Skeletal Muscle Lectures


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Skeletal Muscle Physiology

Objectives
1. Structure & function of skeletal muscle
2. Training for power
• Anaerobic • Aerobic
3. Strength training

Gross Structure
¾ Long multi-nucleated fibers
¾ Levels of organization:
1. Endomysium: wraps each fiber 2. Perimysium: surrounds several fibers (up to 150)
and forms bundles called a fasciculus 3. Epimysium: surrounds all the bundles to form the
entire muscle
¾ Tendons:
• Connective tissue (periosteium of bone to muscle)
9 Origin – more stable bone; Insertion – moving bone

Gross Structure (cont.)
¾ Sarcolemma:
• Muscle cell membrane
¾ Satellite cells:
• myogenic stem cells located within the sarcolemma
9Help regenerative cell growth 9Play a role in hypertrophy
¾ Sarcoplasmic Reticulum:
• Extensive lattice-like network of tubules and vesicles
9Provides structural integrity 9Stores, releases, and reabsorbs Ca2+

Figure 18.1
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Chemical Composition
¾ 75% water
¾ 20% protein
• Myosin • Actin • Tropomyosin & Troponin • Myoglobin
¾ 5% salts, phosphates, ions, and macronutrients

Blood Supply
¾ Skeletal muscle has a rich vascular network
• Enhanced capillarization with training
9Increased capillary-to-muscle-fiber ratio
¾ Flow is rhythmic during aerobic activity
• Vessels compressed during contraction phase • Vessels open during relaxation phase
¾ During sustained contractions > 60% capacity
• Occludes localized blood flow (elevated intramuscular pressure)
• Anaerobic processes supply ATP

Skeletal Muscle Ultrastructure

Skeletal Muscle Ultrastructure

Muscle Fiber Alignment
¾ Long Axis of a Muscle
• From origin to insertion • Determines fiber arrangement
1. Fusiform 2. Pennate 3. Bipennate
¾ Fiber arrangement influences:
• Force generating capacity • Physiological cross sections (PCSA)
9 Sum of cross-sectional areas of all the fibers within the particular muscle

Figure 18.5
2

¾ The degree of pennation directly affects the number of sarcomeres per cross-sectional area

• Allows for packing a large number of fibers into a

small cross-sectional area

9Able to generate considerable power Greater

Rapid muscle

force & power

shortening

Fiber Length:Muscle Length Ratio

Figure 18.4

¾ Fusiform muscles (long fibers) show a longer working range and lower maximum force output
¾ Pennate muscles (short fibers) show a shorter working range & approximately double the force output

Actin-Myosin Orientation
¾ Actin filaments lie in a hexagonal pattern around myosin
¾ Cross-bridges spiral around the myosin where actin and myosin overlap
Figure 18.6

Myosin Thick filament
Myosin molecule

Actin Thin filament
Cross bridge

Actin helix
3

Actin-Myosin Orientation (cont.)
¾ Tropomyosin: lies along actin in the groove formed by the double helix
• Covers cross-bridge binding site
¾ Troponin is embedded at regular intervals along actin
• Interacts with Ca2+ • Moves tropomyosin, revealing binding sites

Tropomyosin

Troponin

Thin filament

Intracellular Tubule Systems
¾ The sarcoplasmic reticulum is distributed around the myofibrils such that each sarcomere has 2 triads
¾ Each triad contains:
• 2 vesicles • 1 T-tubule

Surface membrane of muscle fiber

Myofibrils I band

A band

Segments of sarcoplasmic reticulum
Transverse (T) tubule
I band

Sliding Filament Model
¾ Contraction occurs as myosin and actin slide past one another
¾ Myosin cross-bridges cyclically attach, rotate, and detach from actin filaments
¾ Energy is provided by ATP hydrolysis

Myosin cross bridge

Z line

BINDING Myosin cross bridge binds to actin molecule.

Cross bridge bends, pulling thin myofilament inward.

DETACHMENT Cross bridge detaches at end of movement and returns to original conformation.
BINDING Cross bridge binds to more distal actin molecule; cycle repeated.

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Figure 18.9

Mechanical Action of Cross-bridges
¾ Myosin Cross-bridge contain actin-activated ATPase
¾ Provides ability for mechanical movement
¾ Cross-bridging performs repeated, nonsynchronous pulling or ratcheting

Thin myofilament

Thick myofilament

Figure 18.10

Sarcomere Length-Isometric Tension Curves
Figure 18.11

Link Between Actin, Myosin, & ATP
1. Myosin head bends around ATP molecule and becomes ready for movement
2. Myosin interacts with actin
3. ATP is hydrolyzed
4. Energy release forces the bound sight to move

5

Excitation-Contraction Coupling
¾ The electrical discharge at the muscle that initiates the chemical events at the muscle cell surface
• Release of Ca2+

Neuromuscular Junction (REVIEW)

