KINES 350 – Exercise Physiology PDF

KINES 350 – Exercise Physiology Penn State University Extensor Skeletal muscle fiber types Digitorum...

KINES 350 – Exercise Physiology Penn State University Extensor Skeletal muscle fiber types Digitorum Longus Soleus Cells are labeled for laminin (green) and myosin heavy chain (red) Wang et al., EMBO Reports, 2019 Objectives  Explain how muscles are fiber typed  Be able to discriminate the histological and functional characteristics of slow, oxidative (Type I); fast, oxidative- glycolytic (Type IIa); and fast, glycolytic (Type IIx) muscle fibers  Explain the implications of different muscle fiber type distributions between muscles, athletic populations, and in response to exercise training and aging  Key words – slow twitch, fast twitch, oxidative, glycolytic, muscle biopsy, muscle recruitment, all-or-none, fatigability 1 KINES 350 – Exercise Physiology Penn State University Checking in … switching to Top Hat Physical and functional properties distinguishing muscle fibers  Morphology – Number and proportion of fibers – Cross-sectional area of fibers  Function – Maximal force production  Force per unit of cross-sectional area (i.e., “specific tension”) – Speed of contraction (Vmax)  Regulated by myosin ATPase activity – Maximal power output  force x shortening velocity  High force, fast fibers have high power output – Muscle fiber efficiency  amount of ATP used to generate force  efficient fibers use less ATP to generate a given force 2 KINES 350 – Exercise Physiology Penn State University Physiological and biochemical properties distinguishing muscle fibers  Abundance of contractile protein in the muscle fiber – Actin and myosin  Ability to use oxygen, aka “oxidative capacity” – Number of capillaries, mitochondria – Amount of myoglobin – Abundance of metabolic enzymes  Type of myosin ATPase isoform – Speed of ATP hydrolysis Checking in … switching to Top Hat 3 KINES 350 – Exercise Physiology Penn State University Muscles are biopsied to collect samples for fiber typing  A muscle sample is collected under local anesthesia – Punch (needle) - most common – Open - most controlled  Analytical methods Punch muscle biopsy technique – Morphology  Size, number, etc. – Physiology / function  Force, contraction velocity, fatigability, etc. – Identification of myosin heavy chain isoforms  Immunohistochemistry  Immunofluoresence  Gel electrophoresis Open muscle biopsy technique Fiber type names are based on their classification scheme  Type I fibers RED = Slow-twitch = Slow, oxidative (SO)  Type IIa fibers = Intermediate = Fast, oxidative-glycolytic (FOG) = Fast, fatigue resistant (FR)  Type IIx fibers WHITE = Fast-twitch = Fast, glycolytic (FG) = Fast, fatigable (FF)  Type IIb fibers – Similar to IIx in their characteristics Blue = Type I fibers – not found in humans Green = Type IIa fibers Black = Type IIx fibers Red = dystrophin (sarcolemmal protein) 4 KINES 350 – Exercise Physiology Penn State University Methods to determine (phenotype) skeletal muscle fiber type based on myosin On the left-head side of this representation of the myosin head… Atoms in the myosin heavy chain (MHC) are colored pink. The MHC includes the actin and ATP binding regions Atoms in the light chains are colored pale-orange and pale-yellow. This region binds calcium, which can affect force of contraction  Histochemical staining – Isoforms of myosin ATPase  Functionally similar proteins that have a similar, but not an identical, amino acid sequence – Also used for metabolic enzymes (glycolysis, Krebs cycle, etc)  Immunochemical identification of myosin heavy chain (MHC) isoforms – Antibodies for the myosin heavy chain separated by gel electrophoresis  Immunohistochemistry  Immunofluorescence – Quantitative determination of protein – Capable of identifying mixed (“chimeric”) fibers  These methods do not always align – Fibers may be categorized differently depending on the method of analysis – Differences across species may not be detected – “State of art” is to identify the myosin heavy chains Histochemical (ATPase) staining  ATPase staining performed on serial tissue sections can be used to identify fiber types – Preincubation pH inactivates some myosin ATPase  “Acid reversal” reveals at least two types of fibers with different ATPase activities – Alkaline (pH 10.8) medium stains type II fibers darkly – Acid (pH 4.5) medium stains type I fibers darkly, IIa fibers intermediate and IIb lightest. – Acid (pH 4.3) medium stains type I fibers darkly and type IIa and IIb lightest  Extremely complex and time demanding laboratory procedure A – type IIb D – type IIa I – type I 5 KINES 350 – Exercise Physiology Penn State University Other stains can be used to describe metabolic features of muscle fibers  Glycogen  Glycolytic enzymes  Krebs cycle enzymes  etc. Immunocytochemistry The upper row shows antibody staining for type I fibers. The bottom row shows another antibody staining for type IIa fibers. 