Muscle Structure and Function

1. Muscle Structure and Function Sport Books Publisher

2. Learning Objectives • To describe muscle’s macro and micro structures • To explain the sliding-filament action of muscular contraction • To differentiate among types of muscle fibres • To describe group action of muscles Sport Books Publisher

3. Skeletal muscle Cardiac muscle Smooth muscle Types of Muscle • The human body is comprised of 324 muscles • Muscle makes up 30-35% (in women) and 42-47% (in men) of body mass. Three types of muscle: Sport Books Publisher

4. A. Skeletal (Striated) Muscle • Connects the various parts of the skeleton through one or more connective tissue tendons • During muscle contraction, skeletal muscle shortens and moves various parts of the skeleton • Through graded activation of the muscles, the speed and smoothness of the movement can be gradated • Activated through signals carried to the muscles via nerves (voluntary control) • Repeated activation of a skeletal muscle can lead to fatigue • Biomechanics: assessment of movement and the sequential pattern of muscle activation that move body segments Sport Books Publisher

5. B. Smooth Muscle • Located in the blood vessels, the respiratory tract, the iris of the eye, the gastro-intestinal tract • The contractions are slow and uniform • Functions to alter the activity of various body parts to meet the needs of the body at that time • Is fatigue resistant • Activation is involuntary Sport Books Publisher

6. C. Cardiac Muscle • Has characteristics of both skeletal and smooth muscle • Functions to provide the contractile activity of the heart • Contractile activity can be gradated (like skeletal muscle) • Is very fatigue resistant • Activation of cardiac muscle is involuntary (like smooth muscle) Sport Books Publisher

7. Components of skeletal muscle d) myofibril c) muscle fibre b) muscle fibre bundle a) Muscle belly Sport Books Publisher

8. Muscle Fibres • Cylinder-shaped cells that make up skeletal muscle • Each fibre is made up of a number of myofilaments • Diameter of fibre (0.05-0.10 mm) • Length of fibre (appr. 15 cm) • Surrounded by a connective tissue sheath called Sarcolemma • Many fibres are enclosed by connective tissue sheath Perimycium to form bundle of fibres • Each fibre contains contractile machinery and cell organelles • Activated through impulses via motor end plate • Group of fibres activated via same nerve: motor unit • Each fibre has capillaries that supply nutrients and eliminate waste Sport Books Publisher

9. Muscle Teamwork • Agonist (prime mover): - the muscle or group of muscles producing a desired effect • Antagonist: - the muscle or group of muscles opposing the action • Synergist: - the muscles surrounding the joint being moved • Fixators: - the muscle or group of muscles that steady joints closer to the body axis so that the desired action can occur Sport Books Publisher

10. Bending or straightening of elbow requires the coordinated interplay of the biceps and triceps muscles Sport Books Publisher

11. Longitudinal section of myofibril (a) At rest Contractile Machinery:Sarcomeres • Contractile units • Organized in series ( attached end to end) • Two types of protein myofilaments: - Actin: thin filament - Myosin: thick filament • Each myosin is surrounded by six actin filaments • Projecting from each myosin are tiny contractile myosin bridges Sport Books Publisher

12. High microscope magnification of sarcomeres within a myofibril Sport Books Publisher

13. Contractile Machinery:Crossbridge formation and movement • Cross bridge movement: • - similar to the stroking of the oars and movement of rowing shell • - movement of myosin filaments in relation to actin filaments • - shortening of the sarcomere • - shortening of each sarcomere is additive • Cross bridge formation: - a signal comes from the motor nerve activating the fibre - the heads of the myosin filaments temporarily attach themselves to the actin filaments Longitudinal section of myofibril b) Contraction Sport Books Publisher

14. Contractile Machinery:Optimal Crossbridge formation Longitudinal section of myofibril • Sarcomeres should be optimal distance apart • For muscle contraction: optimal distance is (0.0019-0.0022 mm) • At this distance an optimal number of cross bridges is formed • If the sarcomeres are stretched fartherapart than optimal distance: - fewer cross bridges can form  less force produced • If the sarcomeres are too close together: - cross bridges interfere with one another as they form  less force produced c) Powerful stretching d) Powerful contraction Sport Books Publisher

15. Contractile Machinery:Optimal muscle length and optimal joint angle • The distance between sarcomeres is dependent on the stretch of the muscle and the position of the joint • Maximal muscle force occurs at optimal muscle length (lo) • Maximal muscle force occurs at optimal joint angle • Optimal joint angle occurs at optimal muscle length Sport Books Publisher

16. Muscle tension during elbow flexion at constant speed Sport Books Publisher

17. Contractile Machinery:Tendons, origin, insertion • In order for muscles to contract, they must be attached to the bones to create movement • Tendons: strong fibrous tissues at the ends of each muscle that attach muscle to bone • Origin: the end of the muscle attached to the bone that does not move • Insertion: the point of attachment of the muscle on the bone that moves Sport Books Publisher

18. Fast twitch fibres: Fast Glycolytic (Type IIb) Fast Oxidative Glyc. (Type IIb) Slow twitch fibres: Slow Oxidative (Type I) Muscle Fibre Types Sport Books Publisher

19. A. Slow Twitch Fibers • Suited for repeated contractions during activities requiring a force output of < 20-25% of max force output • Examples: lower power activities, endurance events Sport Books Publisher

20. Slow twitch fibers • High aerobic (oxidative) capacity and fatigue resistance • Low anaerobic (glycolytic) capacity and motor unit strength • Slow contractile speed (110 ms) and myosin ATPase • 10 to 180 fibers per motor neuron • Low sarcoplasmic reticulum development • Large posterior muscles of the neck. Sport Books Publisher

