1. Muscle Structure and Function
Chapter 4
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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 fibers
To describe group action of muscles
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3. 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
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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 create movement of body segments
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5. 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
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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)
Very fatigue resistant
Activation of cardiac muscle is involuntary (like smooth muscle)
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7. Components of skeletal muscle
d) myofibril c) muscle fiber b) muscle fiber bundle a) Muscle belly
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8. 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
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9. 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
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10. Bending or straightening of the elbow requires the coordinated interplay of the biceps and triceps muscles.
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11. 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
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12. High microscope magnification of sarcomeres within a myofibril
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13. 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
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14. Contractile Machinery: Optimal Crossbridge formation
Longitudinal section of myofibril
Sarcomeres should be optimal distance apart
For muscle contraction: optimal distance is ( to 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
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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
Maximal muscle force occurs at optimal joint angle
Optimal joint angle occurs at optimal muscle length
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16. Muscle tension during elbow flexion at constant speed
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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 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
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18. 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)
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19. 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
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20. 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
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21. 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
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22. Muscle Biopsy
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23. 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
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24. 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
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25. Activation of motor unit and its innervation systems
Spinal cord Cytosome Spinal nerve
4. Motor nerve Sensory nerve 6. Muscle with muscle fibers
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26. 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
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27. 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
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28. 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)
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29. 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
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30. 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
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31. 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
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32. 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
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