Basic Exercise Physiology

[vigneshwaran.r]

Hi, all here i post what i know about exercise physiology any corrections, diagrams or any links related to this post, if you know them kindly mention it here.

1. BASIC EXERCISE PHYSIOLOGY

Physical Activity: Any skeletal muscle contraction (concentric and eccentric) that results in increased energy expenditure. Includes all exercise and sport (Isometric muscular contractions do not result in movement because length does not change but require energy expenditure).

Exercise & Sport: Exercise is repetitive physical activity or movements to improve and/or maintain fitness or health. Sport is physical activity or movements involving rules and competition.

Acute effects of physical activity: means (for e.g.) what happens to heart at the end of 10 minutes of cardio or what happens in muscle at end of a workout.

Chronic (adaptive) or long term effects: means (for e.g.) how RHR will be affected at the end of 12 weeks of strength and cardio training program.

Ergometry: Methods used to control or measure work rate or intensity. Experiments done on machines, ergometers in laboratory based experiments. Ergometer measures work rate, enables physiologist to measure energy expenditure.

SI Units (System International Units): Standardized unit of measurement by exercise physiologist.

To convert Into Conversion Factor
mph m.min-1 x 26.8
min/mile mph 60 ÷ min/mile
weight (lb) mass (kg) ÷ 2.2
power (watts) workload – kg/m.min-1 x 6.12
mass (kg) force (Newton) x 9.81
kg/m joules x 9.81
VO2 kcal x 5
kcal joules x 4186
kcal kilojoules x 4.186
VO2 (ml/kg/min) METS ÷ 3.5

Force: is measured in Kilopound (kp). 1kp = force exerted by 1kg mass in normal gravity. SI unit of force is Newton (N). 1kp = 1kg = 9.80665 N.

Work: work = force x distance. Force is measured in Newton (N). Mass is measured in kg.

For example: movement of a 2kg mass over 1 meter distance…..
From the above conversion table, to convert mass into force = x 9.81
2kg x 9.81N = 19.62 N (force). Work = 19.62 N (force) x 1m (distance).
Work = 19.62 Nm (Newton-Meter)
1Nm = 1joule. Therefore 19.62 Nm = 19.62 J. Since 1kg = 1kp, 2kg = 2kp.
Therefore 2kp x 1m = 2kpm = 19.62 J. It means, moving 2kg over 1 meter will require following work to be performed.
Work = 2kp x 1m
= 2kpm (kilopound meters)

Power: also called as work rate. Function of time: Power = work ÷ time.
For example: performance of 150kpm (or kgm) of work in 1 minute will produce work output of: Power = 150 ÷ 1 = 150kpm min-1

SI unit for power is Watts (W). Power output if 150kpm min-1 ÷ 6.12 = 24.51 W-1

Measurement of energy expenditure: Ability to perform physical activity depends upon ability of muscle to convert chemical energy into mechanical energy. This conversion depends upon:

1. Ability of circulatory system to deliver O2 to muscle tissue.
2. Ability of tissue to extract and use O2

Athlete may work at 100% or 50% or some % of maximum capacity of circulatory system or O2 utilization. This work rate or % of max capacity is called Exercise Intensity.
ACSM classifies exercise intensity as either %of Max O2 uptake or Max Heart Rate or in Metabolic Equivalents (METs).

Max O2 uptake = max amount of O2 body can utilize during exercise at sea level.
Work at 100% O2 = Supramaximal (e.g. sprints or 1RM)
1MET = 3.5 ml kg-1 min-1 O2

Other intensity measurements:

Moderate exercise = below lactate threshold. (The intensity at which adequate oxygen is unavailable is referred to as the lactate threshold or anaerobic threshold.)

Heavy exercise = above lactate threshold.

Strength intensity as % of 1RM (limit strength).

Intensity Measurements:

Muscular strength assessment: 1RM – this is generally for advanced lifters/athletes only. For upper body – barbell bench press. For lower body – 450 leg press.
Wilk’s formula – 3 power lifts – add total – apply formula to obtain Wilk’s index.

Muscular endurance: Push-ups (for women modified push-up) max reps. Sit-ups – in 1 minute max.

