1. Chapter 4: Skeletal Muscle System

2. Describe the anatomy and physiology of skeletal muscle
Learning Objectives
Describe the anatomy and physiology of skeletal muscle
Role of muscle fiber types as it relates to different types of athletic performances
Histochemical techniques for identification of muscle fiber types
Explain how skeletal muscle produces movement
Basis of proprioception in muscle and kinesthetic sense
Force production capabilities of muscle and types of muscle actions
Training-related changes in skeletal muscle
Effects of simultaneous high intensity endurance and strength training on adaptations

3. Basic Structure of Skeletal Muscle
Basic Organization of Skeletal Muscle

4. Basic Structure of Skeletal Muscle (continued)
Connective Tissue (CT) and Muscle Organization
Tendons: bands of tough, fibrous CT that connect muscle to bone
Fasciculus: small bundle of muscle fibers
Muscle fiber: long, multinucleated cell that generates force when stimulated
Myofibril: portion of muscle composed of thin & thick myofilaments (actin & myosin)
Actin & myosin: contractile proteins in muscle

5. Basic Structure of Skeletal Muscle (continued)
Connective Tissue (CT) and Muscle Organization (cont’d)
Role of CT
Stabilizes & supports components of skeletal muscle
Surrounds muscle at each organizational level
3 layers of CT in muscle
Epimysium: covers whole muscle
Perimysium: covers bundles of muscle fibers (fasciculi)
Endomysium: covers individual muscle fibers

6. Basic Structure of Skeletal Muscle (continued)
Connective Tissue in Skeletal Muscle

7. Basic Structure of Skeletal Muscle (continued)
Characteristics of Connective Tissue
Sheaths coalesce to form tendons at each end of muscle
Force generated by muscle is transferred to tendon & bone
Epimysium helps prevent spread of signal for muscle activation
Elastic component of CT contributes to:
Force & power production (like recoil of rubber band)
Stretch-shortening cycle
Eccentric action (elongation)
Concentric action (shortening)

8. Basic Structure of Skeletal Muscle (continued)
The Sarcomere
Basic skeletal muscle unit
Capable of force production & shortening
Arrangement of protein filaments gives striated appearance
Components
Z lines: at each end of sarcomere
H zone: in middle of sarcomere, contains myosin
I bands: at edges of sarcomere, contain actin
A band: overlapping actin & myosin
M line: middle of H zone, holds myosin in place

9. Basic Structure of Skeletal Muscle (continued)
The Sarcomere

10. Basic Structure of Skeletal Muscle (continued)
Actions of Sarcomere
As sarcomere shortens:
Actin filaments slide over myosin
H zone disappears as actin filaments slide into it
I bands shorten as actin & myosin slide over each other
Z lines approach ends of myosin filaments
As sarcomere relaxes:
It returns to original length
H zone & I bands return to original size & appearance
Less overlap between actin & myosin

11. Basic Structure of Skeletal Muscle (continued)
Noncontractile Proteins
Provide lattice work for positioning of actin & myosin
Contribute to elastic component of muscle fiber
Titin
Connects Z line to M line
Stabilizes myosin in longitudinal axis
Limits ROM of sarcomere & contributes to passive stiffness
Nebulin
Extends from Z line & is localized to I band
Stabilizes actin by binding with actin monomers

12. Basic Structure of Skeletal Muscle (continued)
Noncontractile Proteins

13. Basic Structure of Skeletal Muscle (continued)
Actin (Thin) Filament
2 intertwined helices of actin molecules
Projects from Z lines toward middle of sarcomere
Active site: where heads of myosin crossbridges bind to actin
Wrapped by tropomyosin & troponin (regulatory protein molecules)
Subunits of troponin
Troponin I: holds to actin
Troponin T: holds to tropomyosin
Troponin C: can bind calcium

14. Basic Structure of Skeletal Muscle (continued)
Actin Filament Organization

15. Basic Structure of Skeletal Muscle (continued)
Myosin Filament
Has globular head, hinged pivot point, & fibrous tail
Heads: made up of enzyme myosin ATPase
Tails: intertwine to form myosin filament
Crossbridge
Consists of 2 myosin molecules, with 2 heads
Interacts with actin
Develops force to pull actin over myosin
Features different isoforms of ATPase

16. Basic Structure of Skeletal Muscle (continued)
Myosin Filament Organization

17. Basic Structure of Skeletal Muscle (continued)
Muscle Fiber Types
Type I (slow-twitch)
Slow to reach peak force production
Low peak force
High capacity for oxidative metabolism
Fatigue-resistant
Endurance performance

18. Basic Structure of Skeletal Muscle (continued)
Muscle Fiber Types (cont’d)
Type II (fast-twitch)
Rapidly develop force
High peak force
Low capacity for oxidative metabolism
Fatigue easily
Sprint, short-term performance

