Download Muscle Fiber

Chapter 4
Muscular Adaptations to Training
Copyright © 2012 American College of Sports Medicine
Muscle Types
• Cardiac
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Heart walls
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Contracts involuntarily with strong force
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Responsible for creating rhythmic pressure & moving blood
• Smooth
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Walls of hollow organs & blood vessels
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Contracts involuntarily, causing constriction
• Skeletal
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40% of body mass
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Contracts voluntarily, rapidly
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Roles of Skeletal Muscles
• Functions
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Produce tension
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Act on bones to produce movement
• Attachments
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Proximal (origin): closest to midline
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Distal (insertion): farthest from midline
• Movement at Joints
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Uniarticular: produce movement at one joint
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Multiarticular: produce movement at two joints
Copyright © 2012 American College of Sports Medicine
Roles of Skeletal Muscles (cont’d)
• Agonist
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A muscle that contracts to perform a specific movement
• Antagonist
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A muscle that opposes agonist movement
• Stabilizer or Fixator
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A muscle that contracts to stabilize point of origin or insertion for
a corresponding muscle
• Neutralizer
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A muscle that contracts to eliminate one movement of a
multiarticular muscle
Copyright © 2012 American College of Sports Medicine
Skeletal Muscle Gross Anatomy
• Skeletal Muscle Is Designed to:
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Generate high levels of force efficiently
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Transmit forces effectively to bone
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Skeletal Muscle Gross Anatomy (cont’d)
• Myofilament
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Basic structural unit of skeletal muscle
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Composed of contractile proteins actin & myosin
• Sarcomere
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Basic functional unit of muscle
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Composed of myofilaments
• Myofibril: bundle of sarcomeres
• Muscle Fiber: bundle of myofibrils
• Fascicle: bundle of muscle fibers
Copyright © 2012 American College of Sports Medicine
Skeletal Muscle Anatomy
Copyright © 2012 American College of Sports Medicine
Muscle Fiber Organization
• Sarcolemma: cell membrane surrounding muscle fiber
• T tubules: propagate action potential into muscle fiber
• Sarcoplasm
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Liquid cytoplasm surrounding myofibrils
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Contains enzymes, fat, glycogen, mitochondrion, nuclei
• Sarcoplasmic reticulum: lattice of conductile tissue
• Terminal cisternae: lateral sacs that store calcium
• Nucleus: contains genetic material, involved in protein
synthesis
Copyright © 2012 American College of Sports Medicine
Muscle Fiber Organization (cont’d)
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Structure of a Sarcomere
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The Myofibrillar Proteins
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Muscle Fiber Organization (cont’d)
• Muscle Contraction: The Sliding Filament Theory
– Thick & thin filaments slide past each other w/o changing length
– Sarcomere shortens in series
– Muscle fibers & muscle belly shorten
– Results in movement & force generation
– Three stages
• Excitation-contraction coupling
• Cross-bridge cycling
• Relaxation
Copyright © 2012 American College of Sports Medicine
Crossbridge Cycling
Copyright © 2012 American College of Sports Medicine
Muscle Fiber Organization (cont’d)
• Skeletal Muscle’s Graded Responses
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Twitch: contraction of muscle in response to a stimulus
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Summation: effect of multiple twitches
• Spatial summation: activation of multiple motor units
thereby contributing to force production
• Temporal summation: motor unit increases frequency of
discharge to increase force
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Tetanus: maximal force production when summation is peaked
Copyright © 2012 American College of Sports Medicine
Skeletal Muscle Characteristics and
Adaptations
• Muscle Fiber Formation: Myogenesis
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Critical to normal muscle function
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Replacement of old/damaged muscle fibers in tissue remodeling
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Satellite cells (stem cells):
• Are released from basal lamina
• Migrate to area of fiber formation
• Proliferate
• Differentiate into myoblasts
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Myoblasts fuse to form myotubules, which mature into new fibers
Copyright © 2012 American College of Sports Medicine
Skeletal Muscle Characteristics and
Adaptations (cont’d)
• Muscle Fiber Types
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Slow-Twitch (ST) or Type I: endurance fibers
• Type I
• Type IC
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Fast-Twitch (FT) or Type II: strength/power fibers
• Type IIC
• Type IIA
• Type IIAX
• Type IIX
Copyright © 2012 American College of Sports Medicine
Fiber-Type Continuum With Force and
Endurance Ratings
Copyright © 2012 American College of Sports Medicine
Skeletal Muscle Characteristics and
Adaptations (cont’d)
• Fiber Types in Athletes
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Ratio of fiber types in a muscle determines muscle’s functional
capacity
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Each muscle has distinct ratio & may favor one type or the other
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Endurance athletes: larger % of type I
• Long- & middle-distance runners
• Cyclists
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Strength/power athletes: larger % of type II
• Sprinters, throwers, weightlifters, jumpers
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Genetic predisposition
Copyright © 2012 American College of Sports Medicine
Skeletal Muscle Characteristics and
Adaptations (cont’d)
• Fiber-Types Transitions
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With RT, transitions take place from IIX to IIA
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Transition causes more force to be produced over time
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Detraining results in transition back to IIX
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No evidence yet of transitions between types I & II
Copyright © 2012 American College of Sports Medicine
Fiber-Type Transitions With Training and
Detraining
Copyright © 2012 American College of Sports Medicine
Skeletal Muscle Characteristics and
Adaptations (cont’d)
• Muscle Hypertrophy
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Increase in muscle size
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Common adaptation to anaerobic training, especially RT
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Larger muscle = stronger muscle
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Results from:
• Increase in protein synthesis
• Decrease in protein breakdown
• Combination of two
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Muscle Growth in the Quadriceps Muscle
Following RT
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Skeletal Muscle Characteristics and
Adaptations (cont’d)
• Muscle Hypertrophy and Fiber Types
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Hypertrophy occurs in both ST & FT muscle fibers
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Higher growth potential in FT fibers
• Factors Influencing Muscle Hypertrophy
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Mechanical
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Circulatory
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Nutritional
• Muscle Hypertrophy and Other Training Modalities
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Sprint & power training increase muscle size to lesser extent
than RT
Copyright © 2012 American College of Sports Medicine
Skeletal Muscle Characteristics and
Adaptations (cont’d)
• Hyperplasia
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Longitudinal splitting of existing muscle fibers
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Results in increased number of muscle cells
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May occur via increase satellite cell proliferation after muscle
damage
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Shown in lab animals, but controversial in humans
Copyright © 2012 American College of Sports Medicine
Skeletal Muscle Characteristics and
Adaptations (cont’d)
• Structural Changes to Muscle
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Structural changes can cause increases in:
• Strength
• Power
• Size
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RT increases:
• Endurance
• # of myofibrils
• Density of sarcoplasm, sarcoplasmic reticulum, & T tubules
• Sodium-potassium ATPase pump activity
Copyright © 2012 American College of Sports Medicine
Skeletal Muscle Characteristics and
Adaptations (cont’d)
• Other Changes to Skeletal Muscle
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Sprint & RT
• Increase anaerobic substrate content
• Increase muscle’s buffer capacity
• Alter enzyme activity
• Up-regulates anabolic hormone receptors (RT)
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Aerobic training
• Increases activity of aerobic enzymes
• Increases mitochondrial & capillary density (decreased by RT)
Copyright © 2012 American College of Sports Medicine