The Skeletal Muscle System Flashcards
(38 cards)
1
Q
Intro to Skeletal Muscle
A
- most abundant tissue in human body
- 40-45% of total body weight
- transforms chemical energy into mechanical energy: bioenergetics –> force
- mechanical energy results in: generation of internal forces, the resistance and absorption of external forces
2
Q
Functions of Skeletal Muscle
A
- body posture
- locomotion
- venous return
- thermogenesis
3
Q
Characteristics of Muscle Tissue
A
- irritability
- contractility
- extensibility
- elasticity
4
Q
Irritability
A
- ability to respond to stimulation
- skeletal muscle is one of the most sensitive and responsive tissues in the body
- only nerve tissue is more sensitive
5
Q
Contractability
A
- ability to shorten
- occurs when muscle tissue receives sufficient stimulation
- some muscles can shorten as much as 50-70% of their resting length
- shortening distance limited by its confinement in the body
6
Q
Extensibility
A
- ability to stretch or lengthen
- muscle itself cannot produce the elongation
- another muscle or force is needed
- determined by connective tissue found in: perimysium, epimysium, fascia surrounding and within
7
Q
Elasticity
A
- ability to return to resting length after stretching or lengthening
- determined by connective tissue in muscle
- a critical component in facilitating output in a shortening muscle action preceded by a stretch: aka stretch-shortening cycle
8
Q
Macroscopic Structure of Skeletal Muscles
A
- organization and connective tissue…
- skeletal muscle fibers are bundled together into groups of fibers called fasciculi
- a muscle fiber itself is comprised of myofibrils that are in turn made up of myofilaments
9
Q
Microscopic Structure of Muscle Fiber
A
- sarcolemma: cellular plasma membrane for muscle, surround individual muscle fiber, large, multinucleated
- SR: stores glycogen and myoglobin, runs longitudinally along myofibril, lateral sacs lie at end of SR, calcium stored in lateral sacs
- transverse tubule: carry electrical signal from sarcolemma to interior of cell, run perpendicular to myofibril
- 2 types of myofilaments: thick (myosin) and thin (actin), repeating pattern of thick and thin along length of myofibril give striated appearance
10
Q
Molecular Structure of Myofilaments
A
- myosin: thick, anchored at midpoint of sarcomere
- actin: thin, anchored at end of sarcomere
- tropomyosin: thin, regulatory protein, blocks binding site on actin
- troponin: thin, regulatory protein, controls position of tropomyosin
- sarcomere: repeating unit of thick and thin filaments
11
Q
Contraction of Muscle Fiber
A
- sliding filament theory of muscle action…
- force for action created during process in which actin slides over myosin
- length of thick and thin filaments does not change
- length of sarcomere decreases
12
Q
Excitation-Contraction Coupling
A
- refers to sequence of events which create a muscular contraction
- initiated by AP
- transmitted to sarcolemma
- ends with contraction of myofilaments
- 3 phases
1. spread of depolarization: AP carried into interior of muscle cell by T-tubules, electrical signal causes release of calcium stored in lateral sacs of SR
2. binding of calcium to troponin: troponin has conformational change which pulls tropomyosin from blocking position on actin so myosin cross-bridge can now bind with actin
3. generation of force: actin-myosin binding creates tension in sarcomere, causes release of stored energy in myosin heads
13
Q
Excitation-Contraction Coupling: Cross-Bridge Cycle
A
- describes generation of force
- consists of…
- myosin binding to actin
- “powerstroke”
- dissociation of actin and myosin
- activation of myosin heads
- changes in sarcomere during contraction…
- Z lines move closer together
- A band does not change in length
- I band shortens and may disappear
- H zone shortens and may disappear
14
Q
Fueling Muscle Contraction
A
- ATP lays several important roles in muscle contraction…
- provides energy to activate or reactivate myosin head prior to binding with actin
- necessary for breaking link between myosin-actin cross bridge, so cycle can be repeated
- also used to return calcium to SR and restore existing membrane potential
- during relaxation…
- calcium pumped int SR by active transport
- troponin no longer keeps tropomyosin from blocking position
- ability of muscle to relax just as important for contraction
15
Q
All-Or-None Principle
A
- when motor unit is stimulated, all muscles in that motor unit contract to fullest extent or they don’t contract at all
- threshold stimulus: minimum amount of excitation necessary to stimulate contraction
16
Q
Muscle Fiber Types
A
- properties of motor units: integrated nomenclature
- divided into 2 types on basis of contractile properties: fast and slow twitch
- FT can be subdivided on basis of metabolic properties: fast oxidative glycolytic, and fast glycolytic
- ST recruited first then FT recruited for more force
- you can change properties of FOG fibers based on training but other two do not change
- each individual has unique makeup of fibers, influences performance capabilities so in turn training habits can influence metabolic characteristics
17
Q
Distribution of Fiber Types and Training
A
