PEP 294: Lecture Notes
VI. Skeletal Muscle
1. Overview
 | Functions of muscle (pp. 146, 141): |
- develops tension
- maintains upright body posture
- moves body limbs
75 muscle pairs (out of 434 muscles) are responsible for body movement
- absorbs shock
| Basic properties of musculostendinous unit (pp. 147-149): |
- extensibility: ability to be stretched
- elasticity: ability to return to normal length
- irritability: ability to respond to stimulus
- contractility: ability to contract
| Types of muscle contraction (activation) (pp. 162-164): |
- concentric contraction: muscle length decreases
- eccentric contraction: muscle length increases
- isometric contraction: muscle length does not change
| Roles assumed by skeletal muscle (pp. 164-166): |
- agonist: responsible for the joint motion (primary & assistant)
- antagonist: acts against the agonist for fine control and balance
- stabilizer: stabilizes a portion of the body against a particular force
- neutralizer: prevents unwanted accessory actions that normally occur when agonist develops concentric tension
- example: Biceps Brachii (elbow flexion & supination) vs. Pronator Teres (pronation)
2. Functional Organization of Skeletal Muscle (p. 149-162)
| Muscle fibers: |
- threadlike shape
- size: wide range of length and diameter
- growth in length and diameter
- no change in number of fibers
- twitch (< 100 ms) vs. tetanus:
(a) twitch: response to a single stimulus
(b) tetanus: state of muscle producing sustained maximal tension due to repeated stimulus
| Fiber types: |
- slow twitch (Type I, Slow-twitch Oxidative; SO):
(a) longer max tension time
(b) lower tension
(c) high endurance
- fast twitch (FT, Type II):
(a) shorter max. tension time
(b) higher tension
(c) fatigue quickly
(d) sub-types:
Type IIa (Fast-twitch Oxidative Glycolytic; FOG)
Type IIb (Fast-twitch Glycolytic; FG)
| Motor unit: |
- single motorneuron + muscle fibers innervated
- functional unit of muscle
- number of fibers in a unit:
(a) smaller: fine movement -- eyes & fingers
(b) larger: gross & powerful movement -- gastrocnemeus & power zone
- sequence of recruitment of motor units -- size principle
(a) SO -> FOG -> FG
(b) smallest available first --> largest last
(c) fatigue development --> recruit new motor units
- fiber composition:
(a) same type in a motor unit
(b) both in a muscle -- ratio varies
3. Mechanical Factors Affecting Muscular Force (p. 167-170)
| Muscle components - Hill's muscle model: |
- contractile component (CC):
- parallel elastic component (PEC):
- series elastic component (SEC): tendons
| Force-length relationship (isometric contraction): |
- force-length curve:
(a) active force-length relationship
(b) passive force-length relationship
- stretch-shortening cycle:
(a) eccentric contraction immediately followed by concentric contraction
(b) stretch --> stretch reflex --> forceful tension development --> effective development of concentric force
| Force-velocity relationship (dynamic characteristic): |
- concentric contraction
- eccentric contraction
| Force-time relationship: |
- force-time relationship
- electromechanical delay (EMD):
(a) 20 - 100 ms
(b) due to stretch of the SEC
(c) more FT fibers = shorter EMD
4. Muscular Strength, Power & Endurance (p.170-177)
| Muscular strength: max. isometric force generated by the muscle |
- factors affecting muscular strength:
(a) ratio of force generation capability / cross-sectional area
(b) muscle composition
(c) innervation
| Muscular power: (muscle force) x (contraction velocity) |
- power-force relationship
- more FT content = higher max power & higher max power velocity
| Muscular endurance: ability to exert tension over a period of time |
- factors affecting muscular endurance:
(a) force & speed requirement of the activity
(b) SO fiber proportion
- fatigue:
(a) muscle level: unable to respond to stimulus
(b) motorneuron level