chemical compound ADENOSINE TRIPHOSPATE (ATP). This compound contains an adenosine molecule with three (tri) phosphate groups attached. ATP stored in the muscles is very small and lasts only 1 – 2 seconds. The body is able to replenish as quick as it is broken down. This is achieved by resynthesising (rebuilding) ATP through the break down of reserve fuels.

The ATP-PC system This system provides energy for activities of short duration (up to 10 seconds) high intensity (85-100% maximal effort) Is an anaerobic system (without oxygen) because the body does not have enough time to deliver the oxygen for energy production in less than 10 seconds. Sometimes called the Alactic System because no lactic acid is produced during this kind of energy production. For this reason it is the predominant system used in short bursts of activity such as 100-metre sprint, high jump, running to first base, fast bowling in cricket and shot put.

When chemical bonds of the ATP molecule are broken, energy is released. Because there is only a limited supply of ATP in muscles, it must be resynthesised from ADP + Pi (Pi is a free phosphate molecule), which also requires energy. This energy is obtained from the breakdown of a phosphate compound stored in muscles called creatine phosphate or PC. As rapidly as ATP is broken down during muscular contractions, it is being resynthesised by from ADP + Pi by the energy provided by the breakdown of PC stored in the muscles. Once the supply of CP runs low and we stop exercising it takes about 2 minutes to replenish the stores. Oxygen is used to rebuild the CP, which is why we breathe heavily after a sprint or high intensity burst.

ATP-PC system A high-energy phosphate bond is broken to release energy to rebuild ATP from ADP and Pi.

The Lactic Acid system This is also a system that requires no oxygen. Also known as Anaerobic Glycolysis, this system becomes active when we require energy beyond the capacity of the ATP-PC system. Provides energy for high intensity activities lasting from 10 seconds up until 2 – 3 minutes in some elite athletes. For this reason it is the predominant system used in longer bursts of high intensity activity such as 400-metre and 800-metre running events.

The Lactic Acid system It is a chemical pathway used to produce ATP from the partial breakdown of glucose with a by-product of lactate (lactic acid). It uses carbohydrates for as its main fuel source for rebuilding ATP. Carbohydrates are broken down in the digestive system into glucose. Most of the glucose is distributed via the bloodstream, into muscles (in large quantities) and the liver (to be stored as glycogen or if in excess as adipose tissue or fat). A process called glycolysis dissolves or breaks down the sugar (glycogen and glucose) into pyruvic acid. During this breakdown, energy is released which is used to resynthesise ATP. (Because oxygen is not present, the breaking down of glycogen is incomplete, resulting in the formation of a waste product called lactic acid). A high amount of lactic acid in the muscles during exertion causes muscular fatigue. The body can tolerate increasing levels of lactic acid production only until the lactate accumulation rate is greater than the body’s ability to remove it. Lactate cannot get out into the bloodstream because the bloodstream is saturated ie. Has a higher concentration of lactate than the muscle cell. When this occurs it is called the Lactate Threshold. This level for everyday people is about 70% of their maximum heart rate and for elite athletes it is 90% of their maximum heart rate. Once an athlete passes this threshold they must reduce or stop their muscular effort. The lactic acid is only dissipated when the athlete ceases exercise.

In all-out efforts such as the 400m run or the 100m swim, these athletes may reach their lactate threshold towards the end of the race, which can be endured when the end is so near, but a team game athlete is placed in a different situation. Lactate threshold can happen at any time during the game due to differing intensities and longer duration so therefore substitution or time outs need to be utilized effectively.

The Aerobic or Oxygen system This system requires oxygen and is also known as aerobic glycolysis. Provides energy for prolonged physical activity of 2 – 3 minutes or more of low to moderate intensity (sub maximal exercise). Energy is produced in the presence of oxygen, which has been transported from the lungs through the blood and into the mitochondria in the muscle cells. It takes the body 2-3 minutes to increase its oxygen consumption from the resting level to a higher level in response to the demands of shorter and more explosive exercise. This system can create 38 molecules of ATP from 1 molecule of oxygen (anaerobic glycolysis can only create 2). This extra amount is possible because the abundance of oxygen allows a more complete breakdown of glucose as pyruvic acid and can be broken down further rather than being converted to lactic acid. For this reason it is the predominant system used in continuous activity of sub maximal intensity such as 1500-metre swimming, long distance running.

