Pod Biomechanics Exam 1
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Subtalar Joint Motion and Axis
Motion: triplanar, supination and pronation
Axis: distal-medial-dorsal to proximal-lateral-plantar. 16 deg from sagittal plane, 42 from transverse, and 48 from frontal
1st Ray Motion and Axis
Motion: Biplanar and coupled. Dorsiflexion with inversion and plantarflexion with eversion.
Axis: 45 deg from frontal and 45 from sagittal, parallel to transverse plane.
STJ Effects on 1st Ray
STJ pronated = increased 1st Ray motion
STJ supinated = decreased 1st Ray motion
5th Ray Motion and Axis
Motion: Triplanar. Pronation and supination.
Axis: oriented similarly to subtalar joint
Central 3 Rays and IPJs Motion and Axes
Motion: Single plane, dorsiflexion and plantarflexion
Axis: Perpendicular to sagittal plane, parallel to front and transverse planes
MTPJs Motion and Axes
Biaxial - vertical axis (transverse plane, ab and adduction) and horizontal axis (sagittal plane, dorsi and plantarflexion)
2 separate motions!
MTJ (talo-navicular and calcaneo-cuboidal joints) Motion and Axes
Biaxial: longitudinal axis and oblique axis.
Longitudinal axis: perpendicular to frontal plane (75 deg from frontal plane). Inversion and eversion
Oblique axis: oblique to sagittal and transverse. 8 deg from sag, 15 from transverse. Dorsiflextion w/ abduction and plantarflexion w/adduction
Newton's 1st Law
Body in motion stays in motion, and body at rest stays at rest, unless acted upon by another force.
vector that has magnitude and direction
Mass that moves from point A to B with a linear velocity (i.e. walking). Linear force = axial force.
Difference b/w speed and velocity
Speed = distance/time. No direction.
velocity = displacement/time with direction.
displacement = distance
the rate at which an object changes its velocity.
a = change in velocity/time
i.e. going from 30mph to 60 mph is acceleration. or, changing direction also considered accel.
Force (Newton's Second Law)
F= mass x acceleration
Momentum = mass x velocity
quantity of motion a mass has.
has force potential.
Change of momentum
It takes a certain force over a time period to change the momentum.
change in momentum = force x time
How do you minimize force (i.e a punch coming at you w/ momentum?). Increase the time (move your head back).
Velocity, acceleration and momentum are all...?
Vectors. Have a direction and speed.
Newton's 3rd Law
When a force is exerted on an object, that object pushes back with an equal and opposite force.
Ground reactive forces from the earth push back on our feet.
How does pronation and fat cushion of our feet minimize the force of hitting the ground?
change in momentum = force x time. They increase the time.
body of mass rotating around axis to produce linear momentum. Degrees per minute.
ang. mom = inertia x angular velocity
inertia = amt. of energy it takes to start or stop an object
same as linear. change in momentum - change in acceleration (bc velocity changes). To produce ang. accel, apply a force called a torque or moment.
= force x distance from fulcrum (moment arm)
if two forces are equal, but one force is applied 2x farther from the fulcrum, it will produce 2x the torque.
Center of Mass
Point in an object where it has its smallest amount of inertia around any axis that contains that point
it's easier to rotate object around an axis if that axis lies within its center of mass.
Why does a balance beam help?
The farther a mass is spread out, the greater its inertia (or ability to resist change in velocity)
Class 1 Lever
Fulcrum is between the force (effort) and the load (inertia)
example: tricep with elbow as fulcrum and hand with the load
Lever mechanical advantage - length of effort arm divided by length of resistance arm
Class 2 Lever
Strongest. Fulcrum at one end, force (effort) at other end with load in the middle. (wheel barrel, or door on hinge)
when effort arm is long, it's strong.
calf is effort, load is body weight and fulcrum is toes.
Class 3 Lever
Weakest bc moment arm is smaller, but fastest.
Fulcrum at end, effort in middle, load at other end. i.e. elbow, bicep and load on hand. (bicep attaches to forearm).
Most joints are Class 3.
Every change in form or function of a bone is followed by adaptive changes (bone remodeling)
What stabilizes a joint? What de-stabilizes a joint?
Compression stabilizes bone
Rotation/angular movements cause instability if excessive or outside normal range (muscle action keeps angulations low and stabilizes joint motion)
soft tissues (ligaments) will elongate under prolonged tension.
- elasticity = returns to normal shape, usually with force of short duration.
- plasticity = doesn't return to shape, long duration force.
Born with ligaments of high elastic content (abnormal collagen/elastin ratio)
Ligaments go through plastic deformation. Pronation in childhood becomes excessive and leads to bone and joint adaptation.
process by which the body can vary muscle contraction in response to external forces. Specialized structures: golgi tendon bodies, muscle spindle, joint proprioception.
