Chapter 8Skeletal Muscle: Structure

and Function

EXERCISE PHYSIOLOGYTheory and Application to Fitness and Performance,

6th edition

Scott K. Powers & Edward T. Howley

Skeletal Muscle

• Human body contains over 400 skeletal muscles– 40-50% of total body weight

• Functions of skeletal muscle– Force production for locomotion and breathing– Force production for postural support– Heat production during cold stress

Connective Tissue Covering Skeletal Muscle

• Epimysium– Surrounds entire muscle

• Perimysium– Surrounds bundles of muscle fibers

• Fascicles

• Endomysium– Surrounds individual muscle fibers

Connective Tissue Surrounding Skeletal Muscle

Figure 8.1

Microstructure of Skeletal Muscle

• Sarcolemma – Muscle cell membrane

• Myofibrils – Threadlike strands within muscle fibers

• Actin (thin filament)• Myosin (thick filament)

• Sarcomere– Includes Z-line, M-line, H-zone, A-band, I-band

• Sarcoplasmic reticulum– Storage sites for calcium

• Transverse tubules

Microstructure of Skeletal Muscle

Figure 8.2

The Sarcoplasmic Reticulum and Transverse Tubules

Figure 8.3

The Neuromuscular Junction

• Junction between motor neuron and muscle fiber• Motor end plate

– Pocket formed around motor neuron by sarcolemma

• Neuromuscular cleft – Short gap between neuron and muscle fiber

• Acetylcholine is released from the motor neuron– Causes an end-plate potential (EPP)

• Depolarization of muscle fiber

The Neuromuscular Junction

Figure 8.4

Muscular Contraction

• The sliding filament model– Muscle shortening occurs due to the movement

of the actin filament over the myosin filament– Formation of cross-bridges between actin and

myosin filaments • Power stroke

– Reduction in the distance between Z-lines of the sarcomere

The Sliding

Filament Theory

Figure 8.5

The Relationships Among Troponin, Tropomyosin, Myosin, and Calcium

Figure 8.6

Energy for Muscle Contraction

• ATP is required for muscle contraction– Myosin ATPase breaks down ATP as fiber

contracts

• Sources of ATP– Phosphocreatine (PC)– Glycolysis– Oxidative phosphorylation

Sources of ATP for Muscle Contraction

Figure 8.7

Excitation-Contraction Coupling

• Depolarization of motor end plate (excitation) is coupled to muscular contraction– Action potential travels down T-tubules and

causes release of Ca+2 from SR– Ca+2 binds to troponin and causes position

change in tropomyosin • Exposing active sites on actin

– Strong binding state formed between actin and myosin

– Contraction occurs

Muscle Excitation,

Contraction, and

Relaxation

Figure 8.9

Figure 8.10

Steps Leading to Muscular Contraction

Muscle Fatigue

• Decrease in muscle force production– Reduced ability to perform work

• Contributing factors:– High-intensity exercise (~60 sec)

• Accumulation of lactate, H+, ADP, Pi, and free radicals

– Long-duration exercise (2–4 hours)• Muscle factors

– Accumulation of free radicals– Electrolyte imbalance– Glycogen depletion

• Central Fatigue– Reduced motor drive to muscle from CNS

Muscle Fatigue

Figure 8.8

Characteristics of Muscle Fiber Types

• Biochemical properties– Oxidative capacity

• Number of capillaries, mitochondria, and amount of myoglobin

– Type of myosin ATPase• Speed of ATP degradation

• Contractile properties– Maximal force production

• Force per unit of cross-sectional area

– Speed of contraction (Vmax)

• Myosin ATPase activity– Muscle fiber efficiency

Characteristics of Individual Fiber Types

• Type IIx fibers– Fast-twitch fibers – Fast-glycolytic fibers

• Type IIa fibers– Intermediate fibers– Fast-oxidative glycolytic fibers

• Type I fibers– Slow-twitch fibers– Slow-oxidative fibers

Characteristics of Muscle Fiber Types

Table 8.1

Comparison of Maximal Shortening Velocities Between Fiber Types

Figure 8.12

Type I

Histochemical Staining of Fiber Type

Figure 8.11

Type IIa Type IIx

Fiber Types and Performance

• Nonathletes – Have about 50% slow and 50% fast fibers

• Power athletes – Sprinters– Higher percentage of fast fibers

• Endurance athletes – Distance runners– Higher percentage of slow fibers

Exercise-Induced Changes in Skeletal Muscles

• Strength training– Increase in muscle fiber size (hypertrophy)– Increase in muscle fiber number (hyperplasia)