Cardiovascular endurance: THR test. Dr. Karvonen’s HRR, Cooper’s 12 minute run/walk/swim/cycle test – age wise, Harvard Step test (Heart rate recovery test) age wise, Borg’s perceived exertion rate – rating 6 to 20. (Rating 15-16 = THR 70% MHR), Rockport walking test.

Energy expenditure: Measurement of energy expenditure enables to calculate Metabolic cost of exercise. Energy expensed by body is expressed as heat production. Heat production is measured in calories. (Calories = amount of heat to raise temperature of 1kg of H 2O by 10C. 1 kilocalorie (kc) = 1,000 calories.

Oxygen cost: Oxygen cost of performing work (VO2) depends primarily on work rate. Oxygen cost reflects metabolic cost of exercise. Oxygen cost is measured through indirect calorimetry. Expired air is collected in Douglas bags which is then assessed for
CO2, O2, total quantity, temperature., etc.

O2 uptake is calculated against atmospheric(A) temperature(T) and pressure(P) in the laboratory and water vapor saturation(S) in the captured air. These ATPS volume is corrected to a standard (S) temperature (T: 273 K or 00 C), standard pressure (P: 760 mmHg) and volume that they would be in dry air (D). This gives STPD volume.

Absolute O2 Cost – liters (Lmin-1) or milliliters (ml min-1) of O2 consumed per minute. Used during activity which is not weight bearing – e.g. cycling.

Relative O2 Cost – milliliters of O2 consumed per kilogram of body mass per minute (ml kg-1 min-1). Used during activity which is weight bearing – e.g. running.

Total O2 Cost of movement includes oxygen cost of rest, oxygen cost of moving legs on the cycle ergometer, or whole body on treadmill, plus oxygen cost of performing work.

True O2 Cost is oxygen cost above that of rest and leg and body movement. On cycle, leg movement is measured against zero load/resistance (0W). Oxygen cost of performing additional work is then measured as difference between O2 uptake at a given work rate and that recorded at 0W.

Caloric Cost of exercise provides estimation of metabolic energy utilized in producing skeletal work. Estimated from oxygen cost of work.1 liter of O2 = 5 calories (20kJ).

Maximal Oxygen Uptake/Consumption (VO2 max) is the maximum rate that the body can consume O2 during physical activity at sea level. Can be increased through cardiovascular training Average man 30-55ml kg min-1 Average woman 25-40ml kg
min-1 Endurance athletes may have VO2 max greater than 80ml kg min-1

[vigneshwaran.r]

2. BIO-ENERGETICS FOR MOVEMENT

At start of any activity energy is from Adenosine TriPhosphate (ATP) in the muscles cross-bridges. When used ATP is broken down into Adenosine DiPhosphate (ADP). Creatine Phosphate/Phospho Creatine (PCr) rapidly replaces ATP and indirectly becomes next source of energy. Anaerobic breakdown of glycogen (glycolysis) to form ATP and lactic acid predominates after PCr. Aerobic processes predominate after 60 seconds.

Energy continuum describes the changes in major energy sources with time when exercise is maximal for each of the time phases. Each process takes place simultaneously. E.g., at 20 seconds of exercise at max intensity, 40% of energy is from PCr, 50% from glycolysis and 10% from aerobic processes. At 40 seconds of exercise at max intensity, 5% of energy is from PCr, 80% from glycolysis and 15% from aerobic processes.

Adenosine Triphosphate:

 High energy phosphate consisting of a neucleoside – adenosine to which are attached 3 phosphate molecules using high energy yielding bonds.

 When 1 phosphate molecule is removed from ATP, energy is produced. This energy contracts muscles and enables other molecules to be synthesized or transported against a concentration gradient or excreted.

 ATP is the prime source of energy. 90% of activity is in ATP pathway.

 ATP is hydrolyzed by enzymes called ATPases which result in formation of Adenosine Diphosphate (ADP) and inorganic phosphate (Pi).

 ATP amount is small approximately 20-30mM kh-1 of dry muscle.

Creatine Phosphate or Phosphocreatine:

 High energy phosphate in which single phosphate molecule is attached to a molecule of creatine.