19. Basic Structure of Skeletal Muscle (continued)
Muscle Fiber Types Compared

20. Basic Structure of Skeletal Muscle (continued)
Myosin ATPase Histochemical Analysis
Differentiates among muscle fiber subtypes
Involves histochemical staining procedure
Process
Thin cross-section of muscle is obtained from biopsy sample
Sample is placed into baths of different pH conditions
Fibers are classified according to staining intensity
Subtypes, from most oxidative (slowest) to least (fastest):
I, IC, IIC, IIAC, IIA, IIAX, IIX

21. Basic Structure of Skeletal Muscle (continued)
Myosin ATPase Delineation of Muscle Fiber Types

22. Basic Structure of Skeletal Muscle (continued)
Myosin Heavy Chain (MHC)
Molecular weight of 230 kD
Associated with 2 light chains (per MHC)
Essential
Regulatory
MHC composition of muscle can profile fiber type composition
High correlation between subtypes I, IIA, & IIX and MHC subtypes I, Ia, & Ix, respectively

23. Basic Structure of Skeletal Muscle (continued)
Myosin Molecule

24. Sliding Filament Theory
Overview
Explains how muscle proteins interact to generate force
Proposed in 1954
Summary
Actin & myosin filaments slide over each other to produce force without the filaments themselves changing length
Sliding of actin over myosin produces change in striation pattern
# of actomyosin complexes formed dictates how much force is produced

25. Sliding Filament Theory (continued)
Steps Mediating the Contraction Process
Electrical impulse is generated at neuromuscular junction
Impulse spreads across sarcolemma into T-tubules
Ryanodine receptors release Ca++ into cytosol of muscle fiber
Ca++ binds to troponin C subunit
Tropomyosin uncovers active sites of actin
Myosin crossbridge heads bind actin, form actomyosin complex
Heads pull actin toward center of sarcomere (power stroke)
Force is produced

26. Sliding Filament Theory (continued)
Sarcoplasmic Reticulum

27. Sliding Filament Theory (continued)
Ratchet Movement Produces Power Stroke

28. Sliding Filament Theory (continued)
Muscle Contraction Steps

29. Proprioception and Kinesthetic Sense
How the body senses where it is in space
Monitored by feedback as to length of muscle & force produced
Proprioceptors: receptors located in muscles and tendons
Info. from proprioceptors is sent to brain (conscious & subcon.)
Learning effect
Ability to repeat a specific motor unit recruitment pattern
Results in successful performance of a skill
Requires practice

30. Proprioception and Kinesthetic Sense (continued)
Muscle Spindles
Proprioceptors in skeletal muscle
2 functions
Monitor stretch or length of muscle
Initiate a contraction when muscle is stretched
Stretch reflex: quickly stretched muscle initiates immediate contraction due to being stretched
Located in intrafusal fibers (modified muscle fibers)

31. Proprioception and Kinesthetic Sense (continued)
Muscle Spindles

32. Proprioception and Kinesthetic Sense (continued)
Golgi Tendon Organs
Proprioceptors in tendon
Main function is to monitor & respond to tension in tendon
Activated by excessive force on tendon
Inhibit action of muscle to prevent injury
New training techniques seek to decrease inhibition by Golgi tendon organs to allow greater force production

33. Proprioception and Kinesthetic Sense (continued)
Golgi Tendon Organs

34. Force Production Capabilities
Types of Muscle Actions

35. Force Production Capabilities (continued)
Terms Used to Describe Resistance Exercise
Isotonic: muscle generates same force throughout ROM
Dynamic constant external resistance: resistance provided by free weights or weight machine that remains constant
Isoinertial: exercise movement with variable velocity & constant resistance throughout ROM
Variable resistance: resistance that changes over ROM
Isokinetic: resistance in which velocity of limb’s movement throughout ROM is held constant by a device

36. Force Production Capabilities (continued)
Force-Velocity Curve

37. Force Production Capabilities (continued)
Training Effects on Concentric Force-Velocity Curve

38. Force Production Capabilities (continued)
Strength Curves

39. Force Production Capabilities (continued)
Length-Tension Relationship

40. Force Production Capabilities (continued)
Force-Time Curve

41. Muscle Adaptations that Improve Performance
Effects of Endurance Training
Increase in delivery of oxygen to muscle, caused by:
Increase in # of capillaries per muscle fiber
Increase in capillary density
Increase in concentration of myoglobin, which increases rate of oxygen transport from capillaries to mitochondria
Enhanced ability for aerobic metabolism, caused by:
Increase in size & number of mitochondria in muscle
Increase in ability to produce ATP

42. Muscle Adaptations that Improve Performance (continued)
Effects of Resistance Training
Hypertrophy
Increase in size of muscle fibers
Results from addition of protein & new myofibrils to existing fibers, making them larger
Requires addition of myonuclei to support increase in muscle fiber size
Hyperplasia
Increase in number of muscle fibers
Occurrence is controversial

43. Muscle Adaptations that Improve Performance (continued)
Compatibility of Exercise Training Programs
Conclusions from studies of concurrent endurance & resistance training:
Strength can be compromised due to endurance training
Power may be compromised more than strength
Anaerobic performance may be decreased due to endurance training
Development of maximal oxygen consumption is not compromised
Endurance capabilities are not diminished by strength training