Exam 1: Biomechanics Concepts

Dr. Ross - Fall 2023

rotary

the rolling or rotation at a joint around a center of rotation (angular motion)

y-direction, up, and to the right

translatory

the glide or translation at a joint (linear motion)

x-direction, down, and to the left

spin

a single point rotates like a top spinning

instantaneous center of rotation (ICR)

axis of a joint shifts in space as it moves

frontal plane

plane: x-y plane dividing the body into front and back

axis: z (anterior/posterior)

motion: abduction/adduction/lat flexion of the trunk

sagittal plane

plane: y-z plane dividing the body into left and right

axis: x (medial/lateral)

motion: flexion/extension + dorsiflexion/plantarflexion

transverse plane

plane: x-z plane dividing the body into top and bottom

axis: y (superior/inferior or longitudinal/vertical)

motion: rotations

degrees of freedom

number of planes a joint can move in

closed kinematic chains

distal segment is fixed to the earth or immovable surface while the proximal segment is free to move

closed kinematic chain example

stand to sit, stance phase of gait (and with a cane planted)

open kinematic chain

distal segment is free to move in space

open kinematic chain example

hand to mouth, sitting knee extension, swing phase of gait

convex-concave principle

convex moving on stable concave: roll and glide occur in opposite directions

concave moving on stable convex: roll and glide occur in the same direction

close-packed position

locked position of a joint

close-packed position conditions

1. joint surfaces are maximally congruent

2. ligaments and capsule are taut and twisted

3. usually at end range

open-packed (loose-packed) position

any unlocked position at a joint where the ligaments and capsule are lax

rotary magnitude

ROM in degrees

translatory magnitude

distance in cm

force equation

F (N) = m(kg) x a(m/s²)

what force is this?

unloaded

what force is this?

tension

what force is this?

compressions

what force is this?

bending

what force is this?

shear

what force is this?

torsion

what force is this?

combined loading

what happens to muscle with strain?

change in length occurs

toe region

take up slack

point B

elastic region

elongation is linear, return to beginning (no change)

point B-C

plastic region

progressive failure of tissue; permanently deformed

point C-D

yield point

when plastic region begins

point C

failure point

fracture, break, or tear occurs

point D

creep and hysteresis

energy is lost as heat when tissue deforms and change in length is permanent

point C-D

Poisson’s ratio

when a tissue is stretched, the tissue elongates and the diameter decreases

decreased diameter leads to increased stress

fatigue

repetitive loading weakens materials

loading rate

faster is more likely to fx so slower leads to creep

internal forces

muscles, ligaments, and bones

external force

gravity and equipment

force vectors

have a base, magnitude, and direction

all muscles are these

positive vector

up, right, or counterclockwise direction

negative vector

down, left, and clockwise direction

action line of a muscle: orientation

begins on the bone @ the point of insertion and goes in the direction of the muscle pull

rotary force orientation

starts at the insertion and is always perpendicular to the bone it inserts on

translatory force orientation

starts at the insertion and runs parallel to the bone it inserts on

center of gravity

hypothetical point at which all mass would appear to be concentrated and where the force of gravity will act → point of balance

anterior S2

line of gravity

a line drawn from COG directly down to the surface

always equal/opposite in magnitude to the gravity reaction force

base of support

feet and the space between (a box drawn around them)

assistive device and BOS

adding an assistive device will increase the BOS

factors affecting stability

height of the COG above BOS, size of BOS, location of LOG within BOS (bending over), and COG of the body

LOG and stability

squatting: LOG stays centered

bending over: LOG shifts anteriorly and decreases stability since there is extra translation

types of force systems

linear, concurrent, and parallel

types of parallel force systems

force couples and levers

linear force system

when 2+ forced act on the same object in the same line (joint compression and distraction)

joint distraction

tensile forces (often gravity)

joint compression

joint reaction forces (often mm or surface contact)

concurrent force system

2+ forces acting at a common point of application, but in divergent directions

composition is through a parallelogram

parallel force systems

2+ forces act on the same object but at some distance from each other and never converging

i.e. bones

force couples force system

2 forces are equal in magnitude and opposite in direction; always produces rotation

lever force system

three forces of a mechanical level and has three components: A, R, E

lever force arms

A = axis

R = resistance (loser)

E - effort (winner) and acting in the direction of rotation

mechanical advantage

= EA/RA

1st class lever

EAR or RAE

EA < = > RA

2nd class lever

ERA and ARE (most efficient)

EA always > RA

3rd class lever

AER and REA (least efficient)

EA always < RA

1st class lever example + mechanical adv

occiput on C1

it depends

2nd class lever example + mechanical adv

bicep curl

> 1.0

3rd class mechanical adv

< 1.0

torque

the ability of a force to cause rotation of a lever

T = f x d

moment arm

shortest distance between the action line and the joint axis

greatest when force is at 90 degrees to the segment

moment arm drawing

drawn perpendicular to the action line and intersects the joint axis

anatomic pulleys

change the direction of pull without changing the magnitude of the force

i.e. sesamoid (patella) bone

synarthrosis

connective tissue binds joints

synarthrosis joint types

fibrous and cartilagenous

fibrous joints

fibrous tissue connects bone to bone

i.e. skull sutures, gomphosis, and tibia+fibula

cartilaginous joints

joined by fibro or hyaline cartilage

i.e. pubis symphesis, growth plates, ribs to sternum)

diarthrosis / synovial joints

ends of bones are free to move + joint capsule

joint capsule

jt. receptors in fibrous outer layer attache to the periosteum with Sharpey’s fibers

what happens with sprains or torn ligaments in diarthrosis joints?

nerve supply is disrupted which decreases proprioception

synovial joint components

joint capsule, joint cavity, synovial membrane, synovial fluid, and hyaline cartilage

synovial fluid

provides nourishment, removes waste, and lubricates by diffusion via movement

hyaline cartilage function

decreases friction and absorbs shock

types of synovial joints

uniaxial, biaxial, and triaxial

uniaxial joint

1 DOF

hinge and pivot joints

example of uniaxial joint

inter-phalangeal and atlas-axis

biaxial joint

2 DOF

condyloid and saddle joint

biaxial joint examples

metacarpal-phalangeal and carpal-metacarpal of thumb

triaxial joint

3 DOF

plane and ball-and-socket

triaxial joint examples

carpal bones and hip/shoulder

connective tissue composition

cellular matrix and extracellular matrix

cellular matrix

fibroblasts mature into fibrocytes, chondrocytes, tenocytes, or blastocytes

extracellular matrix components

ground substance and fibrous proteins

ground substance

GAGS (+) affect hydration and contribute to the strength of collagen to withstand compression

fibrous proteins

keeps extracellular matrix together with collagen and elastin

collagen

accounts for 30% of all protein in body and has tensile strength

type I or II

type I collagen

thick and stiff

type II collagen

thin

elastin

elastic and deforms under force and returns back to original state

importance of fibrous proteins

tells us the strength and stiffness of the tissue

types of connective tissue

dense, articular cartilage, and fibrocartilage

types of dense connective tissue

ligaments, joint capsule, and tendons