mechanics of human movement exam 4

contact forces

forces resulting from physical contact between objects

ex: ground reaction force, friction, fluid resistance, buoyant force

non-contact forces

forces that do NOT result from direct physical contact

ex: gravity (weight), electromagnetism

external force

forces outside the system of interest

internal force

force applied within the tissues of the body (also called "loads")

compression

pressing or squeezing force directed normal to a surface (normal = perpendicular to the longitudinal axis); causes widening and/or shortening

tension

pulling or stretching force directed normal to a surface (normal = perpendicular to the longitudinal axis); causes lengthening and/or narrowing

shear

sliding or tearing force directed parallel to the surface; internal deformation in angular direction

deformation

change in shape; result of stress

stress

distribution of a force on the body; the force divided by the area over which the force acts

Pa (Pascal = 1 N/m^2)

unit for stress

pressure

stress due to a compressive force

unequal

equal tensile forces may result in ______________ (equal/unequal) displacements.

unequal

equal displacements may result in from _____________ (equal/unequal) forces.

complex loading

bending, torsion, and combination

bending

asymmetric loading that produces tension on one side of a body's longitudinal axis and compression on the other

torsion

loading producing twisting of a body around the longitudinal axis; internal shear stress with maximal stress at the surface; beyond physiological limits - can lead to spiral fractures

femoral neck fractures

result of bending loads in the body: common in the elderly due to decreased bone mineral density

common injury mechanism: unexpected load (e.g., stepping off a curb)

combination (complex loading)

one bone may experience: weight-bearing (compression/bending), twisting (torsion), and/or muscle action (tension)

fairly common, during walking both the femur and tibia experience complex loading

strain

the deformation a tissue undergoes scaled to its original length; a measure of deformation (ε)

ε = change in length (final - initial) / original length (initial)

UNITLESS

stress-strain curve

yield point = the most elastic point before injury

ultimate strength = highest stress point

failure strength = rupture/fracture

hard and brittle

what is letter A?

- ductile and soft

- hard and brittle

- ductile and tough

ductile and tough

what is letter B?

- ductile and soft

- hard and brittle

- ductile and tough

ductile and soft

what is letter C?

- ductile and soft

- hard and brittle

- ductile and tough

stiff

what is letter A?

- stiff

- pliant

pliant

what is letter B?

- stiff

- pliant

stiff and ductile

what is letter A?

- brittle

- pliant and ductile

- stiff and ductile

brittle

what is letter B?

- brittle

- pliant and ductile

- stiff and ductile

pliant and ductile

what is letter C?

- brittle

- pliant and ductile

- stiff and ductile

connective tissue

connects and protects; most abundant tissue; most common element is collagen

collagen and elastin

what are the fibrous proteins in connective tissues?

collagen

5 different types; like a rope - may stretch a little when you pull on it, and very good at resisting the pulling forces, but can't push

type I cartilage

thick, rugged fibers that stretch slightly when under tension (tendon, ligaments, bone)

type II cartilage

thinner fibers that provide framework and shape of structures (cartilage)

elastin

resists tension, allows tissue to return to original shape after deformation; similar to a rubber band - stretchy and usually returns to its original shape unless it's been damaged by stretching too much or too fast

ground substance of connective tissues

glycosaminoglycans, solutes, water; helps maintain the integrity of out tissues and helps transport nutrients and waste in and out of our tissues; vital to maintaining the water balance that our healthy tissues need to respond appropriately to loading

fibroblasts and chondrocytes

cells of connective tissues

ligament

dense, parallel collagen; responsible for guiding the motion at a joint and typically acts as "restraints" to prevent unwanted or excessive motion at a joint; bone to bone

tendon

dense, parallel collagen; transmit the tensile forces from the muscle to the bone to either cause movement or increase joint stability; muscle to bone

hysteresis

energy loss

viscoelastic

how tendons and ligaments respond to loading depends on how fast a load is applied; also very resilient with very little hysteresis - this is because they're about 25% collagen

tendon

has parallel bundles of collagen; can only resist loads longitudinally (or in series with the collagen fibers)

ligament

has both parallel and cross-catching bundles of collagen; can bare small tensile loads at oblique angles

passive tension

ligaments prevent excess motion by generating _________ _________.

crimp (slack)

the wavy organization of fibers in tendons and ligaments

cartilage

collagen fibers within jellylike substance; framework is resistant to damage (superficial side is parallel to surface, in the middle is randomly arranged, and deep surface is perpendicular to surface); 60-80% water

creep

constant compressive stress to cartilage results in:

- strain increases as water is squeezed out

- strain reaches a maximum

- strain stabilizes

stress relaxation

constant compressive strain to cartilage results in:

- stress increases as water is squeezed out

- stress reaches a maximum

- stress decreases (relaxes) to a lower value

bone

hardest of the connective tissues; matrix contains calcium and phosphorous; functions - (mechanical) support, levers, protection and (physiological) mineral/energy storage, blood cell formation

minerals

bone composition:

provides compressive strength; ___________ deficiency = bendable bones (e.g., Rickets)

collagen

bone composition:

provides tensile strength; loss of ___________ causes bone to become brittle

cancellous bone

"spongy" bone; found in the ends of long bones, vertebrae; porous (not dense); contains trabeculae; offers structure without the weight - our bones would be too heavy without it

trabeculae

irregular latticework of collagen and inorganic material

cortical bone

"dense" bone; orderly arrangement in the shaft of long bones; very dense (few spaces without collagen and inorganic material); stronger than cancellous bone - resists bending

Wolff's Law

bone will be laid down where needed and reabsorbed where not needed

factors that effect bone remodeling

minerals, vitamins, sex hormones, and mechanical loading

osteoporosis

bone reabsorption outpaces growth

anisotropic

bone behavior varies with direction of loading

viscoelastic

bone behavior depends on the rate and duration of loading

anisotropic

connective tissues such as tendons and ligaments are ______________, meaning that they do not have the same mechanical properties in every direction.

false

true/false

bones without inorganic components (i.e., minerals) become brittle and shatter easily.

force per unit area

stress is defined as the:

a) mass per unit volume

b) force per unit time

c) force per unit area

d) displacement per unit time

e) force per unit mass

less compact than cortical bone, important for shock absorption, and more porous than cortical bones

cancellous bone is: (choose all that apply)

a) less compact than cortical bone

b) important for shock absorption

c) more porous than cortical bones

d) provides the outer shell of long bones

increasing the surface area over which a force is distributed decreases the amount of stress acting on the surface

which of the following is true?

a) increasing the surface area over which a force is distributed decreases the amount of stress acting on the surface

b) decreasing the surface area over which a force is distributed decreases the amount of stress acting on the surface

c) both a and b above are true

d) none of the above are true

shear stress perpendicular to the shaft of the bone

bone is weakest in resisting:

a) tensile stress

b) shear stress perpendicular to the shaft of the bone

c) compressive stress

d) bone is equally strong in resisting all types of stress

tensile

the organic components in bone (e.g., collagen) provide __________ strength.

creep

when articular cartilage is exposed to constant compressive stress, the strain in the cartilage will increase gradually with time, followed by a plateau in the strain. this pattern is called ________.

a) elasticity

b) creep

c) collagen

d) stress-strain curve

injury

loads applied exceed the tissue's strength

acute injury

only one load causing injury; ex: car accident causing a leg break

overuse (repetitive) injury

occurs when people perform the same task over and over again; ex: stress fractures

material and force

deformation is local shape changes due to applied loads defined by: __________________ (properties, size, shape) and ______________ (magnitude, direction, duration)

high

low physical activity results in (low/high) risk for injury.

pathologic underload zone

low stress; when you're injured you can't work out like how you used to

pathologic overload zone

high stress; overloading causes injury - shin splints

eustress

good stress - normal amount when working out

physiologic loading zone

stress level used to live our daily life

physiologic training zone

working out and training our tissues - normal amount of stress

force, repetition, posture

the three biomechanical forces related to injury

single point

the COM (center of mass) is a ___________ ___________ associated with a body where, if supported at that point, the object will remain balanced

limb

in the human body, the COM location is dependent on ___________ position

balance

a person's maintenance of the center of mass within their base of support

base of support

area beneath and between the points of contact a person has with the ground; area bound by the outermost region of the body and support surface or surfaces

stability

capacity of an object to return to its original position (equilibrium) after it has been displaced

stable

when new behavior = old; maintaining balance

unstable

when new behavior ≠ old; a perturbation causes you to not maintain balance

state

balance is the __________ (state/act) of keeping the center of mass within the base of support to maintain upright posture

act

stability is the _______ (state/act) of maintaining, achieving, or restoring a state of balance

mass, friction, height of the COM, and base of support

the four factors that affect stability

how can you increase stability?

increase the size of the base of support OR change the position of the COM near the edge of the base of support (either on the side or opposite of the external force)

walking

trading stability for mobility

true

true/false

when a person lifting weights picks up a weight, that object contributes to a combined center of mass that must be balanced over the person's base of support.

linear

if a force acts directly though an objects center of gravity, _____ acceleration will occur.

a) angular

b) linear

c) parabolic

no

yes/no

can humans maximize both stability and mobility at the same time?