 Hydrolysis of PCr is done through enzyme creatine kinase (ck).

 Above hydrolysis (chemical reaction) is linked to reformation (rephosphorylation) of ATP from ADP.

 CK is found in crossbridges and in mitochondria.

 Hydrolysis of PCr takes place during exercise in crossbridges and during recovery in mitochondria.

 Total creatine pool within muscles – 120 mMkg-1 dry muscle of which PCr is 80% (96 mM kg-1) and actual creatine (Cr) concentration is 20% (24 mMkg-1) under resting conditions. Quantities will change significantly during high intensity exercise.

 This PCr amount will last for 6-8 seconds.

ATP/CP Pathway:

 ATP molecule releases energy for muscle contractions.

 Energy production continuously focuses on rebuilding ATP molecules after they are broken down for energy production.

 All human movements must start with this pathway regardless of its intensity or length.

 Release 1 of 3 phosphate molecules produces this energy. ATP thus turns into ADP.

 Process: ATP = enzyme Myosin ATPase = ADP + Pi + Energy.

 ATP in muscle lasts only for 1.26 seconds (sufficient for 1RM or shot put throw).

 For further activity ADP re-synthesized into ATP.

 Process: ADP + CP = Creatine Kinase = ATP + Creatine.

 CP is in muscle. Depleted in 60 seconds.

 ATP restored in 3 1/2 mins. CP in 8 mins.

 ATP stored more in Type II than Type I muscle fibers because of hypertrophy through training.

Muscle Glycogen: Normal muscle glycogen concentration of 350 mM kg-1 dry muscle. Can be increased through high carb diet but reduced significantly by anaerobic strength endurance or prolonged exercise session or when on low carb diet for 2-4 days.

Glycogenolysis & Glycolysis:

 When muscle breaks down to produce energy, chemical reactions remove glucose molecules under influence of enzyme glycogen phosphorylase.

 This process of breaking glucose molecules from glycogen is Glycogenolysis. Initial by products are glucose 1 phosphate and glucose 6 phosphate.

 Once glucose 6 phosphate is produced glycolysis provides common route for glucose byproducts and blood sugar entering cells. Glycolysis is series of processes which takes place in cytoplasm of cells resulting in formation of ATP and 2 pyruvic acid molecules. Pyruvic acid leads to formation of lactic acid during intense activity and formation of CO2 and water during aerobic activity.

Blood Sugar:

 Glucose delivered to muscles by blood is a useful source of energy. Normal blood sugar concentration is 5mM but may exceed 7-8mM (hyperglycemia) post high carb meal, or reduce below 4mM (hypoglycemia) due to exercising for long periods without ingesting carbs.

 Blood sugar produced in liver via Glycogenolysis of its own glucose stores or from gluconeogenesis where glucose is produced from precursors like lactic acid, alanine, glycerol or pyruvic acid.

Glycolytic Pathway – breakdown of glucose/glycogen:

 After ATP/CP stores are finally depleted, carbs broken down for more ATP.

 ADP is re-converted into ATP using glycogen (in muscle cells) or glucose (in blood) with lactic acid as waste product. This is Non-Oxidative Glycolysis.

 1 glucose (blood) molecule = 2 ATP. 1 glycogen (muscle) molecule = 3 ATP.

 Lactic acid builds faster than it can be flushed out of muscles to point of anaerobic threshold or muscular fatigue. Activity stopped/reduced until acid is removed.

 Lactic acid accumulation ends glycolytic pathway under high intensity conditions at about 80 secs before oxidative pathway starts.

 How well muscle function under glycolytic pathway depend on:
1. How soon you get rid of lactic acid via blood to liver. (Training)
2. How well you tolerate pain caused by lactic acid. (Practice)

Blood lactate levels normalize in 60 mins of activity. Glycolysis (and glycogen) are essential for both anaerobic and aerobic activities.

Aerobic Energy Systems:

 Aerobic energy processes take place in mitochondria of cells. Oxygen must be present to complete the process.

 During steady state exercise, pyruvic acid produced is concerted into acetyl-CoA in the mitochondria and then undergoes oxidation via TCA cycle.

