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Orthopaedic Biomechanics > Lecture 8 - Muscles > Flashcards

Lecture 8 - Muscles Flashcards

1
Q

What makes muscle tissue unique (4)?

A

Extensibility
- ability to stretch in response to a force

Elasticity
- recoil when stretch is removed and return to its resting length

Contractibility
- ability to shorten/contract

Irritability
- ability to respond to a stimulus

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2
Q

Hierarchical structure

A 
Sarcomere
Myofibril
Fibres (joined by endomycium - continuous with cap + nerves)
Fasicles (joined by perimycium)
Muscles
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3
Q

What creates motion?

A
  1. CNS sends signal to motor unit
  2. Muscle shortens and applies force on bones
    - sliding filament theory - relative movement between actin and myosin
    - crosslinks flip over and shorten muscle –> sum of contractions of all individual sarcomeres
  3. Moment generated around joint
    - muscle contracts in the middle and pulls on both bones
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4
Q

Components of a sarcomere

A

Actin - thin filament (5nm)
Myosin - thick filament (15nm)
Titin - maintains structure (like a molecular spring)

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5
Q

Hill muscle model:

What are represented by:

  • contractile element
  • elastic element in series/parallel

What is the force equation relating them?

A

Contractile element = actin/myosin

Elastic element in series = tendon

Elastic element in parallel = connective tissues

F(active) + F(passive) = F(tot)

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6
Q

What affects the force production?

A

Tension depends on:

  • amount of stimulation
  • length of muscle
  • velocity of muscle shortening

Maximum tension (i.e. maximum overlap of actin and myosin) is at resting length –> striation spacing = 2-2.5um

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7
Q

Draw the tension-length curve for a muscle:

A

Active tension:

  • max at ideal length
  • negative parabola
  • -> shortened muscles = less tension produced

Passive tension:

  • kicks in when muscle is lengthened only
  • due to tendons and connective tissues
  • longer –> resists tension
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8
Q

What is active insufficiency?

A

Failure to produce force when the muscle is too short

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9
Q

Difference between an agonist, antagonist and synergist?

Example of an agonist and antagonist?

A

Agonist - responsible for movement
Antagonist - opposes the movement, adds control
Synergist - assists the agonist in performing the movement

Example:
bicep (flexion) tricep (extension)

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10
Q

Concentric contraction

A

Shortening of muscle causes joint movement
–> muscle moment the same direction as joint angle change

e.g. (on chair) quads contract

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11
Q

Eccentric contraction

A

Lengthening of muscle decelerates joint movement
–> muscle moment opposite direction to change in joint angle

e.g. quads contract, but moment due to external weight if more than that due to muscle force - muscle lengthens

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12
Q

Isometric contraction

A

Contraction of muscle with no movement
–> muscle stays the same length

e.g. posture

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13
Q

Of the three types of contraction, which produced the greatest force?

A

Eccentric > isometric > concentric

Due to longer contraction time:

  • more cross bridging
  • more tension in elastic components
  • recruitment of motor neurons
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14
Q

Velocity tension curve - relationship between eccentric and isometric contraction?

A

max T(eccentric) = 1.25 max T(isometric)

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15
Q

Muscle fibre types:

A

Fibre types allow for muscle adaptation

Type I:

  • recruited first
  • slow contraction
  • good blood supply (red)
  • difficult to fatigue
  • endurance
  • small diameter (low tension)

Type IIA:

  • moderately fast contraction
  • variable contractile force
  • long term anaerobic
  • moderately good blood supply
  • intermediate fibres (between I and II)

Type IIB:

  • fast contraction
  • powerful contraction (greater force than type I)
  • anaerobic
  • poor blood supply (white in colour)
  • rapidly fatigued
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16
Q

What affects muscle movement (2)?

A

Length of muscles moment arm

  • |_ distance between muscle and point of rotation
  • smaller moment arm = larger excursion

Length of fibres composing the muscles

  • muscle fibre can shorten by 50-60% of its length
  • total shortening depends on # sarcomeres in series
17
Q

Physiological cross sectional area (pCSA) and its force relation

A

Area |_ to fibre direction

pCSA = muscle volume / fibre length

F(max) = pCSA x 37N/cm2

18
Q

Pennation angle - what is it and how does its value affect muscle architecture?

A

Angle at which the fibres are attached to tendon

Fmuscle = Ffiber x cos(a)

  • allows more fibres to be packed in –> more force
  • high a = shorter fibres, long tendons, static tasks
  • low a = longer fibres, more shortening, motion tasks
19
Q

Four different forms of muscle architecture

A

Fusiform

  • v small pennation angle
  • quick movement
  • easily fatigued

Unipennate

  • larger pennation angle
  • slower movement
  • powerful

Bipennate

  • multiple pennation angles
  • static contraction
  • stability

Multipennate

  • short and long fibres
  • multiple angles
  • stability and movement
20
Q

Muscle adaptation in strength training

A
  • bulk up - high force, low reps
  • increase pCSA
  • bigger fibres, more contractile protein content
  • hypertrophy of fibres (increase size, not #)
  • improved innervation
21
Q

Muscle adaptation in endurance training

A
  • adapt by changing energy supply (not fibre size)

- increase # capillaries and mitochondria

22
Q

Sore muscles- inflammatory response

A
  • tensile stress –> structural injury
  • diffusion of cell contents into interstitial fluid
  • macrophages come clean up
  • elevated pressure, swollen fibres
  • delayed onset (24-48hr)
  • most severe for eccentric contractions
23
Q

What affects muscle strength?

A
  • muscle size
  • moment arm
  • stretch
  • contraction velocity
  • level of fibre recruitment
  • fibre type
24
Q

Determining muscle forces: optimisation techniques (5)

A

Minimise total muscle force
- only muscles with longest lever arms are active

Maximum total muscle stress

  • max stress equivalent for all muscles –> once max stress reached, synergistic muscle activates
  • muscles with longest lever arms are active

Minimise max muscle stress

Minimise muscle power

Maximise endurance

25
Q

EMG:

What does it measure?
How does it measure it?
Determining force from EMG?
What affects EMG signals?

A

Measures
- muscle activity (V) - timing and magnitude, not force

Mesure using

  • surface EMG (summation of action potentials, non invasive)
  • fine wire EMG (specific motor units, invasive - good for small and deep muscles)

Fi = (EMGi x pCSAi) / (EMGpm x pCSApm)
–> pm = prime mover

Factors affecting signals

  • cross-talk between muscles
  • changes in geometry
  • tissue impedance
  • external noise
26
Q

Disuse atrophy

A
  • decrease pCSA
  • affects both type I and II fibres
  • muscle dependent

Orthopaedic Biomechanics (8 decks)

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