The Aerobic or Oxygen system The intensity of these activities does not exceed 85% of the maximal heart rate so enough oxygen will be present after 2-3 minutes to prevent lactic acid from accumulating and the glycogen can be completely broken down. This allows energy to resynthesise a substantial amount of ATP. The waste products carbon dioxide, water and heat are by-products of the complete chemical breakdown of glycogen (and fat). Any activities that exceed intensity levels of 85% of maximal heart rate, promote the accumulation of lactic acid, which results in fatigue. The body cannot sustain activity at this intensity and requires recovery. This is referred to as the anaerobic threshold. Performance improves when athletes can tolerate high levels of lactic acid, which can be achieved through training. Aerobic glycolysis chemical reactions, like all energy systems, are conducted within muscle cells. Aerobic glycolysis however occurs within a specialised component of muscle cells called mitochondria. ATP produced in the mitochondria is transported to myosin cross bridges when required, to provide the energy for muscular contraction.

Aerobic Glycolysis Aerobic glycolysis can be divided into three main stages: Aerobic Glycolysis - this involves the breakdown of glycogen and glucose to pyruvic acid in the presence of oxygen, with some energy being released for ATP resynthesis. Kreb’s Cycle – Sir Hans Krebs discovered that pyruvic acid is broken down into carbon dioxide, with further energy release. Electron Transfer System – this is when water (perspiration), heat and large amounts of ATP are formed. In extreme conditions, fat can be broken down to carbon dioxide and water to provide energy for ATP resynthesis. Fats have the capacity to produce more ATP than carbohydrates ( 1 molecule of fat provides 100 molecules of ATP but at a much slower rate than the release from glucose) although fats require very high amounts of oxygen for conversion. The body tends to exhaust glycogen stores before resorting to fat as an energy source, which can take up to 2-3 hours. The body will use protein as an energy source when glycogen and fat have been completely exhausted, although this rarely occurs.

The relationship between the three energy systems

The relationship between the three energy systems Area On Chart Performance Time Major Energy System(s) Involved Example A Less Than 30 Seconds ATP-PC System 100m Sprint B 30 to 90 Seconds ATP-PC System Lactic Acid System 200m - 400m Sprint 100m Swim C 90 Seconds to 3 Minutes Lactic Acid System Oxygen System Boxing (3m Rounds) 800m Run D Over 3 Minutes Oxygen System Aerobics Class Marathon

Anaerobic and Aerobic Glycolysis

Energy Production at Rest When the body is at rest ATP is produced aerobically through Aerobic Glycolysis. Then energy demand when your body is resting is quite small; therefore enough oxygen is made available without having to use the anaerobic sources. The major energy sources of energy in the resting state are fatty acids (2/3) and carbohydrates (1/3). As the intensity of physical movement increases from the state at rest, the contribution of carbohydrates as a fuel source also increases.

Definitions VO2 Oxygen uptake. The amount of oxygen taken up and used by the body VO2 max The greatest volume of oxygen used by the cells of the body within a given time. Steady state State of stability where the energy demands can be met relatively easily for a prolonged period of time. O2 deficit The amount of oxygen taken in above what is normally used at resting levels. EPOC The amount of oxygen required for pay back in recovery Alactic acid debt EPOC supplies the energy for phosphagen replenishment (anaerobic). Lactic acid debt EPOC that supplies energy for the removal of lactic acid (aerobic). Anaerobic threshold The intensity effort at which lactic acid accumulates. Aerobic threshold Sub-maximal steady state of 130 – 150 bpm. Cardiac output The volume of blood pumped from a ventricle per unit of time. It is the product of HR and SV. Stroke volume The amount of blood pumped out of the ventricles with each beat. Arterio-venous O2 difference The difference in oxygen content between the arterial blood and the mixed venous blood in the right atrium. Tidal volume The amount of air breathed in and out in one breath. Hypertrophy Increased cell size leading to increased tissue size.

Oxygen deficit and EPOC When you begin to exercise, oxygen uptake does not immediately meet demands. An oxygen deficit results as you rely on ATP, PC and anaerobic glycolysis. When oxygen uptake meets the demands, a steady state is achieved. After 2 to 4 minutes of exercise you experience the sensation called “second wind”. Exercise can continue as long as you are able to meet the fuel requirements. After exercise, oxygen returns slowly to resting levels. Recovery oxygen uptake in excess of resting needs is called EPOC (Excess Post Oxygen Consumption). EPOC is used to repay the oxygen deficit, to replace ATP and PC stores, to remove lactic acid and to replace liver and muscle glycogen used during exercise.