 Fat – another major source of energy during prolonged exercise. Can only be used to produce energy using aerobic processes. Essentially, fats are converted to produce through β-oxidation acetyl-CoA and then enter the TCA cycle.

Oxidative Pathway:

 Activity > 2 mins needs oxidative/aerobic pathway.

 Uses O2 to produce ATP via krebs cycle and electron transplant chain.

 Produces more ATP but takes much longer time.

 β-oxidation: Fatty acids break down to provides co-enzymes for krebs cycle.

 Co-enzymes oxidized to reconvert ADP into ATP.

 Aerobic Glycolysis – 1 glucose molecule = 38 ATP.

 1 glycogen molecule = 39 ATP.

 In krebs cycle, pyruvate (produced during glycolysis) converts several coenzymes (which have lost an electron in earlier chemical processes) to their original molecules.

 In rest and oxidative state, 70% of energy from fat. As intensity increases, more carbs used instead of fat because beta oxidation cannot keep pace.

 In fact at upper limit of aerobic pathway, 100% energy is from carbs, not from fat.

 If carbs not available, body will catabolise muscle (protein) to generate energy.

 Lactic acid cleared from muscle via blood to liver where converted into glucose, glycogen.

 Oxidative energy production takes place in mitochondria where glucose broken down. Waste product pyruvate also broken down in mitochondria.

Rates of energy production:

 Maximal rate of ATP production is related to power developed during exercise or activity.

 It is directly dependent on maximal rate of ATP utilization by muscles.

 These energy sources include those within energy continuum
 ATP
 PCr
 Breakdown of muscle glycogen rapidly resulting in lactic acid formation.
 Aerobic oxidation of carbs, fats.

Process Rate of ATP production
mM kg-1 dry muscle/second
ATP ↔ ADP + P1 12
PCr ↔ Cr + P1 8-10
Glycogen → lactic acid 4
Glycogen/glucose → CO2 + H2O (aerobic) 2
Lipid → CO2 + H2O (aerobic) 1

 Rate of ATP utilization reflects rate of ATP hydrolysis by ATPases at crossbridges. Similarly max rate of ATP from PCr is slower than ATP hydrolysis and is limited by rate of PCr degradation by CK. ATP production from anaerobic glycolysis is approx ½ of that for PCr degradation.

 Rate of ATP production through aerobic sources much slower than that from anaerobic sources. However, total amount of energy from aerobic sources is larger. Carbs produce ATP twice faster than fats.

Energy Metabolism:

 It is series of chemical reactions resulting in breakdown of carbs, proteins, fat, by which energy is produced, used, and given off as heat.

 20% of energy is trapped, 80% released as heat. (That is why body heats during exercise).

 4 physical activity types according to energy systems used.

1. Strength Power: Energy from immediate ATP. 1Rep Max (RM). Lasting 0 to 3 secs. E.g., Shot put, high jump, power lift.

2. Sustained Power: Energy from immediate ATP, CP, Glycolytic (non-aerobic). Near-maximal effort or 0 to 4RM. Lasting 0 to 10 secs. E.g., 100mt sprint.

3. Anaerobic Power Endurance: Energy from ATP, CP, Lactic acid. Lasting about 20 secs to 2 mins. Sub-maximal effort. E.g., 200/400mt dash.

4. Aerobic Endurance: Energy from oxidative pathway. Lasting >2 mins duration. (ATP, CP derived from breakdown of glucose, fat, some protein). E.g., long distance running, swimming, cycling, etc.

3 major energy pathways all of which ultimately produce ATP: 1) ATP/CP, 2) Glycolytic, 3) Oxidative.
Energy for Exercise at Varying Intensities:

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High Intensity Exercise: Strength endurance or non-steady-state exercise not likely to be maintained longer than 5 mins before fatigue commences.

 Major sources of energy are: ATP, PCr, and muscle glycogen.

 These reside in muscle cytoplasm and thus can be used rapidly.

 Fats cannot be used because they are located in mitochondria.

