Biomechanics Exam 2

Kinematics

the description of motion without attention to the forces that cause them

How do we measure kinematics?

kinematic data from passive marker-based motion capture (MoCap)

• many 2D perspectives used to construct 3D image

focal length

distance from the lens to the camera sensor

____ focal lengths capture a ____ scene but are subject to ________

• shorter

• wider

• parallax error

parallax error

the line of sight is not exactly perpendicular to the scale - something looks too big/small

frame rate

how many frames are taken per second (fps) by camera

higher frame rate

~60 fps

• smoother motion, more realistic

lower frame rate

~24fps

• choppy, more cinematic

_____ in shutter speed reduces blur

increase

shutter speed

the amount of times the camera shutter is open (how long?)

shutter speed controls:

motion blur & exposure

slower shutter speed

• more motion blur

• brighter image

faster shutter speed

• less motion blur

• darker image

• objects moving at faster rate

how to calculate instantaneous velocity

Central Difference Formula

v(t2) = [s(t3) - s(t1)] / [t3 - t1]

how to calculate average velocity

v = [final - initial position] / [total time elapsed]

position

scalar

• location of object at specific time

velocity

vector

• rate of change in position over time

• derivative of position graph

acceleration

• (+) vs (-)

vector

• rate of change in velocity over time

• (+) = speed up in positive direction

• (-) = slow down or speed up in negative direction

if velocity & acceleration are in the same direction… object (speeds up/slows down)

speeds up

if velocity & acceleration are in the opposite directions… object (speeds up/slows down)

slows down

if velocity & acceleration are perpendicular → the object…

changes direction

position function:

• increasing position

• decreasing position

• @ max./mins.

• curved up

• curved down

• @ inflection pt.

• positive v

• negative v

• v = 0

• position a

• negative a

• a = 0

Gait cycle:

Each leg will be on the ground for __% of cycle

62%

Gait cycle:

Both legs touch simultaneously for __% of cycle

24%

Gait cycle:

Swing accounts for __% of cycle

38%

peak knee flexion occurs at what phase of gait cycle? why?

• swing

• so foot does not drag & hit ground (have to pick up foot)

congruence

the amount of bony contact between 2 surfaces (at joint articulation)

• where 2 bones meet, how well their surfaces meet together

• femoral head, acetabulum

joint stability

resistance to luxation (dislocation)

• position dependent

• prevention of injury

• influenced by:

  • supporting structures (ligaments, muscles, joint capsule)

  • shape & congruence of bone

• trade off with joint laxity

  • more stable → less movement

joint laxity

how much movement is permitted

• trade off with joint stability

  • more movement → less stable

HAT

Head, Arms, Trunk

• weight-bearing movements

  • weight down onto femoral head

Anthropometrics

anatomical measurements of humans

• anteversion, torsion, etc

close packed position

most congruent (usually stable)

• most stable does NOT match with most congruent for hip

loose packed position

not congruent/stable

SI Joint Movement

rotation in the sagittal plane

• sacroiliac joint

Nutation

• anterior pelvic tilt

  • top of pelvis forward

• sacrum tips forward in pelvic girdle

• lumbar EXTENSION

  • arch back (stick out butt)

Counternutation

• posterior pelvic tilt

  • top of pelvis backward

• “tucking” in

• lumbar FLEXION

  • hunch forward (tuck tail)

largest, most congruent, most stable joint

coxofemoral joint AKA

• femoroacetabular joint

• hip joint

what type of joint is the coxofemoral joint? how many DOF? what are they?

• synovial (highly mobile)

• Ball & Socket Joint: 3 DOF

  • frontal: abd/add

  • sagittal: flex/ext

  • transverse: IR/ER

primary function of coxofemoral joint

support the weight of HAT in both static & dynamic upright postures

• walking, running, stair climbing

the neck of the femur angulates:

superiorly, anteriorly, & medially

primary function of greater & lesser trochanter

big muscle attachment points

orientation of femur affects…

loading through joints

the farther away the greater trochanter is from the joint center, the ____ the moment arm

BIGGER

Acetabulum (socket) is a ____ surface

lunate (load bearing)

lunate (load bearing) surface (acetabulum) means

femoral head punches up into socket during upright dynamic movement

• bears a lot of force/load against it

the acetabulum is covered with ____ cartilage that ____ peripherally

articular, thickens

Acetabulum faces:

anteriorly, inferiorly, laterally

the acetabulum is thickened by the ____ which _______

labrum, increases depth

• increases congruency

the labrum of the acetabulum contains

free nerve endings

when the labrum of the acetabulum is loaded…

it deforms around the femoral head (increases congruency)

strain

amount of deformation with respect to a structure

strain can be ____ or ____

compressive or tensile

strain formula

ratio of change in length (unitless)

• change in length / original length

stress

distribution of force within a body (surface area)

• more surface area → more force distribution (good)

stress formula

force / area

stress AKA

“internal pressure”

• small area with high force against it

spondylolisthesis

vertebrae slips forward (shear stress)