Oxygen deficit and EPOC The early portion of EPOC falls very quickly within the first minute or two, whereas the later portion falls at a more gradual rate over a prolonged time. This early portion is called the Alactic acid debt as it is regenerates the stores of ATP and PC and supplies oxygen to blood and other body tissues. The later portion is known as the Lactic acid debt as it removes lactic acid from the body by oxidizing it to carbon dioxide and water. The diagram below shows the use of oxygen during and after a steady state exercise such as a jog or cycle. At the beginning we cannot deliver enough oxygen to meet the requirements of the exercise. During this phase, the oxygen deficit, we rely on the phosphate and lactic acid systems. Eventually the oxygen intake increases as our breathing and heart rate increases to move more oxygen to the working muscles. Once we have reached the oxygen demands we have reached a steady state. At the end of the exercise the heart and lungs continue to deliver oxygen to the body to rebuild the energy supplies used at the start of the run or cycle. This is EPOC.

Physiological responses and adaptations to exercise Responses Adaptations HEART Heart rate increases. Cardiac output increases. SV increases from resting level. Coronary circulation increases. Max HR may be achieved. Resting HR decreases. SV increases during rest & work. Blood supply to heart muscle increases during rest & work. Volume of left ventricle increases after aerobic training. Hypertrophy of the left ventricle after anaerobic training. Max HR remains the same. HR at sub-max workloads falls. Cardiac output at max workloads increases. CIRCULATORY SYSTEM Systolic blood pressure increases. Speed of blood flow increases. Body temperature increases. Arterio-venous O 2 diff increases. Vasodilation occurs. Redistribution of blood flow. Maintained elasticity of artery walls. Diminished fatty deposits. Low risk of high blood pressure and cardiovascular disease. Capillary supply to heart and skeletal muscles increases. Blood volume increases. Hemoglobin count increases. Oxygen-carrying capacity of blood increases.

Physiological responses and adaptations to exercise Responses Adaptations RESPIRATORY SYSTEM Breathing rate increases. TV rises from 0.5L to a max of 5L per breath. Pulmonary diffusion increases. Lung ventilation increases from 7.5L/min to a max of 150L/min. Efficiency of intercostals muscles increases. Elasticity of lungs improves. Lung volumes increase. Pulmonary diffusion increases. MUSCULAR SYSTEM Motor unit recruitment increases, leading to greater strength of contraction. Temp increases due to increased blood flow. ATP production increases. Phosphates in muscle cell increase. O 2 supply to muscles increase. Enzyme activity increases. Glycogen, triglycerides and PC all deplete to produce ATP. Production of LA, CO 2 and other by-products increases. Aerobic Training Effects Capillarisation to muscles increase. Mitochondria increase in size and number. Myoglobin concentration increases. Triglyceride stores increase. Glycogen stores increase. Oxidative enzymes increase. Lactic acid tolerance increases. Red muscle fibres hypertrophy to a small degree. Glycogen sparing as fats are used in preference during sub-max efforts. Anaerobic Training Effects Hypertrophy of muscles occur (size increase of fast twitch). Glycogen stores increase. Capillarisation increases. PC stores increase. Muscle stores of ATP increase.

Physiological responses and adaptations to exercise OTHER Perspiration rate increases. Oxygen consumption increases . Arterio-venous oxygen difference increases slightly at maximal efforts. VO 2 max increases by up to 30%. Recovery HR returns to resting levels faster. Lactate thresholds increases.

Aerobic and anaerobic thresholds These thresholds relate to their respective energy systems. The first stage involves aerobic metabolism and is generally characterized by heart rates below 130 bpm. Blood lactate levels don’t change much from resting levels and ventilation only slightly increases. The second stage or Aerobic Threshold occurs: At about 40 – 60% of VO2 max or a heart rate between 130 – 150 bpm. Anaerobic Threshold : If the exercise intensity is increased, heart rate, blood lactate and ventilation will rise sharply. This effort can be maintained no longer than a few minutes (anaerobic threshold)

Exercise physiology 2011

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Exercise physiology 2011