 Studies show that at end of 6 seconds duration exercise, ATP, PCr, and muscle glycogen have depleted by 9%, 35% and 17% respectively. At the end of 30 seconds exercise, depletions are 44%, 66% and 17% respectively.

 Fatigue is inability to sustain necessary exercise intensity associated with high intensity exercise because of lactic acid accumulation and associated with increase in pH due to H+ ions.

 This is because lactic acid splits into lactate ions and hydrogen ions and it is the hydrogen ions which decrease pH.

 Normal resting pH of muscle is around 7. It can go as low as 6.4 in muscle following repeated limit strength outputs. This reduction impairs of certain regulatory enzymes including glycogen phosphorylase, PFK (Phosphofructokinase) and even ATPases. H+ may also directly affect Ca2+ at the crossbridges by denying them binding sites.

 Lactic acid is not the only fatiguing factor. H+ may also directly affect Ca2+ at the crossbridges by denying them binding sites. Increase in inorganic (Pi) and reduction in PCr also contribute to fatigue in high intensity exercise.

 Though lactic acid is considered as problematical byproduct of high intensity exercise, volume of blood lactic acid concentrations at varying exercise intensities indicates aerobic capacity.

 Most prolonged steady state exercise is performed at intensity between 55 and 65% VO2 max. However most endurance athletes can exercise at 70 to 80% VO2 max for prolonged periods of time without accumulating lactic acid.

 Lactic threshold occurs due to following factors:-

 As exercise intensity increases, more glycolytic muscle fibers which favour lactate production are recruited.
 As exercise intensity increases, more pyruvic acid which is formed is converted into lactic acid.
 As exercise intensity increases, there is limited O2 supply to muscle and hence greater anaerobiosis.

 Reasons for why endurance training pushes lactic threshold:-

 Increased lactate removal and reduced lactate production.
 Increased capillarization of muscle enables greater O2 delivery.
 Increased mitochondrial density – hence more oxidative enzymes reduce lactate production and increase lactate removal.
 Slow oxidative and intermediate fibers are influenced in being more oxidative.

Prolonged steady state exercise: Training determines exercise intensity for prolonged activity.

 Major energy source is initially carbohydrate from muscle glycogen and blood glucose.

 As exercise progress, energy is derived through fatty acids from either in intra muscular triglyceride stores or from adipose tissue. In exercise lasting 60 minutes, at about end of 20 minutes, energy production switches from predominantly carbohydrate to predominantly fat oxidation.

 Why 20 minutes? Because it takes this long for lipolysis to be stimulated only after which fatty acids are released under the influence of hormones adrenaline (epinephrine) increasing in quantity and insulin decreasing.

 Simple way of determining contribution of carb and fat to total oxidation is by measuring oxygen uptake and respiratory exchange ratio (RER) which is also referred as Respiratory Quotient (RQ).

Volume of CO2 produced
RER = ------------------------------------
Volume of O2 consumed

 RER of 1 = only carb as energy. RER of 0.7 = only fat as energy source.

 Fatigue in prolonged exercise is due to hypoglycemia (low blood glucose level – 3mM), muscle glycogen depletion or dehydration.

 Carb loading for 3 days leads to elevated muscle glycogen level and extends activity time till fatigue. Low muscle glycogen leads to quicker fatigue.

 Dehydration leads to low electrolyte concentration in muscle even if blood glucose levels are above minimum (4mM).

Intermittent Exercise: Many sports require athlete to engage in repeated short sessions of high intensity activity interspersed with lower level activities.

 Use of match analysis techniques by studying video footage leads to greater understanding of overall and component intensities and types of sport. Further analytical tools – e.g., heart rate recording during play or practice – now available for analysis of intensities and energy pathways.

 Fatigue likely through hypoglycemia, muscle glycogen depletion and dehydration – as in case of prolonged exercise.

Responses to training:

Adaptation: It occurs in muscles as result of repeated bouts exercise over period of time.

Two types of adaptation:-

 Structural – actin and myosin are modified.
 Functional – changes occur in mitochondrial density and cytoplasmic enzyme activity.

Muscle fiber type: Muscle fibers generate energy and power for work to be undertaken.

