4) Intro to Musculoskeletal Loading

The amount of mass that can be accelerated is proportional to the force acting on it

F = ma

what is a force

Newtons (N) = 1 kg x 1m*s^2

standard international units for force are

vector

force is what type of quantity

magnitude and direction (represented by arrows)

a vector has a

load

a force that acts on the body tissue is often referred to as a

healthy; harmful

tissues all respond to force in diff ways and NEED force to be _____ ; however, forces can also be ____

hyaline/articular cartilage

needs to be loaded to be healthy - squeezes fluid out and absorbs it back in

loading magnitude

Loading rate

loading type

important factors that impact musculoskeletal loads

loading magnitude

how much a tissue is loaded

loading rate

speed at which a tissue is loaded

loading type

the way in which a tissue is loaded

time

The loading rate is a function of

the rate at which they are loaded/how quickly they get loaded

biologic tissues are sensitive to

different loading-rate conditions

tissues behave differently under

• unloaded

• tension

• compression

• bending

• shear

• torsion

• combined loading

types of musculoskeletal loads

tension loading

two forces pull on an object in different directions

tension loading injury example

The lateral ankle ligaments (most commonly torn) are severely tensioned as the foot rotates inward

compression loading

forces that push or pull the surfaces of objects together or brings the end of an object closer

Compression loading example

The humerus is pulled against the glenoid by the deltoid muscle, creating a compressive load between the joint surfaces of the bones

bending load

• deformation of tissue that occurs at right angles to its longitudinal axis

• concave side undergoes compression load

• convex side undergoes tension load

bending load injury example

coxa vara results in an increased bending load on the neck of the femur

shear load

Unaligned parallel forces that move on part of a body in one direction and another part in the opposite direction (created by friction)

shear load injury example

cam type morphology in femoroacetabular impingement syndrome creates abnormal shear load between the femur and acetabulum

torsion load

twisting force applied to tissue around its longitudinal axis (one segment moving one way, other moving the opposite)

torsion load injury example

non-contact ACL rupture (tears by ____) … tibia sticks, femur twists

internal muscle forces

what counteracts the external loads on the body

active muscle forces

passive tissue deformation

viscoelastic

tissues in the human body are

viscosity

fluid-like component to the behavior of tissue ("gooeyness")

- honey = high

- water = low

time-dependent behavior… wants to return to original shape but is time-dependent

fluid-like behavior =

Elasticity

ability of material to return to its original state after loading

it can deform but resist change in structure and shape

healthy tissue can resist loads,

resist the external forces placing the tissue under the load

internal forces that arise w/in the structure under load can

tissue dependent

load response is

tissue stress

- force/load divided by cross-sectional area

- a measure of load or energy that is stored within a tissue

force (newtons) / area (m^2)

pressure =

tissue strain

the amount a tissue deforms under a force or load

usually expressed as a percent change in length or distance

elastic deformation energy

- tissue returns to its original shape and all stored energy is released once unloaded

elastic energy represents

transition between elastic and plastic behavior

- additional load results in a marginal increase in stress

yield point is the

plastic deformation

- overstrained tissue is permanently deformed and energy cannot be recovered once load is released

micro-failure of tissue under continued load results in

creep

continued deformation of a material over time as it is subjected to a constant load