Midterm 2 Flashcards
tensile stress (Pa)
axial stress that tends to pull apart the molecules at the analysis plane
- object elongates
compressive stress (Pa)
axial stress that tends to push or squash the molecules together at the analysis plane
- object shortens
shear stress (Pa)
transverse stress that acts parallel to the analysis plane
- molecules slide past one another
- one force pushes down the other pushes up
simple (uniaxial) loads
one type of stress produced, uniform across plane
complex mechanical loads
multiple types of stress produced, stress varies across the plane
bending (Pa)
counteracting tensile and compressive stress
- object with greater depth and larger CSA can withstand greater bending
torsion (Pa)
twisting forces - shear force
- creates an internal torque
- object with greater diameter (area) can withstand greater torsion
combined loads
combination of loads
1. muscles, tendons & ligaments: carry one type of load (uniaxial tension)
2. bones and cartilage: carry multiple loads (more complex)
linear strain
change in length/deformation as a result of tensile or compressive stress
shear strain
change in orientation of adjacent molecules as a result of molecules slipping past each other due to shear stress
elastic modulus
ratio of stress to strain
elastic behaviour
occurs when an object stretches under tensile load, but return back to original shape when the load is removed (rubber band)
material strength
maximum stress/strain a material is able to withstand before failure
plastic behaviour
when a permanent deformation of the object occurs under a load
yield point
point on stress-strain curve where further stress will cause permanent deformation
yield strength
stress at the elastic limit of a materials stress-strain curve
ultimate strength
maximum stress the material is capable of withstanding
failure strength
stress where failure actually occurs (endpoint, breakage)
ductile vs brittle materials
ductile: can withstand lots of plastic deformation before failure = high failure strain
brittle: require little plastic deformation before failure = small failure strains
hard vs soft material
hard: stiff = large failure stress
soft: pliant = small failure stress
toughness
ability to absorb energy - area under stress-strain curve
- tougher = the more energy requires to reach failure
viscoelastic materials
both viscous and elastic behaviours
(liquid and solid)
ex. bone, tendon, ligament, cartilage, muscle
properties of viscoelastic materials
- strain-rate dependency
- stress-relaxation
- creep
- hysteresis
strain-rate dependency
rate at which you deform/strain a tissue will effect the stress it feels
- faster loading = more stress
stress-relaxation
Decrease in stress under constant strain
(length held constant)
creep
Increase in plastic strain under constant stress
(load held constant)
hysteresis
the amount of energy absorbed during loading and unloading
active element
muscle tissue
passive element
connective tissue
collagen
stiff, brittle, high tensile strength, unable to resist compression
elastin
pliant (soft), extensible, ductile, high failure strain
isotropic
same mechanical properties in every direction
ansiotropic
have different mechanical properties depending on direction of the load
ex. tendon is stronger if tensile load is aligned with tendon fibres rather than transverse
two primary factors affecting mechanical properties of tissues
- activity
- age
activity
- strength of CT increases with use
- stiffness may also increase
***inactivity = decrease in strength of tissues
age
- CT increases in stranght with age until 30 years old, then strength decreases
- bone become brittle and less tough with increasing age
- tendons and ligaments become less stiff
bones
- strongest in compression, weakest in shear
- high tensile strength
- strongest and stiffest material
- ansiotropic
cortical (compact) bone
found in dense and hard outer layers of bone
cancellous (spongy/trabecular) bone
less dense, porous bone that is spongy in appearance and found deep to cortical bone
cartilage
able to withstand compressive, tensile, and shear loads
hyaline cartilage
covers the end of long bones at joints
1. no blood supply, thin to allow diffusion of nutrients
2. made of water and water is not compressible so under compressive load, collagen is under tension
fibrous cartilage
found within some joint cavities
elastic cartilage
found in external ear and in several other organs that are not part musculoskeletal system
tendons and ligaments
major difference is arrangement of their collagen fibres
tendons
collagen fibres are bound together in parallel
- parallel arrangement produces a structure that is very stiff and high in tensile strength
- little resistance to compression or shear
ligaments
collagen fibres are bound together nearly parallel
- have slightly larger elastin component making them less stiff and slightly weaker than tendons
muscle fibres
encased in endomysium
fascicles
bundles of muscle fibres
- encased in perimysium
whole muscle
bundles or muscles fascicles
- epimysium
tendon
woven CT that connects muscle to bone allowing for movement
sarcomeres
basic contractile unit of a muscle
- contain overlapping filaments of myosin (thick) and actin (thin)
resting state
tropomyosin prevents myosin from bonding to actin
z-line
band that anchors actin to each other, boundary of one sarcomere
I-band
area that contains only actin
A-band
area that contains myosin, overlapping with actin
H-zone
area that contains only myosin
M-line
band that anchors myosin to one another
factors affecting force development
- length
- velocity
- physiological cross-sectional area
- muscle geometry
- activation
resting length
greatest active tension - optimal length
contractile element
represents force development by sarcomeres
force-velocity relationship
the greater the shortening velocity, the smaller the force produced
- slower the contraction velocity = more force produced (more time for cross-bridges to form and exert force)
concentric
muscle shortens while generation force (lifting weights)
1. muscle contraction or flexion - muscle is moving body part closer to center body
2. muscle tension exceeds the external load
AS VELOCITY INCREASES, FORCE DECREASES
eccentric
muscle lengthens while generating force (lowering weight or body)
1. lengthening or extension - muscle is moving body part away
2. external load exceeds muscle tension = muscle lengthens
AS VELOCITY INCREASES, FORCE INCREASES
physiological cross-sectional area
adding sarcomeres in parallel makes a muscle stronger
activation of muscle fibres
force produced by a muscle is directly proportional to number of fibres that contract at any given time
- more fibres contracting = more force produced
fine/precise control
smaller number of muscle fibres per motor neuron
coarse control
larger number of muscle fibres per motor neuron
hennemans size principle
motor units are recruited from smallest (slow-twitch) to largest (fast-twitch)
- beneficial for fatigue prevention + fine + coarse control
sarcomere arrangement
length-wise= strong muscle
height-wise= fast muscle
***longer the muscle length, the easier the ability to create tension
shorter moment arm
more force required with same torque
larger joint angle
faster the muscle
longer moment arm
less force required with same torque
smaller joint angle
slower the muscle
series element
muscle fibres aligned end-to-end so the forces are additive
1. greater ROM
2. greater force production
3. limited strength
***typically longitudinal muscle
parallel element
muscle fibres aligned side-by-side, running parallel to each other
1. greater strength (not faster)
2. limited ROM
force-length relationship - optimal length
generate highest force due to best overlap of myosin and actin to form cross-bridges
force-length relationship - too short
force production is reduced cause there is limited overlap between myosin and actin thereby reducing cross-bridge formation
force-length relationship - too stretched
force production is reduced cause myosin and actin are pulled too far apart, limiting cross-bridge formation
passive tension
as a muscle length increases (overstretch), active force is decreasing and passive force component comes into play and increases total force production of the muscle