• Endurance training– Increase in oxidative capacity

• Alteration in fiber type with training– Fast-to-slow shift

• Type IIx IIa • Type IIa I with further training

– Seen with endurance and resistance training

Effects of Endurance Training on Fiber Type

Figure 8.13

Muscle Atrophy Due to Inactivity

• Loss of muscle mass and strength– Due to prolonged bed rest, limb immobilization, reduced

loading during space flight• Initial atrophy (2 days)

– Due to decreased protein synthesis• Further atrophy

– Due to reduced protein synthesis• Atrophy is not permanent

– Can be reversed by resistance training– During spaceflight, atrophy can be prevented by

resistance exercise

Age-Related Changes in Skeletal Muscle

• Aging is associated with a loss of muscle mass– 10% muscle mass lost between age 25–50 y– Additional 40% lost between age 50–80 y– Also a loss of fast fibers and gain in slow fibers– Also due to reduced physical activity

• Regular exercise training can improve strength and endurance– Cannot completely eliminate the age-related loss

in muscle mass

Types of Muscle Contraction

• Isometric– Muscle exerts force without changing length– Pulling against immovable object– Postural muscles

• Isotonic (dynamic)– Concentric

• Muscle shortens during force production

– Eccentric• Muscle produces force but length increases

Muscle Actions

Table 8.3

Type of exercise Muscle Muscle Action Length

Change

_________________________________________________

Dynamic Concentric Decreases

Eccentric Increases

Static Isometric No Change

Isometric and Isotonic Contractions

Figure 8.14

Speed of Muscle Contraction and Relaxation

• Muscle twitch– Contraction as the result of a single stimulus– Latent period

• Lasting ~5 ms

– Contraction • Tension is developed• 40 ms

– Relaxation• 50 ms

• Speed of shortening is greater in fast fibers– SR releases Ca+2 at a faster rate– Higher ATPase activity

Muscle Twitch

Figure 8.15

Force Regulation in Muscle

• Force generation depends on:– Types and number of motor units recruited

• More motor units = greater force• Fast motor units = greater force

• Initial muscle length– “Ideal” length for force generation– Increased cross-bridge formation

• Nature of the neural stimulation of motor units– Frequency of stimulation

• Simple twitch • Summation• Tetanus

Relationship Between Stimulus Strength and Force of Contraction

Figure 8.16

Length-Tension

Relationships in Skeletal

Muscle

Figure 8.17

Simple Twitch, Summation, and Tetanus

Figure 8.18

Force-Velocity Relationship

• At any absolute force the speed of movement is greater in muscle with higher percent of fast-twitch fibers

• The maximum velocity of shortening is greatest at the lowest force– True for both slow and fast-twitch fibers

Muscle Force-Velocity Relationships

Figure 8.19

Force-Power Relationship

• At any given velocity of movement the power generated is greater in a muscle with a higher percent of fast-twitch fibers

• The peak power increases with velocity up to movement speed of 200-300 degrees•second-1

– Power decreases beyond this velocity because force decreases with increasing movement speed

Muscle Force-Power Relationships

Figure 8.20

Receptors in Muscle

• Provide sensory feedback to nervous system– Tension development by muscle– Account of muscle length

• Muscle spindle

• Golgi tendon organ

Muscle Spindle

• Responds to changes in muscle length• Consists of:

– Intrafusal fibers • Run parallel to normal muscle fibers (extrafusal fibers)

– Gamma motor neuron • Stimulate intrafusal fibers to contract with extrafusal

fibers (by alpha motor neuron)

• Stretch reflex– Stretch on muscle causes reflex contraction

• Knee-jerk reflex

Muscle Spindles

Figure 8.21

Golgi Tendon Organ (GTO)

• Monitor tension developed in muscle– Prevents muscle damage during excessive force

generation

• Stimulation results in reflex relaxation of muscle– Inhibitory neurons send IPSPs to muscle fibers

Golgi Tendon Organ

Figure 8.22