 Three types based on metabolic and contractile properties:-

 Slow oxidative – SO or Type I fibers have many mitochondria and have a high capillary network supplying blood to them. They are engaged in prolonged, low intensity exercise.
 Fast glycolytic – FG or Type IIb fibers have fewer mitochondria and blood supplying capillaries and thus used in high intensity, short duration bursts.
 Fast oxidative glycolytic – FOG or Type IIa.

 In endurance training, muscle fibers are recruited at lower levels of exercise intensity for prolonged period of time. Energy derived from aerobic sources.

 SO and FOG fibers likely to be recruited rather than FG. Adaptations will occur in only these fibers.

 Adaptations – muscle fiber capillarization, increase in number of cell mitochondria which contain enzymes for TCA cycle and β-oxidation of fatty acids. This leads to improved aerobic capacity.

Muscle Fiber Characteristics:

Contractile SO (Type I) FOG (Type IIa) FG (Type IIb)
Speed of contraction Slow Fast Fast
Force production Low Intermediate High
Fatiguability Low Intermediate high

Metabolic SO (Type I) FOG (Type IIa) FG (Type IIb)
Glycogen stores Low High High
Glycolytic enzyme
activity Low High High
Oxidative enzyme activity High Intermediate low

Sprint training: involves repeated bouts of high intensity efforts interspersed with appropriate recovery periods.

 Since activity is high intensity, short duration (explosive), FG and FOG fibers are mainly recruited to produce force.

 Type of energy source used for each sprint depends on the time – e.g., 6 seconds or 45 seconds or 60 seconds intervals.

 Effect is that enzyme activity for that particular energy source is enhanced with training. Repeat 5-secs bouts are likely to enhance PCr system while repeat 30-secs bouts will improve glycolytic energy system.

3. SKELETAL MUSCLE CONTRACTION & CONTROL

Gross muscle structure: specialized to generate force and thus produce movement.

 Components: 70% water, 22% proteins, 6% lipids.

 Major functions: produce movement, assist in maintain posture, and produce heat.

 These functions due to ability of skeletal muscle to - respond to stimuli, conduct wave of excitation, modify length, and regenerate growth.

 Single muscle constructed of muscle fibers bundled together by collagenous tissue.

 This tissue of 3 layers – endomysium (layer around individual muscle fibers), perimysium (collects bundles of fibers into fascicles), epimysium (protective cover around entire muscle).

 This continuous fascia connects muscle fibers to tendons and to Periosteum of bone.

 This entire unit known as musculotendinous unit.
Muscle fibers:

 Skeletal muscle made thousands of elongated cylindrical cells – muscle fibers or myofibers.

 Vary in length between 1-400 mm and diameter 10-100μm.

[vigneshwaran.r]

Sarcolemma and Sarcoplasm:

 Each myofibers surrounded by plasma membrane – sarcolemma. Allows passive and active transport into the cell and is important factor for muscle excitability.

 Fluid enclosed within cell by sarcolemma is sarcoplasm. Contains fuel sources like lipids, glycogen, organelles like cell nuclei, mitochondria and enzymes, contractile proteins required for muscle contraction.

Myofibrils:

 Myofibrils contained within sarcoplasm. Cylindrical shape, approx 1-2 μm in diameter running lengthwise through muscle fiber. Each fiber can have several hundred to thousand myofibrils.

Myofilaments:

 Myofibrils consist of a bundle of smaller structures called Myofilaments.

 Thin filament – protein actin – approx 6nm in diameter. Thick filament – protein myosin – approx 16nm in diameter. Both represent contractile mechanism in muscles.

 Actin is arranged in two strands inter-twined in helical structure. Within helical grooves are two strands of protein tropomysin upon which are regular intervals sits protein troponin.

 Troponin complex has 3 subunits – troponin I- which binds to actin, troponin C- which binds to calcium ions, troponin T- which binds to tropomysin.

 Thick filament has mostly protein myosin. It has 2-chained helical tail which at one end terminates in 2 large globular heads. These heads, during contraction, known as cross-bridges.

 Contain an ATP binding site which is essential for muscle contraction.

