Mechanics of Human Movement: Unit 4

Center of Mass

The average location of the mass of an object

also called center of gravity

Balance

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

Stability

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

Balance vs. Stability: balance

the state of keeping the COM within the base of support to maintain upright posture

Balance vs. Stability : stability

the act of maintaining, achieving or restoring a state of balance

Four factors that affect stability

Mass, friction, height of the center of mass, base of support

How does mass affect stability?

the greater the mass, the greater the stability

ex: football player: harder to move, giving them an advantage in blocking or tackling

How does friction affect stability?

Greater friction between two surfaces requires greater force to initiate or maintain motion

ex: soccer cleats increase friction so players can sprint and cut without slipping

How does the height of the center of mass affect stability?

lower=more stability; harder to tip over

higher=less stability; easier to tip over

ex: gymnastics lower their COM when landing to increase balance and control

How does base of support affect stability?

wider base=more stable

smaller base=less stable

ex: basketball players use a wide stance while guarding opposing players

Mobility

body's ability to move through a range of motion smoothly

Stability vs. Mobility

stability: you want stability when you're squatting heavy or balancing on one leg

mobility: you want mobility in your hips to squat deep, in your shoulders to reach overhead, and in your ankles to walk/run properly

Balance of stability and mobility

you want an equal mix of the two

too much stability=stiffness

too much mobility without stability: wobbly and prone to injury

axial skeleton

skull, vertebrae, ribs (74 bones)

appendicular skeleton

extremities, shoulder girdle, pelvis (126 bones)

biomechanical functions of the skeletal system

levers and support

What elements compose human bone?

Minerals, water, protein (in order of largest to smallest quantity)

Cortical bone

compact, low porosity(measure of void space/ spongy)

makes up the shaft of long bones

Trabecular (cancellous) bone

less compact, high porosity

system of columns of bone

main columns run based on direction of load

makes up the ends of long bones and vertebrae

Anisotropic

bone that has different mechanical properties, like strength, depending on the direction of load

Bone is _____________ in __________________.

strongest, compression

Bone is ______________ in ___________.

weakest, shear

Strength in _____________ is between compression and shear.

tension

Tension, compression, shear in increasing tensile order

shear-->tension-->compression

Wolff's Law

Bone strength increases and decreases as the functional forces on the bone increase and decrease

bone in a healthy person will adapt to the loads under which it is placed

How does bone density change with age?

as age increases bone mineral density decreases (less bone on the inside)

How is trabecular bone affected in osteoporosis?

loss of trabeculae thickness

some transverse trabeculae disappear

Exercises to assist with bone strength

weight-bearing endurance activities (tennis, jogging, resistance exercise)

moderate to high intensity

30-60 min/day

Joint

a point where two or more bones meet

Functions of joints

allow movement, provide stability/support

Joint stability

resistance to movement in planes other than those defined by specific joint

there will always be some movement(laxity)

Tendons

attach muscle to bone

transmit tensile load from muscle to bone to produce joint stability or motion

provide mechanical pulleys

motor control

Ligaments

connects bone to bone

mechanical joint stability

guide joint motion

contribute to proprioception

Composition of Ligaments

Type 1 collagen fibers

not completely parallel

bears tensile load in long direction

can bear smaller tensile loads in other directions

Ligaments have more _________ than ___________.

elastin, tendons

Composition of Tendons

parallel fibered collagenous network

type 1 collagen sustains large tensile loads

more stiff, so it can transfer force to bone without tearing

2% elastin

Tendons can't deal with ___________________.

compression

How do muscles and a well-fitting joint influence joint stability?

Muscles contract to pull bones closer together, making a joint stable by holding it in place

Basic Injury Model for human tissue

demonstrates that the level of stress imposed on a body ranges from a low to high stress continuum

undesirable effects occur when the level of imposed stress is too high or too low

Pathologic underload zone

the body is physically doing nothing

tissues still experiencing distress (atrophy)

Physiologic loading zone

level of imposed stress to maintain muscle strength

Physiologic training zone

increased level of imposed stress on body to build tissue

body becomes accustomed to this level and it becomes the new normal

return to physiologic loading zone(maintain, grow, new normal, and repeat)

Pathologic overload zone

high level of imposed stress causing substantial damage to tissue

What part(s) of the basic injury model is/are part of the distress and increased injury risk category?

Pathologic underload zone

What part(s) of the basic injury model is/are part of the eustress category?

Physiologic loading zone & Physiologic training zone

What part(s) of the basic injury model is/are part of the distress (injury) category?

Pathologic overload zone

Acute injury mechanics

it only takes one instance(applied load) to go from healthy tissue to non-healthy tissue

Prolonged injury mechanics

happens over time from long-term tissue exposure to stress

Repetitive injury mechanics

repeated application of a relatively low magnitude load

Overuse injury model

as repetition increases, failure tolerance decreases

prolonged loading: eventually something will break due to decreased tolerance overtime

minor injuries add up and can cause complete breaks if repetitive strain occurs

External(extrinsic) risk factors for injury

intensity of performance, playing surface, equipment, skill level, rules

Internal(intrinsic) risk factors for injury

skeletal alignment, muscle strength, muscle endurance, joint flexibility, injury history, psychological factors

What are the two fibrous proteins?

collagen and elastin

Collagen

Type 1 fibers: thick fibers that elongate a little when stretched

Type 2 fibers: thinner fibers that provide a framework for maintaining general shape and structure

in ligaments, tendons, and skin

Elastin

resists tensile stretching forces

has more give when elongated (like a rubber band)

returns to original shape after deformation

Ground Substance

water saturated matrix/gel

hydrates tissues to maintain their biomechanical integrity

captures water for nutrient transport and getting rid of waste to prevent buildup

Stress

external forces resisted by internal forces and cause deformations to the body

Stress Equation

Stress = internal force/cross sectional area

units: N/m2

Strain

quantification of the deformation of a meterial

produced by tensile or compressive stress

Strain Equation

Strain = change in length/original length

Tension

stress that occurs when force is applied to an object to pull it apart

pulling in opposing directions

Shear occurs when

two internal surfaces slide against each other

Compression Stress

load pushes material more tightly together

object shortens in direction of these external forces

True or False: Collagen and elastin can resist compression.

false

Compressive stress is highest when the cross-sectional area is at its ___________.

lowest

Plasticity

permanent deformation of an object after deformation

Elasticity

the ability of an object to return to its original shape after deformation

Stiffness

the magnitude of force required to deform a material a certain length

Stiff

applying a large load causes a small deformation

Pliant

applying a small load causes a large strain

Yield Strength

stress at the elastic limit

no breakage of material occurs, but permanent damage occurs beyond this point

Ultimate (Mechanical) Strength

the maximum stress a tissue is capable of withstanding before failure

microdamage occurs

Failure Strength

stress where failure occurs

What does failure mean in human mechanic terms?

complete tearing or breaking of a tissue

what region does a tissue enter when it reaches the ultimate strength state?

Plastic region

Compression occurs when

ends are pushed together

Bending occurs when

tension and compression are applied

Torsion occurs when

an object is being twisted in one direction while another force is twisting in the opposite direction

Combined loading is a combination of

compression and torsion