Control of muscle mechanical output Flashcards
What happens during single muscle fibre isometric contraction?
No change in muscle length (but produces force) - when holding something.
Myosin heads trying to push actin but not moving = produce isometric force.
What happens during single muscle fibre concentric contraction?
Muscle fibre is actively shortening (performing mechanical work)
What are the different types of contraction?
1) Isometric
2) Concentric
3) Eccentric
What is isometric contraction used for?
In everyday life to hold positions
What does force depend on?
1) Number of sarcomeres in parallel
2) Length of sarcomere (and number of cross-bridges)
= force-length relationship important for isometric contraction
Which muscle fibre can generate the most force?
THICK - more sarcomeres which are parallel
What is the force-velocity relationship?
?
What does velocity depend on?
The number of sarcomeres in series (arrangement of sarcomeres)
What makes a muscle fibre stronger than others?
More sarcomeres parallel (stacked) - but has a low velocity
What makes a muscle fibre faster than others?
More sarcomeres in series (next to each other) - but has a high velocity
What does it mean if sarcomeres are in parallel?
- Forces add up
- Each element feels only part of the whole force
- Movements do not add up
- Each element experiences the whole movement
What does it mean if sarcomeres are in series?
- Forces do not add up
- Each element feels the whole force
- Movements do add up
- Movement is shared between the elements
Single muscle fibre power?
- Power is not a synonym of force
- Power = force x velocity
- Power can be measured during concentric contractions, not isometric conditions
What is the influence of fibre types?
Fibre type is defined by the myosin heavy chain isoform composition.
Different isoforms e.g.
- (S) Type I
- (F) Type IIa or Type IIx
What happens during single muscle fibre eccentric contraction?
Muscle is being actively lengthened (performing negative work or absorbing energy)
What is the maximum voluntary contraction (MVC) dependent on?
The type of single muscle fibre.
- Different apparatus is used to measure different muscle groups.
Type II fibres
Strong but can’t sustain contraction
Type I fibres
Sustain contractions for longer
What is the peak power order for the 3 types of fibres?
type IIx > type IIa > type I
Usain Bolt and type IIx fibres
- 99% type IIx fibres
- High power, force + velocity
Mo Farah and type I fibres
- type I fibres only as he doesn’t need to produce a lot of force or velocity
- just need fibres w/ a lot of mitochondria which are able to sustain contractions for longer
How can muscle force be determined?
1) Physiological cross-sectional area
- Anatomical CSA
- Angle of Pennation
2) Antagonist co-contraction
3) Neural activation
- Recruitment
- Firing rate
- Motor unit synchronisation
4) Fibre type
What needs to be considered when looking at the whole force of a muscle?
Need to consider CSA (or detection of fibre)
How can co-contraction be measured?
- Vastus Lateris EMG
- Biceps Femoris EMG
What does co-contraction of antagonists do?
Reduces measured external force of agonists (slows down movement - a break)
Force = agonist Break = antagonist
What does the force-velocity relationship in-vivo show?
At high velocities, a high force cannot be produced
What does neural activation of muscles consist of?
- brain used to make movements
- to use a muscle to push > activate brain > signal to synaptic cleft > muscle fibres
- every time you activate muscle, you have neural activation (which differs - many activate different motor units + recruitment may be different; spatial or temporal)
What happens to neural activation as a person ages?
Weaker - cannot activate muscles as all motor units can’t be recruited
