Contractile mechanics

Question 1

what is the difference between unfused and fused tetanus

Answer 1

unfused = caused by repeated stimulation pulses at higher frequencies that results in twitches summating
fused = caused by repeated stimulation pulses at higher frequencies than unfused that results in fusion - smoothing out of force

Question 2

what is the differences in twitches/tetanus in the different fibre types

Answer 2

fast
- activates and deactivates sooner
- can reach higher peak force
- requires higehr stimulation frequencies in order to reach fused tetanus
slow
- activates and deactivates slower
- requires lower stimulation frequencies to reach fused tetanus

Question 3

what does the amount of force a sarcomere can produce depend on

Answer 3

depends on the amount of overlap between actin and myosin filaments
(active force-length relationship - due to sliding filaments)
- changes depending on sarcomere length

Question 4

what is the average resting sarcomere length

Answer 4

2-2.2 um

Question 5

are all the sarcomere lengths the same in whole muscle
- what does this result in

Answer 5

no - heterogenous (sarcomere non - uniformity)
results in smoothing of the whole muscle force length relationship (averaging effect of force - length relationship)

Question 6

what occurs during the ascending limb of the force length relationship

Answer 6

length and force increases
- as force increases, length increases (cross bridge cycling)

Question 7

what occurs during the descending limb of the force length relationship

Answer 7

as length increases past the plateau region, force decreases
- not enough overlap for cross bridges to form

Question 8

why do we normalise force and length

Answer 8

to allow comparison across muscles of different sizes

Question 9

what is the optimal length vs normalised muscle length

Answer 9

optimal length = length where max isometric force occurs
normalised length = all values divided by optimal length value

Question 10

what is the optimal force vs normalised muscle force

Answer 10

optimal force = max isometric force
normalised = all values divided by optimal force value

Question 11

what causes passive forces at the sarcomere level

Answer 11

primarily due to titin (protein)
- resistive force - muscle wants to recoil
- titin stretches like a spring and produces recoil force to move it back to a shorter length

Question 12

what causes passive forces at the whole muscle level

Answer 12

due to ECM
- endo, peri, and epimysium
- also other structures

Question 13

how is total muscle force found

Answer 13

given by adding the passive and active forces

Question 14

what is the path of total force on a graph

Answer 14

total force increases (active force increases)
dips and decreases (between active and passive max)
increases again after (passive force increases)

Question 15

what does the max shortening velocity depend on

Answer 15

depends on fibre type properties
- faster fibres / muscle shave faster shortening velocities

Question 16

when can muscles produce force (concentric vs eccentric)

Answer 16

muscles can only produce force in concentric movement
in eccentric:
- muscle is still producing force to shorten but the force to lengthen over rides it so the net movement is lengthening

Question 17

what is the relationship between shortening velocity and force production

Answer 17

faster the muscle shortens, the lower the force will be
- max shortening veleocity produces barely any force
- depends on fibre type

Question 18

what is the huxley muscle model

Answer 18

based on dynamics of a cross bridge
limitations
- don’t have good descriptions of everything included in the model
- don’t account for things outside the fibre (muscle shape, function, volume)

Question 19

what is the geometric muscle model

Answer 19

useful in understanding importance of muscel shape change and affect on force
can account for different shape change factors (constant volume, thickness, and area)

Question 20

what is the 3D continuum model

Answer 20

accounts for volume, ECM, and 3D shape
can account for non uniformity (understand more complexity)

Question 21

what is the basis of a hill type muscle model

Answer 21

the model muscle is a scaled up single muscle fibre
consists of 3 elements:
1. CE = contractile element
2. PEE = parallel elastic element
3. SEE = series elastic element

Question 22

what is the difference between a series and parallel arrangement

Answer 22

series = all components are equal to each other
parallel = all components add together

Question 23

what is the contractile element and what is it combined with

Answer 23

contractile element = active muscle force
in parallel with parallel elastic element
(added together)

Question 24

what is the parallel elastic element and what is it combined with

Answer 24

PEE = passive muscle force
in parallel with contractile element
(added together)
in series with series elastic element
(equal to)

Question 25

what is the series elastic element and what is it combined with

Answer 25

SEE = tendon force
in series with both the contractile element and the parallel elastic element (total muscle force)
(equal to)

Question 26

what is the activation component

Answer 26

scales the active force
1 = max force contraction
everything below 1 = submax contraction

Question 27

what does muscle force depend on

Answer 27

contraction history
- previous active lengthening and shortening

Question 28

what is the muscle force response following active lengthening

Answer 28

force enhancement
(increase in force)

Question 29

what is the muscle force response following active shortening

Answer 29

force depression
(decreases)

Question 30

why does force enhancement occur

Answer 30

likely due to the behaviour of titin