1. Chapter 8 Skeletal Muscle: Structure and Function
EXERCISE PHYSIOLOGY
Theory and Application to Fitness and Performance, 6th edition
Scott K. Powers & Edward T. Howley

2. 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

3. Connective Tissue Covering Skeletal Muscle
Epimysium
Surrounds entire muscle
Perimysium
Surrounds bundles of muscle fibers
Fascicles
Endomysium
Surrounds individual muscle fibers

4. Connective Tissue Surrounding Skeletal Muscle
Figure 8.1

5. 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

6. Microstructure of Skeletal Muscle
Figure 8.2

7. The Sarcoplasmic Reticulum and Transverse Tubules
Figure 8.3

8. 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

9. The Neuromuscular Junction
Figure 8.4

10. 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

11. The Sliding Filament Theory
Figure 8.5

12. The Relationships Among Troponin, Tropomyosin, Myosin, and Calcium
Figure 8.6

13. 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

14. Sources of ATP for Muscle Contraction
Figure 8.7

15. 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

16. Muscle Excitation, Contraction, and Relaxation
Figure 8.9

17. Steps Leading to Muscular Contraction
Figure 8.10

18. 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

19. Muscle Fatigue
Figure 8.8

20. 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

21. 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

22. Characteristics of Muscle Fiber Types
Table 8.1

23. Comparison of Maximal Shortening Velocities Between Fiber Types
Figure 8.12

24. Histochemical Staining of Fiber Type
Type I
Type IIa
Type IIx
Figure 8.11

25. 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

26. 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

27. Effects of Endurance Training on Fiber Type
Figure 8.13

28. 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

29. 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

30. 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

31. Muscle Actions Type of exercise Muscle Muscle Action Length Change
_________________________________________________
Dynamic Concentric Decreases
Eccentric Increases
Static Isometric No Change
Table 8.3

32. Isometric and Isotonic Contractions
Figure 8.14

33. 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

34. Muscle Twitch
Figure 8.15

35. 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

36. Relationship Between Stimulus Strength and Force of Contraction
Figure 8.16

37. Length-Tension Relationships in Skeletal Muscle
Figure 8.17

38. Simple Twitch, Summation, and Tetanus
Figure 8.18

39. 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

40. Muscle Force-Velocity Relationships
Figure 8.19

41. 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 degrees•second-1
Power decreases beyond this velocity because force decreases with increasing movement speed

42. Muscle Force-Power Relationships
Figure 8.20

43. Receptors in Muscle Provide sensory feedback to nervous system
Tension development by muscle
Account of muscle length
Muscle spindle
Golgi tendon organ

44. 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

45. Muscle Spindles
Figure 8.21

46. 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

47. Golgi Tendon Organ
Figure 8.22