Muscle design

Question 1

in a work loop, what is the net work per cycle

Answer 1

difference betweeen shortening work and lengthening work

Question 2

what does it mean if the work loop is moving CCW

Answer 2

net work per cycle = positive

Question 3

what does it mean if the work loop is moving CW

Answer 3

net work per cycle = negative

Question 4

what are the different roles a muscle can have in vivo and what could their work loops show

Answer 4

motors - positive net work
brakes - shows hysteresis
springs
struts - large length change with minimal force and large change in force over short length change

can have different roles across different conditions / tasks

Question 5

why is the activation rate of fast fibres important

Answer 5

can impact work per cycle across cycle frequencies

Question 6

what is the difference in net work and cycle frequencies between fast and slow fibres

Answer 6

net work is higher from fast fibres
operating range of freqeuncies is higher in fast fibres
- fast fibres do more work

Question 7

what is the activation of slow and fast fibres at a cycle frequency of 1 Hz

Answer 7

similar for both
- fast fibres reach peak activation and deactivate quicker

Question 8

what is the force of slow and fast fibres at a cycle frequency of 1 Hz

Answer 8

higher and decreases faster in fast fibres
takes longer to get back to zero in slow fibres
- force peaks at slowest velocity (peak length - v=0)
- velocity is highest when the length crosses the x axis (where F=0)

Question 9

what is the work loop of slow and fast fibres at a cycle frequency of 1 Hz

Answer 9

work per cycle = higher in fast fibres
faster decline in force from fast fibres (fast activation and deactivation)
both show CCW rotation (positive net work)

Question 10

what is the activation of slow and fast fibres at a cycle frequency of 3 Hz

Answer 10

fast fibres activate and deactivate faster
don’t get to max activation in slow fibres at that speed
muscle is active during lengthening period
- mostly in slow fibres ^ because they can’t deactivate fast enough
(don’t want the activation during lengthening)

Question 11

what is the force of slow and fast fibres at a cycle frequency of 3 Hz

Answer 11

slow fibres
- have high force during lengthening (eccentric)
fast fibres
- more force during shortening
- can activate and deactivate fast enough

Question 12

what is the work loop of slow and fast fibres at a cycle frequency of 3 Hz

Answer 12

slow fibres = CW rotation = negative work loop cycle
- due to slow act/deact and high force during lengthening
fast fibres = CCW rotation = positive work loop cycle

Question 13

what is the difference in longer muscles

Answer 13

longer muscle can produce force over a greater range of absolute lengths and at higher absolute velocities

Question 14

what is the difference in muscles with a greater CSA

Answer 14

muscle with greater CSA will produce more absolute force
(ex: doubling CSA will double the max isometric force)
- max isometric stress is fairly constant across muscles of different sizes

Question 15

what is the PCSA (physiological cross sectional area)

Answer 15

area perpendicular to the direction of the fibres

Question 16

what is caused by greater pennation in muscles

Answer 16

results in greater PCSA
more fibres can be packed in parallel for a muscle with a given volume
- therefore more force produced

Question 17

why does pennate muscle have a greater force

Answer 17

there are more benefits from the increase in PCSA than the decrease in cos(theta)

Question 18

what is the architectural gear ratio

Answer 18

= muscle belly velocity / fibre velocity

Question 19

what is the result of pennation angle increasing

Answer 19

fibres rotating to greater angles allows the fibres to contract slower relative to the muscle belly
- allows the fibres to produce more force (force-velocity relationship)

Question 20

do parallel and pennate muscles have the same muscle belly velocity

Answer 20

yes - same muscle belly velocity

Question 21

what is the difference between the parallel and pennate muscle fibre velocity

Answer 21

parallel fibres = same as the muscle belly velocity
pennate fibres = velocity is slower than the belly (can stay almost the same length during rotation)

Question 22

what can larger pennation help mitigate

Answer 22

helps mitigate the effects of greater tissue inertia that comes with increases in muscle size

Question 23

what occurs to other muscle variables when length increases 2x

Answer 23

length, velocity, acceleration = 2x increase
force = 4x increase
volume, mass = 8x increase

Question 24

what does larger muscle size result in

Answer 24

results in larger mass relative to force that is available to accelerate it
- less work can be done per cycle (shifts work looop down)
- net force decreases

Question 25

what is the difference in work penalty for larger muscles in parallel vs pennate

Answer 25

parallel = greater work penalty for larger mass
pennate = lower work penalty for larger mass