Characteristics of Muscle

1. Characteristics of Muscle • Irritability • Ability to respond to stimulation • Contractility • Ability to shorten when it receives sufficient stimulation • Unique to muscle tissue • Extensibility • Ability to stretch/lengthen beyond resting length • Protective mechanism • Elasticity • Ability to return to resting length after being stretched • Protective mechanism

2. Groups of Muscles • Muscles typically act in unison – not individually • Fascia • Sheet of fibrous tissue • Compartmentalizes groups of muscles

3. Individual Muscle Organization • Belly • Thick central portion • Epimysium • Outside covering of a muscle • Fascicles • Bundles of muscle fibers • Perimysium • Dense connective sheath covering a fascicle • Fibers • Cells of a skeletal muscle

4. Individual Muscle Organization (cont.) • Endomysium • Very fine sheath covering individual fibers • Sarcolemma • Thin plasma membrane branching into muscle • Myofibrils • Rod-like strands of contractile filaments • Many sarcomeres in series • Sarcoplasma • Cytoplasm of muscle cell • Sarcoplasmic reticulum • Specialized endoplasmic reticulum of muscle cells

5. Individual Muscle Organization (cont.) • T-tubules • Extension of sarcolemma that protrudes into muscle cell • Also called, transverse tubule • Myosin • Thick, dark filament • Actin • Thin, light filament • Sarcomere • Unit of myosin and actin • Contractile unit of muscle

6. Fiber Organization • Fusiform • Parallel fibers and fascicles • High speed of contract, force production • ACS = PCS • Anatomical Cross-Section (ACS) • Physiological Cross-Section (PCS) • Sartorius, biceps brachii, brachialis

7. Fiber Organization • Penniform • 3 Types • Unipennate • Off one side of tendon • Semimembranosus • Bipennate • Off both sides of tendon • Gastrocnemius • Multipennate • Both varieties • Deltoid • PCS > ACS

8. Fiber Type • Type I • Slow twitch, oxidative • Red (because of high myoglobin content) • Endurance athletes • Type IIa • Intermediate fast-twitch, oxidative-glycolytic • Type IIb • Fast twitch, glycolytic • White • Sprinters, jumpers

9. Muscle Attachment • 3 ways muscle attaches to bone • Directly • Via a tendon • Via an aponeurosis • Tendon • Inelastic bundle of collagen fibers • Aponeurosis • Sheath of fibrous tissue • Origin • More proximal attachment • Insertion • More distal attachment

10. Characteristics of a Tendon • Transmits muscle force to associated bone • Can withstand high tensile loads • Viscoelastic stress-strain response • Myotendinous junction • Where tendon and muscle join

11. Functions of Muscle • Produce movement • Maintain postures and positions • Stabilize joints • Other functions • Support and protect visceral organs • Alter and control cavity pressure • Maintain body temperature • Control entrances/exits to the body

12. Role of Muscle • Prime mover • Muscle(s) primarily responsible for a given movement • Assistant mover • Other muscles contributing to movement • Agonist • Muscles creating same joint movement • Antagonist • Muscles opposing joint movement • Stabilizer • Holds one segment still so a specific movement in an adjacent segment can occur • Neutralizer • Muscle working to eliminate undesired joint movement of another muscle

13. Muscle Actions • Isometric • Tension produced without visible change in joint angle • Holding arms out to sides • Concentric • Muscle visibly shortens while producing tension • Up phase of a sit-up • Eccentric • Muscle visibly lengthens while producing tension • Lowering phase of squat

14. Stretch-Shortening Cycle • Pre-stretch • Quick lengthening of a muscle before contraction • Generates greater force than contraction alone • Utilizes elastic component of muscle • Prestretch & Fiber Type • Type I • Slower pre-stretch best because of slow cross-bridging • Type II • Faster pre-stretch best because of fast cross-bridging