Axon of motor neuron Axon terminal

Action potential propagation in motor neuron

Vesicle of acetylcholine Acetylcholine receptor site
Acetycholinesterase

Voltage-gated calcium channel
Action potential propagation in muscle fiber

Voltage-gated Na+ channel
Chemically gated cation channel

Motor end plate

T tubule Surface membrane of muscle cell

Troponin Tropomyosin

Actin

Cross-bridge binding

Myosin cross bridge

Relaxation
¾ Ca2+ is actively pumped back into SR
¾ Troponin allows tropomyosin to interfere with actin-myosin interaction

Muscle Fiber Type
¾ Two distinct fiber types identified by characteristics:
1. Contractile 2. Metabolic

Slow-Twitch Fibers: TYPE I
¾ Low myosin ATPase activity ¾ Slower Ca2+ release and reuptake by SR ¾ Low glycolytic capacity ¾ Large number of mitochondria

6

Fast-Twitch Fibers: TYPE II
¾ High capacity to transmit AP
¾ High myosin-ATPase activity
¾ Rapid release and reuptake of Ca2+ by SR
¾ High rate of cross-bridge turnover
¾ Capable of high force generation
¾ Rely on anaerobic metabolism
• ATP-PCr • Glycolysis

Fast-Twitch Subdivisions
¾ IIa Fibers
• Fast shortening speed • Moderately well-developed capacity for both
anaerobic and aerobic energy production
¾ IIb Fibers
• Most rapid shortening velocity • Rely on anaerobic energy production

Fiber Type Differences Among Athletic Groups
¾ Large individual difference in fiber type distribution
¾ Endurance athletes:
• > TYPE I fibers
• Some as high as 90–95% in gastrocnemius
¾ Speed and power athletes:
• > TYPE II fibers
¾ Middle distance athletes:
• More even fiber distribution

Fiber Type vs. VO2max
Figure 18.17

Muscular Strength Training

Measurement of Muscle Strength
¾ Cable tensiometry:

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Measurement of Muscle Strength (cont.)
¾ Dynamometry
• One-repetition maximum (1-RM) • Estimations of 1-RM

Measurement of Muscle Strength (cont.)
¾ Computer-assisted, electromechanical, and isokinetic methods
• Isokinetic dynamometer

Strength-testing considerations
¾ Standardize pre-testing instructions
¾ Uniformity of warm-up
¾ Adequate practice
¾ Standardize testing protocol
• Body position, size & composition • Joint angles • Reps (pre-determined minimum number of trials) • Scoring criteria (select tests with high reproducibility)

Figure 22.3

Gender Differences
¾ Several applied approaches to determine whether or not a gender difference exists:
¾ Based on evaluation of:
1. Muscle’s cross-sectional area 2. Absolute basis of total force exerted 3. Architectural characteristics 4. Relative to body mass or FFM

Greater CSA = greater strength
Figure 22.4
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Equal strength per CSA

Absolute muscle strength greater in men
*muscle mass distribution

Little difference in strength when expressed in relative terms
Figure 22.6

Training for Strength Improvement
¾ Muscles need to be trained close to its current force-generating capacity
• Overload Principle
¾ Systematic approach to the Overload Principle:
• Progressive resistance training • Isokinetic training • Isometric training

Types of Muscle Contractions
¾ Progressive resistance, isokinetic & isometric training relies on 3 different muscle actions:
1. Concentric action
9 Muscle shortening
2. Eccentric action
9 Muscle lengthening
3. Isometric action
9 No net change in muscle length

Types of Muscle Contractions

Concentric

Eccentric

Isometric

9

Resistance Training for Children?

Progressive Resistance Training
¾ Progressive resistance exercise general recommendations (ACSM Guidelines):
1. Between 3-RM to 12-RM to improve strength 2. Additional 1-RM lifts once per week may
significantly increase strength 3. One set is effective if 10-RM is used
9 Produce most of the health benefits 9 Increase compliance 9 2 – 3 days/week is most effective
4. Faster rate of movement improves strength over a slower rate (generally)

Periodization
¾ Incorporates 4 distinct phases:
1. Preparation phase
9 Modest strength development 9 Focus on high volume, low intensity
2. First transition phase
9 Emphasis on strength development 9 Focus on moderate volume, moderate intensity
3. Competition phase
9 Selective strength development 9 Focus on low volume, high intensity
4. Second transition phase (active recovery)
9 Recreational activities & low intensity workouts

Macrocycles, Mesocycles & Microcycles

4 Phases of Periodization

Practical Recommendations (Program Initiation)
¾ Avoid maximal lifts initially ¾ Use 12-RM to 15-RM initially ¾ Increase weight after 2 weeks
• Use 6 – 8 RM • Progress gradually
¾ Work larger muscle groups first & progress to smaller muscle groups
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Skeletal Muscle Lectures