EXSS 385 NEW SLIDES FINAL

functions of bone biomechanics

• protect vital organs

• support soft tissue

• produce RBCs

• reservoir for minerals

• provide attachment sites for skeletal muscles

• acts as system of machines to recieve muscle torques and make movement possible

factors of bone biomechanics influencing mechanical properties

• structure

• geometry

• mode of loading

• rate of loading

• frequency of loading

• muscle activity

• age

bone biomechanics → structure composition

• collagen (25-30% dry weight)

• mineral (60-70% dry weight)

• ground substances (5% dry weight)

• water (20-25%)

structure → collagen

• tensile strength

• provides flexability (relatively little)

structure → mineral

• calcium & phosphate

• gives compressive strength

structure → ground substance

• gel like substance surrounding collegen fibers

• gives compressive strength

bone biomechanics → geometry

cross sectional area proportional to ultimate failure point

• increased area = increased ultimate failure point

polar moment of inertia

quantity used to predict an objects ability to resist torsion

increased polar moment of inertia meaning

the mass is distributed away from neutral axis

decreased polar moment of inertia meaning

the mass is distributed close to neutral axis

anisotropic mode of loading

stiffness & strength depend upon mode of loading

what mode of loading is failure point HIGHEST

during compression

• 193 MPa

what mode of loading is failure point SECOND HIGHEST

during tension

• 133 MPa

what mode of loading is failure point LOWEST

during shear

• 68 MPa

viscoelasticity

mateirals may behave in both elastic and viscous manners with different rates of loading

• rate-sensitive loading

viscoelastic

stiffness & strength depend upon speed of applied load

what does loading rate influence

influences fracture patterns and soft tissue damage

• energy release

what components make a high loading rate

• increased stiffness

• ncreased ultimate failure point

• increased energy storage prior to failure point

viscous fluid

time-dependent deformation

elastic solid

returns to original shape

stiffness if there’s higher loading rate

increased stiffness

stiffness if there’s lower loading rate

decreased stiffness

physical effects of decreased loading rate

• cracking (no fragments)

• little to no soft tissue damage

physical effects of increased loading rate

• comminuted (fragments displaced)

• extensive soft tissue damage

frequency of loading

interplay of load, repetition, & injury

effects of frequency of loading

• bone fatigue

• fatigue fracture

bone fatigue

weakening of bone during repeated loading

factors of bone fatigue

• magnitude of load

• number of load applications (+ in a given time)

high number of load applications in a given time

fatigue process outpaces the repair-remodeling process

fatigue fracture (stress fracture)

• high load, few repetitions

• low load, many repetitions

fatigue-injury curve

as repetition increases, the load tolerance of tissues decreases, leading to higher injury risk

what do muscles produce

may produce tensile or compressive stresses on bone

• offsets mechanical stresses in opposite directions

bone density with age

there is progressive decrease in bone density as age increases

collagen and mineral content with age

collagen and mineral content decrease as age increases

bone mass and size with age

bone mass and size decrease as age increases

stiffness and strength with age

stiffness and strength decrease as age increases

bone remodeling

alters size, shape, & structure based on the imposed mechanical demands

Wolff’s Law

bone tissue is gained or lost depending upon the level of stress sustained

Wolffs Law → mechanical stress

• high mechanical stress causes high bone tissue production

• low mechanical stress causes low bone tissue production

what is Wolff’s law affected by

affected by activity level and implants

stress shielding

reduction in bone density (osteopenia) by removing typical stress from the bone by an implant

factors influencing active muscle force production

• neural factors

• fiber type

• mechanical factors

• muscle architecture

• muscle stiffness

neural factors

how strongly and frequently the nervous system signals the muscle

how neural factors affect active muscle force

• muscle fiber activation & discharge rate

• motor unit recruitment

muscle fiber activation & discharge rate TYPES

• twitch

• summation

• tetanus

TWITCH muscle fiber activation

response of muscle to single stimulus

• one single activation hump

SUMMATION muscle fiber activation

the overall effect of added stimuli

• multiple activation humps that add off eachother

TETANUS muscle fiber activation

sustaied maximal tension due to high frequency stimulation

• straigh line thats very high due to continuous activation

fiber type

• fast twistch fibers (type IIa & IIb)

