Muscle locomotion in locomotion

Question 1

What is force?

Answer 1

Load, resistance, stress

Question 2

What are requirements for terrestrial locomotion?

Answer 2

Move body centre of mass (COM)
Support body weight against gravity, should be as stable and economical as possible
Move limbs into position for stance

Question 3

How do Newton’s laws relate to motion and forces?

Answer 3

Gravity exerts a downward accleration (constant) 9.81 ms-2 - force is proportional to mass x gravity (mg)

1. To remain standing up, the legs must exert a balanced force upwards
   - F vertical = mg + F legs = 0
   - F legs = - Mg
2. When legs push against the ground, the ground pushes back - ground reaction force (GRF or F or G)

Question 4

How do we measure F?

Answer 4

Measured by a force plate

• push down = vertical force
• push up = horizontal force
• vector sum = resultant force

Question 5

What role to ground reaction forces play in steady locomotion?

Answer 5

Average vertical force must support bodyweight
- average Fv = mass x gravity
More legs that are on the ground = lower force per leg
Horizontal (fore-aft) forces average zero to maintain average speed
- average Fk =Ma = zero

Question 6

How do forces vary with speed and gait?

Answer 6

Peak force increases with speed as duration of contact (tc) decreases
Magnitude of peak force 𝛂1/tc

Question 7

What is the average force that a horses front limbs experience?

Answer 7

Walk ~ 0.5 x body weight
Trot ~ 1 x body weight
Canter ~ 2.5 x body weight

Question 8

How do the GRF and limb posture determine the required muscle force and work?

Answer 8

Muscles contract to resist these locomotor forces
- to do this they must produce work & power to move the limb at an appropriate speed
Work = force x displacement, required work to move mass
Power = work/time, requires power to do work quickly

Question 9

What does a muscle do?

Answer 9

Produces force (in tension only) - contraction means generate force 
Produces movement 
Muscles do not flex they contract 
- joints flex and extend

Question 10

What are the three possible motions of muscle during “contraction”?

Answer 10

Shortening = positive work, most energetically expensive 
Isometric = no work, contract to resist load, cost proportional to force 
Lengthening = negative work, most economic but also most likely to leave muscle injury

Question 11

How does muscle function relate to it’s anatomy?

Answer 11

• muscle cells (fibres) are made from sarcomeres in series (end-to-end)
• muscle tissue is made from fibres arranged in parallel (side-by-side)
• musculoskeletal level systems, trade-off between force and displacement
• work = force x distance, for the same work:
• large force, small distance
• small force, large distance

Question 12

What is the moment arm?

Answer 12

Fout = output side where you want to move a mass
Fin = input side where muscles act
Force x moment arm = torque, balance on to sides of the joint
- Fin x r = Fout x R, Torque in = Torque out
- push down (small force input), effective mechanical advantage : r/R

Question 13

What is a musculoskeletal lever system?

Answer 13

Muscle force required to resist ground reaction force (Fg) - force x distance balance on two sides of the joint

• Fmuscle x r = Fg x R, Fmuscle = Fg x (R/r)
• Fg = Fmuscle x (r/R)

Question 14

What is the effective muscle advantage?

Answer 14

EMA = r/R
If R decreases EMA increases, resulting in lower muscle force required to support Fg but also less speed and foot displacement

Question 15

What is the relationship between EMA and R?

Answer 15

If R decreases EMA increases, resulting in lower muscle force required to support Fg but also less speed and foot displacement

Question 16

What is the relationship between EMA and r?

Answer 16

If r decreases, EMA decreases and there is a greater muscle force required to support Fg but also greater foot displacement and speed

Question 17

How does scaling with isometry affect dimensions?

Answer 17

Strength proportional to cross - sectional area (L2)
Loading is proportional to body mass and volume (L3)
CSA: mass decreases with an increase in size - if animals get larger without changing shape, they become relatively weaker

Question 18

Why does size matter?

Answer 18

Area increases slower than mass/volume
Force required = mxg
Strength of bone and muscles: ≈ cross -sectional area
Large animals are generally weaker (force limited)

Question 19

What does scaling of limb posture cause?

Answer 19

A posture shift

Question 20

What does scaling of limb effective mechanical advantage cause?

Answer 20

Allows muscle forces to scale similar to bone and muscle areas
Upright graviportal limb posture decreases the required muscle forces
- restricted in slower gaits
An elephant uses 1/20th more energy to move 1g mass a given distance than a mouse