biomechanics

evolution by natural selection

• variability in a population

• heritable trait

• overproduction of offspring

• differential survival and reproduction

• result: change in gene frequencies that correlated with a change in morphology, behavior, physiology, etc.

soilds

care about how far they are deformed (elasticity)

fluids

care about how fast they deform (viscosity)

laminar flow

Re < 10

turbulent flow

40 < Re < 200,000

no slip condition

viscous fluid does not slip of a solid surface

creates a boundary layer, where fluid on the solid transitions from 0 m/s to the actual moving velocity of the fluid

Reynolds number equation

Re = (pUI)/u

p = densiyt

U = velocity

I = size

u = dynamic viscosity

swimming and reynolds number

Low Re: fish fight viscosity, stop moving when fins stop

High Re: can glide through water after fins stop bc of interia

skin drag (friction drag)

consequence of the no slip condition and the viscosity of water/air

pressure drag

conservation of energy—> in steady flow, sum of various energies remain constant

velocity increases = pressure decreases

biomaterials

• slimes: low stiffness, high extensibility

• rubberlike materials: have high resilience—able to store and release energy efficiently (abductin)

• fiberous biomaterials: bear tensional loads, very stiff

• crystalline composites: stiffest biomaterial, calcium carbonate

abductin

rubberlike material (highly resilient)

deforms when adductor muscle closes shell

pushes shell apart when adductor muscle relaxes

hydrostatic skeleton

common in soft-bodied invertebrates in which fluid transduces the action of muscle

antagonistic muscles

one contracts, other relaxes

levers

can amplify for force (bent arm with load in hand)

can amplify force (stick under boulder)

torque

= radius x force

work loop

when a muscle goes through a cycle of contraction and re-extension

parallel muscle fibres

long fibers, can contract long

produce low forces

bulge out

pennate muscles

short fibres, short contracting distance

produces higher forces

doesn’t bulge

why pennate muscles and parallel muscles shorten with different speed and force

parallel muscles are longer and shorten faster but generate less forc

pennate muscles are shorter and contract slower but produce more force and has more fibers per volume

LaMSA

has a latch (releases) component and spring (energy load) component

positive allometry

Y increases at a faster rate than x, b>1

negative allometry

Y increases at a slower rate than x, b <1

muscle contraction isotonically

you are hanging a load off a muscle

load is constant, length changes

negative power output of a muscle

it is absorbing energy, such as walking down a hill or landings a jump

muscle is lengthening