Copyright © 2016 Wolters Kluwer • All Rights Reserved
Copyright © 2014 Wolters Kluwer • All Rights Reserved

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

Basic Structure of Skeletal Muscle
• Basic Organization of Skeletal Muscle
3

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
4

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
5

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

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)
7

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
8

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

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
10

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
11

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

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
13

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

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
15

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

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
17

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
18

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

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

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

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
22

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

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
24

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

Sliding Filament Theory (continued)
• Sarcoplasmic Reticulum
26

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

Sliding Filament Theory (continued)
• Muscle Contraction Steps
28

Proprioception and Kinesthetic Sense
• Proprioception
• 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
29

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)
30

Proprioception and Kinesthetic Sense (continued)
• Muscle Spindles
31

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
32

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

Force Production Capabilities
• Types of Muscle Actions
34

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
35

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

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

Force Production Capabilities (continued)
• Strength Curves
38

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

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

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
41

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
42

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
43