Triad:

 Sarcoplasm also contains hollow membranous system linked to sarcolemma. Assists in conducting neural commands through muscle network.

 System includes – Sarcoplasmic reticulum, terminal cisternae – sacs or pouches, transverse tubules – T-tubules.

 Sarcoplasmic reticulum runs lengthwise along fiber. Surrounds myofibril and at specific points it enlarges into lateral sacs or terminal cisternae.

 Running perpendicular to Sarcoplasmic reticulum are transverse tubules which open to the outside of the fiber.

 One transverse tubule plus two terminal cisternae (one on either side of tubule) from triad.

 Triad essential for rapid communication between sarcolemma and contractile apparatus required for successful muscle contraction.

Sarcomeres:

 Myofibril is constructed from series of sarcomeres added end to end.

 They give fiber striated appearance under microscope.

 Refraction of light under electron microscope helps identify sarcomere structure – different bands, zones,

 At rest, sarcomere is approximately 2.5μm in length and separated from next sarcomere by narrow zones of dense material, Z lines or Z bands.

 The zone containing thick filaments and some intervening thin filaments is A band or dark band.

 Area between A bands is called I band or light band. Contains mainly thin filaments.

 Area within A band where there is no overlapping of thin filaments is H zone. In center of this zone is M line or M band.

 Thin filaments attached to Z lines while thick filaments are attached to M band.

 During muscle contraction myosin cross-bridges pull on actin filaments so that they slide inwards towards H zone.

 The sarcomere shortens as Z bands move towards each other – but length of myofilament does not change. As thin filaments meet at center of sarcomere, H zone narrows or disappears.

 Shortening of muscle fibers by sliding of myofilaments is called sliding filament theory.

Nervous System: Nervous system senses change inside/outside body; interprets changes, initiates responses – muscular contractions, glandular secretions.

 CNS: brain, spinal cord. It receives messages, interprets them and sends back responses.

 PNS: 1) Relays messages from CNS to the body – efferent system: ****tic system – responsible for voluntary action; Autonomic system: processes, activates involuntary action. 2) Relays messages from body to CNS – afferent system – via Proprioceptors in joints, muscles, tendons, inner ear, which pick up messages about body. Exteroceptors along skin surface which receive signals about light, heat, touch, pressure, etc., Interoceptors in blood vessels and viscera, report internal signals of hunger, thirst, pain, pressure, fatigue, nausea, etc.,

 Neuron is cell responsible for conductive nervous impulses. Consists of large cell body containing nucleus, cytoplasmic processes called dendrites, responsible for conducting impulses into cell and a long process called axon which is responsible for conducting impulses away from cell.

Nerve impulse: Often referred to as electrical message sent along neuron. In fact, it is alteration in the difference ionic charge across neuron cell membrane.

 At rest, this charge is negative inside cell and positive outside. This is termed resting membrane potential.

 The impulse is carried down a neuron by reversing this charge (positive inside, negative outside), a process called depolarization.

 The charge across membrane at rest or during depolarization depends upon concentration of ions each side of the membrane. Main ions are sodium (Na+) and potassium (K+).

Synapse: is space between end of neuron and its target.

 Nerve impulse is carried from neuron to neuron or from neuron to muscle or gland.

 Impulse is carried across synaptic cleft by a chemical neuro-transmitter which stimulates the post synaptic membrane in order for impulse to be conducted further.

Neuro-muscular Junction: A neuron stimulating muscle tissue synapses with the sarcolemma of a muscle fiber. This synapse is called neuromuscular junction.

Motor unit: The neuron stimulating muscle tissue and the muscle fibers it innervates are together called motor unit.

Excitation – contraction coupling: The nervous impulse arriving at a muscle is transmitted down the T tubes to the triads where a series of chemical events causes muscular contraction. This conversion of an impulse into attachment of cross-bridges is termed excitation – contraction coupling.

Cross-bridge cycle: The attachment of the myosin head to actin filament and its rotation causing the filaments to slide across each other is cross-bridge cycle. ATP powers this cycle and depends on calcium ions (Ca2+) from the sarcopasmic reticulum.

