Chapter 4 Muscular Adaptations to Training

Chapter 4

Muscular Adaptations to Training

Copyright © 2012 American College of Sports Medicine

Muscle Types

• Cardiac

– Heart walls

– Contracts involuntarily with strong force

– Responsible for creating rhythmic pressure & moving blood

• Smooth

– Walls of hollow organs & blood vessels

– Contracts involuntarily, causing constriction

• Skeletal

– 40% of body mass

– Contracts voluntarily, rapidly

– Contains many nuclei—which is critical to the exercise adaptation process


Roles of Skeletal Muscles

• Functions

– Produce tension

– Act on bones to produce movement

– Produce body heat

– Maintain posture

– Assist with communication—aids in ventilation

• Attachments

– Proximal (origin): closest to midline

– Distal (insertion): farthest from midline

• Movement at Joints

– Uniarticular: produce movement at one joint

– Multiarticular: produce movement at two joints


Roles of Skeletal Muscles (cont’d)

• Agonist

– A muscle that contracts to perform a specific movement

• Antagonist

– A muscle that opposes agonist movement

• Stabilizer or Fixator

– A muscle that contracts to stabilize point of origin or insertion for a corresponding muscle

• Neutralizer

– A muscle that contracts to eliminate one movement of a multiarticular muscle


Skeletal Muscle Gross Anatomy

• Skeletal Muscle Is Designed to:

– Generate high levels of force efficiently

– Transmit forces effectively to bone


Skeletal Muscle Gross Anatomy (cont’d)

• Myofilament

– Basic structural unit of skeletal muscle

– Composed of contractile proteins actin (thin) & myosin (thick)

• Sarcomere

– Basic functional unit of muscle

– Composed of myofilaments

• Myofibril: bundle of sarcomeres

• Muscle Fiber (muscle cell): bundle of myofibrils

(diameter of 10-100 um and up to 30 cm long).

• Fascicle: bundle of muscle fibers


Schematic Drawing of a Muscle

• Figure 1.1 (next slide)

– Schematic drawing of a muscle illustrating three types of connective tissue:

• Epimysium (the outer layer)

• Perimysium (surrounding each fasciculus, or group of fibers)

• Endomysium (surrounding individual fibers)

• Fascia: endomysium, perimysium, and epimysium


Skeletal Muscle Anatomy


Muscle Fiber Organization

• Sarcolemma: cell membrane surrounding muscle fiber

• T tubules: propagate action potential into muscle fiber

• Sarcoplasm

– Liquid cytoplasm surrounding myofibrils

– 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—

The muscle fiber has many peripherally located nuclei.


Muscle Fiber Organization (cont’d)


Structure of a Sarcomere


The Myofibrillar Proteins



• Muscle Contraction: The Sliding Filament Theory

• Model of muscle contraction proposed by Hugh Huxley &

Andrew Huxley in the early 1950’s.

– 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 [the electrical signal propagated along the motor nerve must be carried along the muscle fibers—at a high speed]

• Cross-bridge cycling [numerous cross-bridges must form until the sarcomere shortens]

• Relaxation [begins with a reduction in neural stimuli]


Crossbridge Cycling



• Skeletal Muscle’s Graded Responses

– Twitch: contraction of muscle in response to a stimulus

– 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

– Tetanus: maximal force production when summation is peaked


Skeletal Muscle Characteristics and

Adaptations

• Muscle Fiber Formation: Myogenesis

– Critical to normal muscle function

– Replacement of old/damaged muscle fibers in tissue remodeling

– Satellite cells (stem cells):

• Are released from basal lamina

• Migrate to area of fiber formation

• Proliferate

• Differentiate into myoblasts

– Myoblasts fuse to form myotubules, which mature into new fibers

– Myostatin inhibits the process of myogensis


Myostatin Inhibition



Adaptations (cont’d)

• Muscle Fiber Types

– Slow-Twitch (ST) or Type I: (red) endurance fibers

• Type I

• Type IC

– Fast-Twitch (FT) or Type II: (white) strength/power fibers

• Type IIC

• Type IIA

• Type IIAX

• Type IIX


Fiber-Type Continuum With Force and

Endurance Ratings




• Fiber Types in Athletes

– Ratio of fiber types in a muscle determines muscle’s functional capacity

– Each muscle has distinct ratio & may favor one type or the other

– Endurance athletes: larger % of type I

• Long- & middle-distance runners

• Cyclists

– Strength/power athletes: larger % of type II

• Sprinters, throwers, weightlifters, jumpers

– Genetic predisposition


The Gastrocnemius is predominantly FT while the soleus is predominantly

ST.




• Fiber-Types Transitions

– With RT, transitions take place from IIX to IIA

– Transition causes more force to be produced over time

– Detraining results in transition back to IIX

– No evidence yet of transitions between Types I & II


Fiber-Type Transitions With Training and

Detraining




• Muscle Hypertrophy

– Increase in muscle size

– Common adaptation to anaerobic training, especially RT

– Larger muscle = stronger muscle

– Results from:

• Increase in protein synthesis

• Decrease in protein breakdown

• Combination of two


Muscle Hypertrophy

• P-65. “Some have postulated that muscles grow mostly while the athlete is working out. Although the stimulus for muscle growth is the workout, muscles actually grow during the recovery period postexercise.”

• Protein breakdown is common while one is lifting but protein synthesis (and muscle growth) occurs during the recovery period.


Muscle Growth in the Quadriceps Muscle

Following RT




• Muscle Hypertrophy and Fiber Types

– Hypertrophy occurs in both ST & FT muscle fibers

– Higher growth potential in FT fibers

• Factors Influencing Muscle Hypertrophy

– Mechanical [tension/force produced by muscle fibers]

– Eccentric muscle actions [p-66]

– Creatine and water content of skeletal muscle

– Circulatory [blood flow/ and-or restriction of blood flow]

– Nutritional [diet, nutrient timing, supplements and anabolic drug use]

• Muscle Hypertrophy and Other Training Modalities

– Sprint & power training increase muscle size to lesser extent than RT




• Hyperplasia

– Longitudinal splitting of existing muscle fibers

– Results in increased number of muscle cells

– May occur via increase satellite cell proliferation after muscle damage

– Shown in lab animals, but controversial in humans




• Structural Changes to Muscle

– Structural changes can cause increases in:

• Strength [F X D]

• Power [F X D/T]

• Size [Hypertrophy]

– RT increases:

• Endurance

• # of myofibrils

• Density of sarcoplasm, sarcoplasmic reticulum, & T tubules

• Sodium-potassium ATPase pump activity




• Other Changes to Skeletal Muscle

– Sprint & RT

• Increase anaerobic substrate content

• Increase muscle’s buffer capacity

• Alter enzyme activity

• Up-regulates anabolic hormone receptors (RT)

– Aerobic training

• Increases activity of aerobic enzymes

• Increases mitochondrial & capillary density (decreased by RT)


Chapter 4 Muscular Adaptations to Training

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