• slow twitch fibers (type I)

type I slow twitch fiber → shortening speed

slow

type I slow twitch fiber → size

small

type I slow twitch fiber → force production

low

type IIa fast twitch fiber → shortening system

fast

type IIa fast twitch fiber → size

large

type IIa fast twitch fiber → force production

high

type IIb fast twitch fiber → shortening system

fast

type IIb fast twitch fiber → size

large

type IIb fast twitch fiber → force production

high

fast twitch vs slow twitch → peak force

fast twitch fibers have a higher peak force, but lasts shorter

fast twitch vs slow twitch → rate of force production

fast twitch fibers have higher rate of force production

all fibers within a motor unit

all fibers within a motor unit are the same time

all fibers within a muscle

all fibers within a muscle have a mixture of fiber types

what does motor recruitment increase

muscle stiffness

ordered recruitment → Hennemans size principal

1. type I recruitment first (lowest threshold)

2. type IIa recruited second

3. type IIb recruited last (highest threshold)

Hennemans size principal allow for

allows for controlled, smooth gradation of force

• reduction in tension

can fiber types change

fiber types can change with training

• type distribution is largely genetic

muscle architecture

arragement of contractile components that affects force production, excursion, & velocity

• ex: pennation angle

muscle arrangement types

• parallel

• series

muscle arrangement → parallel

side to side arrangement

parallel fiber architecture

represent muscle fibers as parallel springs

addition of parallel springs

K total = K1+ K2 + K3…..

muscle arrangement → series

end to end arrangement

types of muscle fiber architecture

• longitudinal

• unipennate

• bipennate

• fusiform

muscle fiber architecture → longitudinal

fibers run straight along the length of the muscle

• ex: esophagus

whats the “plus” of longtiduinal architecture

good for long excursions

muscle fiber architecture → unipennate

fibers insert at an angle on one side of a tendon

• ex: lumbicals

whats the “plus” of a unipennate architecture

more fibers can pack into a given area to increase force capactity

muscle fiber architecture → bipennate

fibers attack on both sides of a central tendon

• ex: gastrocnemus

whats the “plus” of a bipennate architecture

allows even MORE fibers to fit in to generate high force

muscle fiber architecture → fusiform

fibers are wider in the middle and narrower at the ends

• ex: biceps brachii

whats the “plus” of a fusiform artchitecture

can move a lot and shorten quickly

muscle stiffness

passive elastic properties of the muscle

motor unit recruitment

how many motor units the nervous system activates

what is a motor unit?

a single motor neuron and all the muscle fibers it innervates

• 3 to 2,000 fibers innervated (innervation ratio)

• “all or none” principal

motor unit importance

• functional unit of muscle

• smallest unit of muscle contraction

• all muscle fibers respond as one

muscle force proportional to

• proportional to the # of motor units recruited

• proportional to stimulation/firing rate

what does synchronization of firing impulses increase

may increase muscle force

angle of pennation (θ)

alignment of muscle fibers relative to line of pull

if pennation angle (θ) = 0 degrees meaning

the fibers are aligned with the line of pull

resultant force in less pennated fiber arrangements

increase in resultant force directed along the line of pull

2 ways force can be directed relative to the muscle

1. directed parallel to the longitudinal axis

2. directed at an angle (of pennation) to longitudinal axis

F long =

Fcos(θ)

• F = x Newtons

for the same muscle volume, longitudinal arragements produce…

less force than pennate arrangements

why do longitudinal arragements produce less than pennate arragements

there are more crossbridge formations available

• because more fibers outweighs less resultant force

stiffness

force response to a mechanical stress

stiffness equation

stiffness (k) = (ΔF)/(ΔL)

what do muscle fibers possess

stiffness

in what ways can stiffness be controlled

• intrinsic stiffness

• reflex mediated stiffness

• joint stiffness

intrinsic stiffness

prepatory muscle activation

reflex mediated stiffness

reflex activation