15. Plyometrics • Conditioning protocol that utilizes pre-stretching • Single-leg bounds, depth jumps, stair hopping

16. One- and Two-Jointed Muscles • Muscles can cross one or two joints • One-Jointed Muscles • Brachialis, pectoralis major • Two-Jointed Muscles (biarticulate) • Save energy • Gastrocnemius, hamstrings, biceps brachii

17. Two-Jointed Muscles RF: Better leg extensor, H: better hip extensor

18. Motor Unit • Group of muscles innervated by the same motor neuron • From 4 to 2000 muscle fibers per motor unit • Action potential • Signal to contract from motor neuron • Neuromuscular junction • Also called end plate • Where action potential from neuron meets muscle fiber • Conduction velocity • Velocity at which action potential is propagated along membrane

19. Muscle Contraction • Resting potential • Voltage across the plasma membrane in a resting state • Excitation-Contraction Coupling • Transmission of action potential along sarcolemma • Twitch • Rise and fall reaction from a single action potential • Tetanus • Sustained muscle contraction from high-frequency stimulation

20. Muscle Contraction(cont.) • Depolarization • Loss of polarity • Repolarization • Movement to the initial resting (polarized) state • Hyperpolarization • State before repolarization

21. Sliding Filament Theory • A.F. Huxley • Seeks to explain production of tension in muscle • Myosin & actin • Create cross-bridges • Slide past one another • Cause the sarcomere to contract

22. Mechanical Model of Muscle • A.V. Hill • 3 Component Model • Contractile (CC) • Converts stimulation into force • Parallel elastic (PEC) • Allows the muscle to be stretched • Associated with fascia surrounding muscle • Series elastic (SEC) • Transfers muscle force to bone

23. Factors Influencing Muscle Force • Angle of attachment • Force-time characteristics • Force increases non-linearly due to elastic components • Length-tension relationship • Force-velocity relationship

24. Electromyography • Correct placement of electrodes is critical • Electrodes oriented parallel to muscle fiber • Prepare skin by… • Shaving • Abrading • Cleaning with alcohol • Signal must be amplified • Sample rate ≥ 1000 Hz required

25. Electromyography (cont.) • Rectification • Taking the absolute value of raw signal • Linear envelope • Time domain • Frequency domain • Electromechanical delay (EMD) • Temporal delay between onset of EMG signal & development of muscle tension

26. Muscle Fatigue • Fatigue results from… • Peripheral (muscular) mechanisms • Central (nervous) mechanisms • When motor unit fatigues… • Change in frequency content • Change in amplitude of EMG signal • Sufficient rest restores initial signal content and amplitude

27. Clinical Gait Analysis • EMG used to investigate which muscle group is used in a certain phase of gait • Determine activation order • Raw or rectified EMG signal is used

28. Ergonomics • Ergonomics • EMG used to investigate effects of… • Sitting posture • Carrying loads

29. Principles of Training • Genetic predisposition • Training specificity • Intensity • Rest • Volume

30. Strength Training and the Nonathlete • ACSM • 2 days per week • 8–12 exercises per day • Counteracts atrophy of muscle and bone • Elderly • Children • High-intensity not recommended • Epiphyseal plates susceptible to injury under high loads

31. Training Modalities • Isometric • No visible movement • Rehabilitation • Isotonic • Same weight throughout range of motion (ROM) • Isokinetic • Same velocity, varied resistance • Close-linked • Isotonic, in which one segment is fixed in place • Variable resistive • Supposedly overloads muscle throughout ROM

32. Injury to Skeletal Muscle • At risk • Two-jointed muscles at greatest risk of strain • Eccentrically contracted to slow limb movement • Hamstrings, rotator cuffs • Fatigued or weak muscles • When performing unique task for first time • Already injured • Prevention • Warm-up • Build up when starting new program • Recognize signs of fatigue • Give body adequate rest

33. Summary • Characteristics of muscle tissue • Irritability, contractility, extensibility, elasticity • Often act in compartmentalized groups • Fiber organization • Fusiform, penniform • Fiber types • Type I, IIa, IIb • Functions of muscle • Produce movement, maintain postures, stabilize joints, and others