- best athletes involved in endurance activities typically have higher percentage of ST fibers
- best athletes involved in power activities typically have higher percentage of FT fibers
- training may alter metabolic capabilities of muscles: but not contractile properties
- changes may be significant enough to change classification of FT fibers
- FG FOG
- FT fibers can change based on dominant form of training
18
Q
Movement: Result of Muscle Contraction
A
- muscle tension: force developed when contracting muscle acts on object
- load: force exerted on muscle (aka resistance)
- tension and load are opposing forces
- in order for motion to occur force of tension must exceed force of load
- contraction: initiation of tension producing process of contractile elements without muscle-not all muscle actively produces motion
- torque: capability of force to produce rotation of limb around a joint-basis for all movement in human body
19
Q
Classification of Muscle Actions
A
- Isometric contraction: tension produced does not result in change of muscle or joint angle
- isotonic contraction: constant external resistance, muscular action produces tension to overcome given external load
- isokinetic contraction: velocity of action is constant, muscular tension continually changes during available ROM
- concentric: shortening-results in positive external work, primarily responsible for acceleration in movement, cross-bridge cycling involves primarily pulling together actin-myosin
- eccentric: lengthening-results in negative external work, primarily responsible for deceleration in movement, cross-bridge cycling involves primarily pulling apart actin-myosin
20
Q
Why can eccentric contraction produce more force than concentric?
A
- only ~50% available cross-bridges cycle during concentric
- during eccentric some cross-bridges do not cycle but are continually pulled backward-myosin heads don’t rotate forward and actin-myosin remain bound
21
Q
Why does eccentric work have lesser energy cost than concentric?
A
- fewer motor units recruited –> use less oxygen –> less energy costs
- some cross-bridges don’t cycle –> less ATP broken down for energy (positive work estimated to use 3-9 times energy used in negative work)
22
Q
Static Contraction vs. Dynamic Contraction
A
- static: muscular action which produces an increase in muscle tension but does not cause meaningful limb displacement or joint displacement-does not result in skeletal movement, more accurate description than isometric
- dynamic: tension produced varies as muscle shortens-accommodates change in muscle length and/or joint angle, occurs throughout ROM while moving constant external load, more accurate description than isotonic
23
Q
Muscular Force Development
A
- contractions produce tension but amount of tension not the same for all contractions
- affected by number of factors: neural, mechanical
- true for action on lever of fiber or whole muscle
- all-or-none principle
- twitch: muscle stimulated then allowed to relax
- temporal summation: full relaxation not allowed, produces slightly greater tension
- useful tetany: individual twitches blend together
- fused tetany: produces greatest tension
- gradation of response: whole muscle contractions do not occur in all-or-none fashion; instead can be graded into little force or great force
24
Q
Neural Activation
A
- rate coding: frequency of stimulation; as frequency increases, so does force production
- number coding (recruitment): some motor units always contracting even at rest which creates motor tone and occurs in alternating fashion
- more motor units recruited when greater force is needed
- occurs due to size principle: SO, FOG, then FG
- deactivation occurs in reverse sequence (largest relax first)
25
Mechanical Factors Influencing Muscle Contractions: Length-Tension-Angle Relationship
- basic relationship described by inverted U
- amount of tension possible related to overlap of actin-myosin
- some variation exists in specific whole muscles based on anatomical position (strength curves)
26
Mechanical Factors Influencing Muscle Contractions: Force-Velocity Relationship
- similar for single fiber and whole muscle
- shortening velocity of muscle increases as concentric force developed by muscle decreases (muscle can shorten fastest when load is lightest, maximal velocity occurs in unloaded situation, maximal load results in no velocity
- during eccentric action as velocity increases, force tension increases: greatest tension always occur during eccentric actions; phenomenon contributes to: muscular soreness, strengthening, injury
27
Mechanical Factors Influencing Muscle Contractions: Elasticity-Force Rleationship
- both muscle and tendon have elastic properties
- when muscle is stretched then quickly contracted, the resultant contraction is stronger if there had been no pre-stretch: aka stretch-shortening cycle or plyometric action, provides greater mechanical efficiency --> lesser metabolic costs
- most evident in activities such as walking, running, and cycling: ex in class vertical jump with and without countermovement
28
Mechanical Factors Influencing Muscle Contractions: Cross-Sectional Area/Architecture Design
- applies to single fiber and to whole muscle