• when shear forces go against each other

• 1 vertebrae goes forward and the other 1 goes posteriorly

bending

• neutral axis (straight up & down) - no stress

• tension & compression occur at once along neutral axis

bending: ____ vs ____

• tension (stretched)

• compression (squished)

torsion

twisting motion along neutral axis

• not good for soft tissue

why do we need strong ligaments to maintain integrity & stability of SI joint?

the fault line (where 2 loads try to slip past each other) between femoral head & acetabulum (hip joint) is relatively vertical

• upper body load needs to be transmitted across the joint

• can be done by shear forces parallel to surface

• 1 force down & 1 up (counter each other)

Wolf’s Law

adaptation to stress over time

where is Wolf’s Law seen?

trabecular structure that exists inside bone

• you can see stress lines in the spongy bone

• continuous stress applied to it increases bone density → increase integrity / strength

Stress Line A - internal structure on superior aspect of acetabulum

primary load bearing surface for upright movements

• weight-bearing forces of HAT come down onto femoral head

Stress Line F - vertical lines passing through the ischium

load bearing while sitting

Stress Line G - curved lines along the pubic ramus

only 1 with compressive & tensile forces

• compressive: anterior of pubic ramus (concave)

  • medial thrust of femur while stepping

• tensile: outer part of pubic ramus (convex)

  • mass of upper body pressing into pelvis

Femoral Stress Lines: A. Medial Compressive System

axial loading (bending)

Femoral Stress Lines: B. Lateral Tensile System

more superior lines

• convex (out)

Femoral Stress Lines: C. Zone of weakness

• NO trabecular system

• potential injury

Femoral Stress Lines: F. Trochanter System

muscle pulling on bone

• attachment point (insertion)

mechanical vs anatomical axial loading (Femoral Bending Stress)

• mechanical: line of action coming straight down from HAT or GRF

  • HAT coming superiorly

  • Knee joint rxn force coming inferiorly

• anatomical: curvature of femur, neck angulation (point out & down)

directions of tension & compression in femoral bending stresses

• tension: laterally & anteriorly

  • convex out

• compression: medially & posteriorly

  • concave in

3 capsular ligaments (hip stability)

• ischiofemoral

  • taught in hip extension (spirals)

• iliofemoral

  • fan shaped Y

  • taught in extension

  • strongest

• pubofemoral

  • makes Z with iliofemoral

  • taught in extension AND hip ABduction

intracapsular ligament (hip stability)

ligamentum teres

• secondary stabilizer

• attaches at fovea

• conduit for blood & innervated for pain sensation

3 Hip Positions:

• max bony congruence

• most stable position

• least stable position

• max bony congruence

  • flexion, abduction, external rotation (frog legged)

• most stable position

  • extension, slight abduction, internal rotation

• least stable position

  • flexion with adduction

Center Edge Angle

• larger vs smaller

angle b/w vertical & line drawn from center of femoral head to bony edge of acetabulum

• larger: larger load bearing surface, higher resistance to superior dislocation

• smaller: smaller load bearing surface, increased risk of superior dislocation (less bony contact)

Inclination Angle

• Coxa Vara vs Coxa Valga

how flat/steep the neck is (neck to shaft angle)

• Vara = smaller medial angle

  • trochanter farther from joint center → longer moment arm → less muscle force necessary

  • cantilever neck

• Valga = larger medial angle

  • trochanter closer to joint center → smaller moment arm → greater muscle force necessary

  • higher compressive forces

Angle of anteversion/torsion in femur

formed by the intersection of the long axis (angle at which femoral head goes out) & the transverse axis of the femoral condyles (lateral/medial) at the distal (knee) end

• anteverted hip = more medial twisting (internal rot.) = toe IN gait

• retroverted hip = more lateral twisting (external rot.) = toe OUT gait

Anteverted hip

• toe IN gait

• coxa valga

  • knees knocked in, higher medial angle

• IR

• adductor moments @ knee & ankle (abnormal torque)

  • over compressing medial compartment of knee

  • GRF passes more medially

Retroverted hip

• toe OUT gait

• ER

• propulsive ability compromised

• posterior positioning of GRF during propulsion

  • reduce lever arm of triceps surae

  • use lateral aspect of calf more (forces outward)

  • peroneus longus & brevis

circumduction

outward swing to clear leg during swing phase

• overcome weak hip flexors

• failure to clear leg/foot forward off floor → swing it outwards

hip hiking

lifting pelvis on side of swing leg with spinal & abdominal muscles

• creates pelvic tilt

• weak hamstrings

steppage

exaggerated hip & knee flexion bc of foot drop

• poor dorsiflexion control

vaulting

going up on toes of stance leg to help clear swing leg

preventing anterior translation of tibia

hamstrings

preventing posterior translation of tibia

• patella / quadriceps

• popliteus

Q Angle

angle between 2 intersecting lines

• center of patella to ASIS of pelvis

• patella to tibial tuberosity (vertical)

Q angle is larger in cases of _____ and _____

genu valgum and femoral anteversion (shorter inclination angle at hip → more bent → valgus collapse at knees)

Q angle of ____ or ____ is abnormal

20 degrees or more

center of rotation for Axial Rotation

medial intercondylar tubercle of the tibia