Muscle Structure and Function Chapter 4 Sport Books Publisher

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 fibers To describe group action of muscles Sport Books Publisher

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

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 create movement of body segments Sport Books Publisher

B. Smooth Muscle Located in the blood vessels, respiratory tract, iris of the eye, gastro-intestinal (GI) tract Contractions are slow and uniform Functions to alter the activity of various body parts to meet the needs of the body at that time Fatigue resistant Activation is involuntary Sport Books Publisher

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) Very fatigue resistant Activation of cardiac muscle is involuntary (like smooth muscle) Sport Books Publisher

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

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

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

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

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 Longitudinal section of myofibril (a) At rest Sport Books Publisher

High microscope magnification of sarcomeres within a myofibril Sport Books Publisher

Contractile Machinery: Crossbridge formation and movement Cross bridge formation: - A signal comes from the motor nerve activating the fiber - Heads of the myosin filaments temporarily attach themselves to the actin filaments 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 Longitudinal section of myofibril b) Contraction Sport Books Publisher

Contractile Machinery: Optimal Crossbridge formation Longitudinal section of myofibril Sarcomeres should be optimal distance apart For muscle contraction: optimal distance is (0.0019 to 0.0022 mm) At this distance an optimal number of cross bridges is formed If the sarcomeres are stretched farther apart 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

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 Maximal muscle force occurs at optimal joint angle Optimal joint angle occurs at optimal muscle length Sport Books Publisher

Muscle tension during elbow flexion at constant speed Sport Books Publisher

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 point of attachment of the muscle to the bone that does not move Insertion: the point of attachment of the muscle on the bone that moves Sport Books Publisher

Muscle Fiber Types Fast twitch fibers: Slow twitch fibers: Slow Oxidative (Type I) Fast twitch fibers: Fast Glycolytic (Type IIb) Fast Oxidative Glyc. (Type IIb) Sport Books Publisher

A. Slow Twitch Fibers Suited for repeated contractions during activities requiring a force output of less than 20 to 25 percent of max force output Examples: lower power activities, endurance events Sport Books Publisher

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

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

Muscle Biopsy Sport Books Publisher

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

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

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 fibers Sport Books Publisher

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

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 fibers to contract Every motor unit has a specific threshold that must be reached for such activation to occur Sport Books Publisher

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

Intramuscle Coordination Trained athletes have not only a larger muscle mass than untrained individuals, but can also exploit a larger number of muscle fibers Athletes are more restricted in further developing strength by improving intramuscular coordination Trained individuals can further increase strength only by increasing muscle diameter Sport Books Publisher

Intermuscle Coordination The interplay between muscles that generate movement through contraction (agonists) and muscles responsible for opposing movement (antagonists) is called intermuscle coordination The greater the participation of muscles and muscle groups, the higher the importance of intermuscle 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

Intermuscle Coordination High-level intermuscle 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 intermuscle coordination Sport Books Publisher

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 makeup Enzymes adapt within hours, cardiovascular adaptation within 10 to 14 days Sport Books Publisher