- maximum force capability is limited by cross-sectional area
- muscles designed for high force generation arranged so as to maximize cross-sectional area: pennate, bipennate, and multipennate
- muscles designed for maximum velocity are arranged in fusiform manner
29
Muscular Fatigue and Soreness
- despite all benefits of exercise unpleasant outcomes can occur due to unaccustomed muscular activity
- results in temporary loss of muscle function
30
Muscular Fatigue
- muscle fatigue results in cessation of work or inability to maintain a given intensity of work
- complex process which includes failure at one or more of the sites along the chain of events
- central fatigue: malfunction of nerve cells or inhibition of voluntary effort in CNS; may be influenced by psychological factors and motivation which influence level of effort, particularly if pain is associated with continuing activity
- peripheral fatigue: refers to fatigue at sites beyond CNS; includes sites in PNS and within muscle (neuromuscular junction, sarcolemma-T-tubules-SR system,myofilaments)
- may include electrical signal failure or biochemical/metabolic changes
- 2 primary hypotheses for contractile cause of fatigue within muscle: depletion (exhaustion) hypothesis, accumulation hypothesis
- fatigue currently thought to be related to type of activity
- dynamic resistance exercise: with high intensity, low reps FG fibers predominate, so a decrease in PC the likely cause; with high reps or high total volume of work FOG fibers predominate so the accumulation of H+ likely cause
31
Muscular Soreness
- familiar consequence of physical exertion
- immediate onset soreness
- marked by pain during and immediately after exercise
- thought to be caused by stimulation of pain receptors by metabolic by-products of cellular respiration (H+)
- generally relieved by discontinuing exercise, or it subsides shortly thereafter
- delayed onset muscular soreness (DOMS)...
- increases in intensity in 24 h after exercise
- peaks from 24-48 h post-exercise then declines for next 5-7 days
- generally considered an adult phenomenon
- primary theories: mechanical trauma theory, lateral ischemia theory
32
Eccentric Contraction Importance
- more likely to endure higher workload-greatest tension of muscle, more likely to bring to end range and tear or stretch fibers --> elongates them --> increases strength
- should say we're going to the gym to lower weights, all about deceleration and lengthening)
33
Laboratory and Field Methods
- dynamometers: hand grip is classic example, can measure maximal force or percentage of MVC
- constant-resistance equipment: used to measure maximum strength (1 RM) or muscular endurance (% of 1 RM)-submax values may be expressed as absolute or relative values; resistance offered by free weight or weight machines
34
Filed Tests
- manual muscle test
- calisthenic activities: used primarily to assess muscular endurance, and to lesser extent muscle strength, common tests include push ups, sit ups or curl ups, pull ups; many tests are norm referenced
- vertical jump/standing board jump/medicine ball toss: used to test explosive power, a definite ATP-PC system component to these tests
35
Influence of Age and Sex on Muscle Function: Children and Male-Female
- children:
- strength increases evident from infancy thru maturity
- during early childhood, virtually no difference in strength between sexes
- boys show accelerated gains at late adolescence
- girls maintain slow rise or decrease slightly
- male-female differences: due largely to greater muscle mass of males
- magnitude of differences between sexes dependent on: units used to express strength, region where strength is measured, training status of individual
36
Influence of Age and Sex on Muscle Function: Elderly
- significant declines occur from mid to old age: in both males and females, but rate of loss varies among muscle groups, activity levels, etc.
- hypotheses for the causes of age-related changes in strength: loss of muscle mass, loss of mechanical or contractile properties-both ST and FT fibers lost, but FT fibers are lost more quickly (particularly FG)-reduced activation of motor units
- practical meaning of these changes in the concern: weakened respiratory muscles -->restric aerobic capacity, weakened LE muscles --> instability, difficulty restoring balance, risk of falls; independent living may be at risk-get in/out of chairs, carry groceries,remove caps from medicine or food jars
- resistance exercise programs are most effective method for slowing functional decay: numerous studies show benefits of resistance training among the elderly
37
Take Home Messages
- skeletal muscle is basis for all human movement
- muscle has plasticity meaning it can change based upon the demands placed upon it
- muscle mechanics are the basis for all human movement
- an understanding of muscle mechanics allow the physical therapist to best understand factors which influence performance
- the physical therapist also must understand factors such which contribute to muscular recruitment, fatigue, and soreness
38
Difference Between Hurt and Harm
-you can hurt but it may not be any sort of damage or negative effect on your body whereas harm most definitely is