21. B) Fast Twitch Fibers • Significantly greater force and speed generating capability than slow twitch fibres • Well suited for activities involving high power • Examples: sprinting, jumping, throwing Sport Books Publisher

22. Fast Twitch muscle fibers • Type IIa Moderate aerobic (oxidative) capacity and fatigue resistance • High anaerobic (glycolytic) capacity and motor unit strength • Fast contractile speed (50 ms) and myosin ATPase • 300 to 800 fibers per motor neuron • High sarcoplasmic reticulum development • Fast twitch type IIa fibers are suited to speed, strength and power type activities, such as moderately heavy weight training (8-12 reps) and fast running events such as the 400metres. Sport Books Publisher

23. Fast Twitch Muscle fibers. • Type IIb Low aerobic (oxidative) capacity and fatigue resistance • High anaerobic (glycolytic) capacity and motor unit strength • Fast contractile speed (50 ms) and myosin ATPase • 300 to 800 fibers per motor neuron • High sarcoplasmic reticulum development • Found in the arms. Sport Books Publisher

24. Fast Twitch muscle fibers. • Fast twitch IIb fibers contract extremely rapidly, create very forceful muscle contractions and fatigue quickly.  Fast twitch IIb fibers are also ‘white fibers’ but unlike IIa fibers they can only use anaerobic energy sources.  • Like type IIa fibers the fast twitch type IIb fibers are also suited to speed, strength and power type activities.  Heavy weight training (1-3 reps), power lifting, and 100metre sprints are examples of activities that predominantly require IIb fibers. Sport Books Publisher

25. The Muscle Biopsy • Used to determine muscle fibre type 1. Injection of local anesthetic into the muscle being sampled 2. Incision of approximately 5-7mm is made in the skin and fascia of the muscle 3. The piece of tissue (250-300mg) removed via the biopsy needle is imbedded in OCT compound 4. The sample is frozen in isopentane cooled to –180C Sport Books Publisher

26. Muscle Biopsy Sport Books Publisher

27. Muscle Biopsy Sport Books Publisher

28. The Histochemistry • The biopsy samples are first sectioned (8-10 μm thickness) • Sections are processed for myosin ATPase: Fast twitch fibres – rich in myosin ATPase (alkaline labile) Slow twitch fibres – low in myosin ATPase (acid labile) • Sections are processed for other metabolic characteristics Sport Books Publisher

29. Nerve-Muscle Interaction • Skeletal muscle activation is initiated through neural activation • NS can be divided into central (CNS) and peripheral (PNS) • The NS can be divided in terms of function: motor and sensory activity • Sensory: collects info from the various sensors located throughout the body and transmits the info to the brain • Motor: conducts signals to activate muscle contraction Sport Books Publisher

30. Activation of motor unit and its innervation systems • Spinal cord 2. Cytosome 3. Spinal nerve • 4. Motor nerve 5. Sensory nerve 6. Muscle with muscle fibres Sport Books Publisher

31. Motor Unit • Motor nerves extend from the spinal cord to the muscle fibres • Each fibre is activated through impulses delivered via motor end plate • Motor unit: a group of fibres activated via the same nerve • All muscle fibres of one particular motor unit are always of the same fibre type • Muscles needed to perform precise movements generally consist of a large number of motor units and few muscle fibres • Less precise movements are carried out by muscles composed of fewer motor units with many fibres per unit Sport Books Publisher

32. All-or-none Principle • Whether or not a motor unit activates upon the arrival of an impulse depends upon the so called all-or-none principle • An impulse of a certain magnitude (or strength) is required to cause the innervated fibres to contract • Every motor unit has a specific threshold that must be reached for such activation to occur Sport Books Publisher

33. Intra-muscle Coordination • The capacity to apply motor units simultaneously is known as intra-muscle coordination • Many highly trained power athletes, such as weightlifters, wrestlers, and shot putters, are able to activate up to 85% of their available muscle fibres simultaneously (untrained: 60%) • Force deficit: the difference between assisted and voluntarily generated maximal force (trained: 10%, untrained: 20-35%) Sport Books Publisher

34. Intra-muscle Coordination cont. • Trained athletes have not only a larger muscle mass than untrained individuals, but can also exploit a larger number of muscle fibres • Athletes are more restricted in further developing strength by improving intra-muscular coordination • Trained individuals can further increase strength only by increasing muscle diameter Sport Books Publisher

35. Inter-muscle Coordination • The interplay between muscles that generate movement through contraction (agonists) and muscles responsible for opposing movement (antagonists) is called inter-muscle coordination • The greater the participation of muscles and muscle groups, the higher the importance of inter-muscle coordination • To benefit from strength training the individual muscle groups can be trained in relative isolation • Difficulties may occur if the athlete fails to develop all the relevant muscles in a balanced manner Sport Books Publisher

36. Inter-muscle Coordination cont. • High-level inter-muscle coordination greatly improves strength performance and also enhances the flow, rhythm, and precision of movement • Trained athlete is able to translate strength potential to enhance inter-muscle coordination Sport Books Publisher

37. Muscle’s Adaptation to Strength Training • Individual’s performance improvements occur through a process of biological adaptation, which is reflected in the body’s increased strength • Adaptation process proceeds at different time rates for different functional systems and physiological processes • Adaptation depends on intensity levels used in training and on athlete’s unique biological make-up • Enzymes adapt within hours, cardiovascular adaptation within 10 to 14 days Sport Books Publisher