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KNES 361 Final > Biomechanics of Skeletal Muscle > Flashcards

Biomechanics of Skeletal Muscle Flashcards

1
Q

Muscle Action

A

Muscles have active contractile component that develops force-

Active force dependent on:
Neural factors
Mechanical factors
Fiber type
Muscle architecture

Muscle force transmitted through tendon to bone-

Muscle force creates joint torque or moment → motion

Joint torque/moment dependent on:
Muscle force
Moment arm / Lever arm
Joint position (angle of pull)

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2
Q

Breakdown of Skeletal Muscle

A

Epimysium
Perimysium
Endomysium
Sarcolemma

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3
Q

Sarcomere

A

Basic contractile unit of muscle that develops force

Actin and Myosin (myofilaments) cycling

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4
Q

Sliding Filament Theory

A

↑ cross-bridge formation = ↑ force

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5
Q

Muscles “do work”

A

Work refers to the product of force and displacement (Work = Force x distance)
Displacement = the parallel displacement component relative to the force applied

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6
Q

Motor Unit

A

Single motor neuron and all the muscle fibers it innervates
3 to 2,000 fibers innervated (innervation ratio)
Functional unit of muscle
Smallest unit of muscle contraction
All muscle fibers respond as one
“All or None”

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7
Q

Synergist

A

Two or more muscles working together to produce a movement

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8
Q

Agonist

A

Cause or assist movement

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9
Q

Stabilizer

A

Active in one segment so that a movement in an adjacent segment can occur

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10
Q

Antagonist

A

Perform movement opposite of agonist

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11
Q

Neutralizer

A

Active to eliminate an undesired joint action of another muscle

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12
Q

Concentric action

A

Shortening of fibers to cause joint movement

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13
Q

Eccentric action

A

Lengthening of fibers to control or resist joint movement

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14
Q

Isometric action

A

Minimal change in fiber length

No joint movement

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15
Q

Factors Influencing Active Muscle Force Production

A

Neural Factors

Fiber type

Mechanical Factors

Muscle Architecture

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16
Q

Neural Factors Affecting Active Muscle Force

A

Activation & Discharge Rate

Motor unit recruitment

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17
Q

Muscle Fiber Activation & Discharge Rate

A

Twitch
Summation
Tetanus

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18
Q

Twitch

A

response of muscle to single stimulus

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19
Q

Summation

A

the overall effect of added stimuli

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20
Q

Tetanus

A

sustained maximal tension due to high frequency stimulation

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21
Q

Motor Unit (m.u.) Recruitment

A 
Muscle force is proportional to number of m.u.’s recruited
# crossbridge formations

Muscle force is proportional to rate of stimulation (or firing)
Rate of crossbridge cycling

Synchronization of firing impulses may increase muscle force
Important in fatiguing exertions

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22
Q

Fiber Type Comparison

A

Fiber type affects muscle force, rate of force production,
& recruitment order

*look at chart

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23
Q

Fast Twitch vs Slow Twitch

A

FT peak force > ST peak force

FT rate of force production > ST rate of force production

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24
Q

Fiber Type (cont.)

A

All fibers within a m.u. are the same type
Within a muscle, however, there are a mixture of fiber types

Ordered recruitment (Henneman’s size principal)
Type I recruited 1st (lowest threshold)
Type IIa recruited second
Type IIb recruited last (highest threshold)

Reduction in tension accomplished in reverse order
Allows for controlled, smooth gradation of force
Largely genetic, but may change with training

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25
Q

Mechanical Factors Affecting Muscle Force

A

Length

Velocity

26
Q

Force-Length Relationship

A 
Optimal (~resting length)
(max crossbridges → ↑ force)
Shortened = Crossbridge overlap
(↓ crossbridges → ↓ force)
Lengthened = Crossbridges 
are pulled apart
(↓ crossbridges → ↓ force)
27
Q

Force-Length Relationship:
Concentric vs. Eccentric

A

Same muscle length:
ForceECC > ForceCON
More crossbridges in ECC
ECC uses less energy
No ATP used to break crossbridge like in CON
Stretched – therefore more elastic energy stored  More force
Less Friction

F-L relationship occurs during concentric & eccentric muscle action

28
Q

Force-Length Relationship:
Active & Passive

A

Stretch beyond resting length causes:
↓ Active tension
↑ Passive tension

Total tension may be greater at extremes of motion
Due to passive tension

29
Q

Force-Velocity Relationship

Why Concentric F-V?

A

More crossbridges in release stage at a given time
Concentric - some CBs stay bound to too long (braking effect)
In eccentric – pulling apart makes it easier for crossbridges to cycle

30
Q

Concentric

A

Shortening

CB Resists = “Negative Braking effect”

31
Q

Eccentric

A

Resisting Lengthening

CB Assists: “Positive braking effect”

32
Q

F-V Impact

A

At a given level of activation, muscles contracting more slowly produce greater force than muscles contracting more quickly
To achieve a given force, muscles contracting slowly need less activation as muscles contracting quickly

33
Q

Muscle Power

A

Power refers to the rate at which work is performed
Power = Work / Time
OR
Power = Force x Velocity
Muscle power is especially important to functional and athletic tasks in comparison to strength – examples?

