EXSS385 Exam 3

balance

body’s equilibrium is controlled for a given purpose

equilibrium

system is not changing speed or direction

Stability

difficulty with which equilibrium can be disturbed (how hard it is to disturb equilibrium)

two practical applications of statics

analyzing balance (pre vs. post injury), y-balance test

what does a y-balance test do

assesses dynamic balance, functional symmetry, and neuromuscular control

how do we balance distance and weight?

lower mass moves further distance from AOR and vice versa

define a lever system and the real life examples of each

axis of rotation in which a rigid segment rotates; lever arm = bone, AOR = joint, motive torque = muscle, resistive torque = gravity

first class lever system

effort, fulcrum, resistance (EFR)

second class lever system

fulcrum, resistance, effort (FRE)

second class lever system details

always good mechanical advantage (MA > 1)

third class lever system

fulcrum, effort, resistance (FER)

third class lever system details

always poor mechanical advantage, MA < 1, most common concentrically

Most common lever system in the body and why

third, sacrifices force for speed and range of motion for better movement efficiency

mechanical advantage equation

MA = (effort arm)/(resistance arm); i.e. distance between fulcrum and effort / distance between fulcrum and resistance

why are we stronger during eccentric muscle contraction?

muscle doesn’t need to overcome the load but rather only control it, as weight/gravity is the driving force (muscle becomes resistance and weight/gravity becomes the effort)

three components of a free body diagram and what they represent

point = axis, line = bone, arrows = force vectors

fluid

any substance that tends to flow/continuously deform when acted on by a shear force

describe the motion of fluids

depends on velocity of object and fluid; relative velocity of a moving body relative to the fluid equals the subtraction of wind velocity from the velocity of the body (Vrelative = Vbody - Vwind)

laminar flow

low velocity relative to fluid medium flow of adjacent fluids

turbulent flow

high velocity (rougher surfaces take less velocity to become turbulent)

three properties that determine the magnitude of fluid force

density (more dense = more fluid force), specific weight (higher weight = more fluid force), viscosity (more viscous = more fluid force)

what is buoyancy force

fluid force that acts vertically upward

specific weight

weight/volume, greater specific weight = greater buoyancy

float vs. sink (Archimedes’ principles)

if buoyant force acting on a body = weight of fluid displaced by the body, then float. If weight > buoyant force, then sink

describe drag and how the coefficient of drag is relevant

force caused by the dynamic action of a fluid that acts in the direction of the free-stream fluid flow; smaller drag coefficient (C_d) = object travels through fluid easier (more aerodynamic)

surface drag with example

friction between layers of fluid; swimmer with more hair experiences more drag because a rougher surface means more friction

form drag with example

resistance created by a pressure differential between the lead and rear sides of a body moving through a fluid; swimmer with head up creates more drag than dead down because more surface area presented to water

wave drag with example

resistance created by the generation of waves at the interface between two different fluids, such as air and water; swimming closer to surface creates more drag because more waves are generated

describe the three types of loading

compression (squish, force going against), tension (stretch, force pulling apart), shear (translational/slide, force rubbing against)

describe the two combination loads

torsion (rotational shear with twist along the axis) and 3 point bending (tension and compression)

the five factors that determine mechanical behavior

structure (what it’s made of), geometric characteristics (how it’s put together), mode of loading (type of loading), rate of loading (stiffness increases when rate increases), frequency of loading (how often; fatigability)

anisotropic

different mechanical properties depending on direction of loading; tissues are anisotropic (falls under geometric characteristics)

fatigability and fatigability curve

measure of the weakening of tissue subject to repetitive loading; load (y) vs. reps (x) with an injury curve; for humans, rest resets the curve

area under the curve in the elastic section (before yield curve) represents

usable energy (stored energy)

stress/strain curve

stress (y) = load, strain (x) = deformation

slope of stress/strain

represents stiffness (greater slope = greater stiffness)

area under the curve of stress/strain

energy

failure point

tells strength/amount of load a tissue can withstand

elasticity

the ability to reform after a stress has been applied

yield point

tells whether something is elastic or plastic (point shows when something is no longer elastic/able to return to original shape)