6 KINES 350 – Exercise Physiology Penn State University Immunofluorescence imaging Blue: type I Green: type IIA Purple: IIX Red: type IIB While more expensive and technically complex to perform, immunofluorescence requires only a single section and significantly simplifies imaging and automated fiber typing N.B. - With 4 different MHC, this muscle cannot be from a human Characteristics of Muscle Fiber Types (Table 8.2) Slow-twitch Fast-twitch a Fast-twitch b/x Type I Type IIa Type IIb/x Slow, Oxidative Fast, Oxidative-glycolytic Fast, glycolytic Number of mitochondria High High/moderate Low SR Calcium handling least more extensive extensive Motor units 10-180 fibers 180-300 fibers 300-800 fibers Motor unit force Low High High Nerve conduction slow fast fast Contractile speed (Vmax) slow Fast Fast Specific Tension Moderate High High less frequent Recruitment most frequent (tonic) least frequent (phasic) (phasic) Fatigability Low High/Moderate High Predominant energy system Oxidative Oxidative/Glycolytic Phosphagen ATPase activity least more most Oxidative capacity High Moderate Low Glycolytic capacity Low High Highest Efficiency High Intermediate Low 7 KINES 350 – Exercise Physiology Penn State University Checking in … switching to Top Hat Compared to SO/I fibers, FG/IIx fibers …  contract 3x faster  generate almost >50% more force  produce 6x more power  are similar in size – In normal muscle, there is less than 12% difference in the largest mean fiber diameter between all three muscle fiber types 8 KINES 350 – Exercise Physiology Penn State University Distribution of fiber types in muscles  One muscle biopsy is not representative of the entire body  Type I is generally the most prevalent fiber type in postural muscles – Typical for a sedentary individual (gastrocnemius)  50% SO  25% FOG  25% FG Distribution of fiber types among athletes  In general, highest proportion of SO/I fibers in endurance athletes  SO/I prevalence is positively associated with maximal aerobic Bergh et al, Med Sci Sports Exerc 10:151, 1978 power Jessie Diggins  Considerable variation among Olympic gold medalist 2018 athletes 9 KINES 350 – Exercise Physiology Penn State University Developmental and sex considerations  Normal adult muscle fiber size is reached between the ages of 12 and 15 years – Can be further increased with resistance exercise training  Both type I and type II (IIa and IIx) adult muscle fibers are larger in men than women – In men, type II fibers are usually larger than type I – In women, type I fibers are usually larger than type II  These differences are largely attributable to [testosterone] Caster Semenya (RSA) 2012 & 2016 Olympic Gold Medalist Women’s 800m Checking in … switching to Top Hat 10 KINES 350 – Exercise Physiology Penn State University Sprint Fiber type is one of many variables associated with athletic success  28 world-class cyclists – Sprint test: 30 sec “all out” (Wingate) – Endurance test: 15 km time trial Endurance – vastus lateralis biopsy  Power athletes – Sprint and team pursuit – Percentage of Type II MHC an important predictor of sprint success  Endurance athletes – Road cyclists – Higher percentage of Type I MHC – O2 transport better predictor than fiber type van de Zwaard et al, FASEB J, 2018 Skeletal muscle is plastic Genetic, trophic, and environmental factors affect muscle composition  Major determinants of muscle fiber proportions ! – Genetics  identical twins have similar fiber types – Innervation  All fibers can be modified by changing innervation  Major determinants of muscle fiber sizes – Larger with exercise training (aka, hypertrophy) – Smaller with disuse and disease (aaka, trophy)  Overall impact of environment / lifestyle / disease  Smaller decrease in Type II number, and increase in cross-sectional – Exercise training (endurance or resistance) – shift from fast to slow area and shortening velocity  For both fiber types, peak force and power are reduced  Increase in strength, muscle mass and fiber cross-sectional area  FG/IIx fiber prevalence decreases – Aging – shift from fast to slow  FOG/IIa fiber prevalence and cross-sectional area increases  Sarcopenia – Decrease in strength, muscle mass and fiber cross-  SO/I fibers may increase with endurance training sectional area – Disuse (e.g., spinal cord injury, spaceflight) – shift from slow to fast  increase in percentage (not necessarily number) of SO fibers  selective loss of FG/IIx fibers  Large (30%!) and variable decreases in Type I cross-sectional area, peak force and velocity – Disease – shift from fast to slow  Distribution with cancer and diabetes patients is similar to aging 11 KINES 350 – Exercise Physiology Penn State University Take home messages  Human skeletal muscle fiber types can be divided into three general classes of fibers based on their biochemical and contractile properties. – Two categories of fast fibers: type IIx and type IIa – One type of slow slow fiber: type I  Although classifying skeletal muscle fibers into three general groups is a convenient system to study the properties of muscle fibers, it is important to appreciate that human skeletal muscle fibers exhibit a wide range of contractile and biochemical properties. – The biochemical and contractile properties of type IIx, type IIa, and type I fibers result in a continuum of muscle performance instead of three neat packages.  Muscle fiber type and its distribution are plastic. Contractile, biochemical, and molecular properties adapt to stress; they can be modified by (dis)use, aging and disease Attributions  Original presentation prepared by – Edward T Howley, PhD, U Tennessee – Brian B Parr, PhD, U South Carolina, Aiken – Scott K Powers, PhD, EdD, U Florida  Copyright – McGraw-Hill Education  This presentation has been extensively modified and tailored exclusively to this class. It is for your educational use and should not be shared for other purposes. 12