34
Q

Activation History

Stretch-Shorten Cycle

A

↑ muscle work when shortening immediately follows stretching
Squat jump vs. Counter-movement jump

If stretch is held too long before shortening the effects are lost

Basis is not well understood
Stored elastic energy
Reflex activation

35
Q

Muscle Architecture

A

Arrangement of contractile components (sarcomere) affects force production, excursion, & velocity

Muscle fiber arrangement:
Parallel: side to side arrangement
Series: end to end arrangement

36
Q

Longitudinal

A

Esophagus

37
Q

Unipennate

A

Lumbricals

38
Q

Bipennate

A

Gastrocnemius m.

39
Q

Fusiform

A

Wider in middle than ends (biceps brachii m.)

40
Q

Muscle Architecture & Force

A

Arrangement of contractile components (sarcomere) affects force production

Muscle force is proportional to number of fibers / crossbridges active in parallel
Parallel = greater force production
Series = greater shortening velocity

41
Q

Parallel Muscle Fiber Arrangement

A

side to side arrangement

42
Q

Series Muscle Fiber Arrangement

A

end to end arrangement

43
Q

Muscle Displacement and Velocity

A

Muscle displacement & velocity are proportional to number of fibers / crossbridges in series
Each fiber undergoes a change in length

Δ LengthTOTAL= ∑ ΔLengthSERIES FIBERS

↑ Δ LengthTOTAL in same time → ↑ Velocity

44
Q

Muscle Force

A

Muscle force is proportional to number of fibers / crossbridges active in parallel
Muscle Force = ∑ForcePARALLEL FIBERS
Muscle Force = Avg ForceSERIES FIBERS

45
Q

Angle of Pennation

A

Alignment of muscle fibers relative to line of pull
Pennation θ = 0° → Fibers aligned with line of pull

↑ resultant force directed along line of pull in less pennated fiber arrangements

↑ overall force in less pennated (longitudinal) arrangements???

46
Q

Muscle Architecture

A

For the same muscle volume, longitudinal arrangements produce
less force than pennate arrangements

47
Q

Energy

A 
The capacity to do work
Scalar quantity
We are most interested in mechanical energy (associated with motion and position)
Kinetic: energy of motion 
Potential: energy of position
48
Q

Types of energy

A

Mechanical, chemical, heat, sound, light, etc

49
Q

Mechanical Energy

A

Strain or Elastic Energy
Special form of potential energy

Energy due to deformation

This type of energy arises in compressed springs, squashed balls ready to rebound, stretched tendons inside the body, and other deformable structures (like muscles!!)
Tennis Ball Bounce

50
Q

Stiffness

A

Force response to a mechanical stretch
Muscle fibers possess stiffness
Stiffness (K) = ΔF/ΔL

51
Q

Stiffness Recruitment

A

Stiffness can be controlled
Preparatory muscle activation (intrinsic stiffness)

Reflex activation (reflex mediated stiffness)

Co-activation (joint stiffness)

52
Q

How do reflexes maintain muscle stiffness?

A

Muscle spindle excitation facilitates increased motor unit recruitment
Change in muscle length
Change in rate of muscle lengthening

Increased number of fibers aligned in parallel

Increased muscle stiffness

53
Q

Co-Activation and Joint Stiffness

A

↑ number of fibers aligned in parallel
Agonist & antagonist muscles

Antagonist muscle activity → ↑ agonist muscle activity
Offset antagonist muscles → ↑ number of fibers aligned in parallel

↑ joint compression → ↑ friction

54
Q

Role of Stiffness

A

Stiffness creates stability
Musculoskeletal injury (joint stability)
Balance (postural stability)

55
Q

Preventing Joint Injury

A

Biomechanical stability is required to prevent joint injury

Ability of a loaded structure to maintain static equilibrium after perturbation around the equilibrium position (Bergmark, 1989)

56
Q

Joint Biomechanical Stability

A

Perturbation to Joint
(e.g. anterior tibial shear force) > Limited Athrokinematic Motion > Return to Resting Position > Maintenance of Joint Biomechanical Stability

57
Q

Biomechanical Stability Factors

A

Sufficient potential energy (PE) is needed to maintain biomechanical stability after perturbation (Bergmark, 1989)
System able to return to equilibrium position

Quantifying Biomechanical Stability
Work (J) performed during perturbation ≤ PE (J) inherent to the system → Stable system

58
Q

Forms of Potential Energy (PE)

A

PE due to object height above reference
PE = mgh
(m = mass, g = gravitational acceleration, h = height)

PE due to elastic deformation
PE = ½kx2
(k=stiffness, x = distance stretched)

59
Q

Elastic Energy, Stiffness & Stability

A

PE in form of elastic energy is most important for musculoskeletal applications
PE = ½kx2

↑ stiffness → ↑ PE → ↑ stability
Thus, stiffness creates biomechanical stability

60
Q

Ankle Stiffness

A 
Stiffness of the evertor muscles can limit excessive inversion
Peroneus longus
Peroneus brevis
↑ stability
↓ injuries
61
Q

Summary

A

Stiffness creates joint stability

Active muscle stiffness most important

Insufficient stiffness → inability to maintain stability
Joint injury

62
Q

Too much of a good thing?

A

could increase injury risk of bone and muscle injury

KNES 361 Final (4 decks)

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