Proprioception: Neural control of contraction and consequent movement of body and limbs depend upon CNS receiving feedback about where the body parts are in relation to one another.

 This feedback is provided to CNS by receptors in the periphery termed proprioceptors.

 They provide CNS with orientation and speed of limb movements.

 Proprioceptors can be free nerve endings, Golgi-type receptors or pancinian corpuscles.

 Free nerve endings are sensitive to touch and pressure and thus stimulated at beginning of movement. Become less sensitive as movement continues.

 Golgi-type receptors are position receptors located in ligaments around joints.

 Pancinian corpuscles are located in tissue surrounding joints. Detect rate of rotation.

 Skeletal muscle contains its own receptors to assist in fine motor control – chemoreceptors, muscle spindles, Golgi tendon organs.

 Chemoreceptors are free nerve endings that provide feedback to CNS regarding pH, K+ concentration and O2 and CO2 tension in muscle.

 Muscle spindle provides feedback about muscle length in static and dynamic contractions.

 Golgi tendon organs located within muscle tendon, monitor muscle tension.

 Muscles requiring fine motor control – e.g., hand – have greater number of muscle spindles. Spindles run parallel to muscle fibers and have neurons that synapse at spinal chord. Thus receptors feedback to PNS without involving higher brain centers.

 This feedback arc is mytatic or stretch reflex.

Major Difference Between Skeletal Muscle Fiber Types:

Characteristics Type IIb Type IIa Type I
Force production High High Slow
Predominant energy system Anaerobic Combination Aerobic
Fatigue resistance Low High/Moderate High
Myosin ATPase High High Low
Oxidative enzyme activity Low High High
Phosphocreatine stores High High Low
Glycogen stores High High Low
Mitochondrial density Low High High
Fiber diameter Large Large Small
Capillary density Low Medium High
Speed of shortening High Intermediate Low
Nerve conduction velocity High High Low

Fiber type distribution:

 Average composition of muscle fibers in legs and arms – 45 to 55% slow twitch fibers and equal number of Type IIa and Type IIb fast twitch fibers.

 Other muscles may vary considerably in composition according to function. Postural muscles will have more fatigue resistant slow twitch fibers.

 Endurance athletes have larger proportion of slow twitch fibers in muscles generally used for their sport – e.g., 90 to 95% in Gastrocnemius in long distance runners and skiers. Weight lifters, sprinters have greater percentage of fast twitch fibers while middle distance runners have equal distribution.

 Muscle fibers adapt their molecular composition in response to training. One type of fiber can transition into another.

Thermoregulation: Maintenance of constant internal body environment – Homeostasis.

 Works on feedback systems like a thermostat – e.g., temperature control. If temperature exceeds 98.6F due to activity, brain signals increased sweating rate. In cold climate, shivering raises temperature.

 Physiological and biomechanical processes are very temperature sensitive. Small changes may lead to injury, illness or death. Internal heat balance is tightly controlled – Homeothermic.

 Heat is generated through internal metabolism. More heat is produced when metabolism rises.

 Heat gain and loss pathways – conduction, convection, radiation depending on the temperature difference between skin and environment. Effective for heat loss in thermo-neutral environment.

 Core temperature – around 370C. Any deviation will initiate heat loss or gain measures.

 Important to assess peripheral and core temperature. Easier to measure peripheral (skin) than core (under tongue, in armpit, eso****us, rectum, etc. which are distal from hypothalamus).

 Conduction: heat transfer from one object to another through direct contact.

 Convection: heat transfer between body and a liquid or gas that moves around the warm or cool body.

 Radiation: loss of heat through movement of infrared rats from skin to environment. Net gain of heat through reverse radiation.

 Evaporation: of water from body surface leads to heat loss – sweating is critical to heat loss.

 Loss of 1 liter of water results in transfer of about 2500 KJ of heat to environment.

Heat loss Heat gain
Cutaneous vasoconstriction Cutaneous vasodilation
Sweating Horripilation (goose bumps)
Shivering
Anti-diuretic hormone Catecholamines
Aldosterone Thyroxine

[LearnToGetFit]

Thanks for the notes. I sold my exercise physiology book many years ago and now I can use your notes as a reference.