What are the 4 stabilizers of the foot?
Muscle, bone, ligaments and proprioception
External forces on the foot?
Internal forces on the foot?
External: body mass in motion, gravity
Internal: rotation/angular movement around joint
What are the two types of equilibrium?
- 1. Static
- 2. Steady motion (moving at a constant velocity)
instability occurs when a force displaces the object from its original position (equilibrium)
- Balance is improved when:
- -center of mass is lower
- -base of support is wider
- (crouched low with feet spread apart = most balanced position)
Momentum is conserved in the system. The only negligible loss of kinetic energy is in heat and friction. KE is, for the most part, conserved.
Momentum is conserved but kinetic energy is lost to a form outside the system.
I.e. foot hitting earth. Foot velocity changes (decelerates), earth moves but because of its mass its unperceivable.
the energy a moving body has just because it's moving. Energy is ability to do work.
KE = 1/2mv^2
A body's unrealized potential to do work. Something not in motion has PE, but when it moves it has KE.
Forces on the Foot
- 1. Body weight
- -free falling mass
- 2. Earth as it reacts to foot
- -vertical force
- -horiz. force (friction)
- 3. Internal forces
- -tension, compression, rotation
Stress-strain curve of the bone
Can apply increasing force on a bone, and up to the yield point, it will bounce back (elastic region). Beyond the yield point (plastic region) the bone is deformed/injured.
Highest point of PE in gait
Middle of single support (one foot on ground, one at highest point in air)
Highest point of KE in gait
How to reduce stress/strain
- Decrease body weight
- Minimize how far center of mass falls
- Maximize time of applied force to change momentum
- smooth walking (i.e. limping is less energy efficient
Factors that absorb shock and reduce stress
fat pad and skin of heel, STJ pronation, MTJ pronation, knee flexion (increases time to minimize force), ankle plantarflexion, phasic muscle contraction
Supinated position. Doesn't pronate, loss of shock absorption.
abnormal internally rotated femur due to tight soft tissues. treatable.
externally rotated femur due to tight soft tissues. treatable.
lack of normal external twisting of femur. increased angle. leads to internally rotated femur (intoeing). Not very treatable since the problem is the bone.
Too much external rotation of femur. smaller angle. outtoeing. Not very treatable since it's the bone.
Angle of Inclination or Cervicofemoral angle
Angle of neck to shaft of femur.
- Increased: coxa valgum/genu varum (bow-legged)
- Decreased: coxa varum/genu valgum (knock-kneed)
From birth to adult, decreases from 135-140 to 126-128 degrees
Age ranges for progression of angle of inclination
- Birth - 2: bowlegged
- 2 - 4: straight
- 4 - 7: knock kneed
- 7 - 12: straight
- 13 - 18: knock kneed
- Adult should be straight or slightly knock kneed
knees are hyperextended. Due to ligamentous laxity. affects girls more. sagittal plane.
Ankle Joint Dorsiflexion
- Birth - up to 75 degrees.
- By age 15, decreased to 10 degrees.
Knees and hips straight but feet are rotated either out or in. Tibia, like the femur, starts inwardly rotated and needs to rotate externally.
Could also be caused by position of fibula and tibia to each other (fibia should be slightly posterior to tibia)
Normal development of malleolar torsion
At birth they are straight. Progressively externally rotate until they are 15 to 18 degrees external. Thus, it's normal for fibula to be posterior to tibia.
Lack of malleolar torsion
excess malleolar torsion
out-toe, duck feet
Pseudo-lack of malleolar torsion
soft-tissue problem at the KNEE that causes intoeing, even though it looks like an ankle problem
Developmental foot rotation
In the womb, foot is adducted and inverted. Children are flat footed and everted. Adult should have vertical heel and straight forefoot.
torsion twist from 20 degrees to 40 in a valgus direction (lack of twist=forefoot varus, too much twist=forefoot valgus)
transverse external rotation from 35 to 20 deg (lack of rotation=forefoot adductus, too much=forefoot abductus)
twist and rotation from varus to vertical.
- lack of twist/rotation = inverted/varus
- excess twist/rotation = everted/valgus
the motion of the body is known, and the forces must be extrapolated from position-time curves
adding forces to making something do what u want it to do
Cocheba's definition of Moment
measures the tendency for a structure to rotate
tibialis posterior muscle decreases internal rotation of the foot! (answer on test)
Tendency of bone to bend
i.e. stress fractures : no outward physical change, nothing visible. but when it breaks, it becomes an external moment. Test Q!
Results in visible motion (has an outright physical change). Most common form of a moment in kinematics.
Ground reactive force on calcaneus
If GRF on lateral calcaneus, will cause eversion. If on medial, will cause inversion.
GRF pushes back with equal and opposite force!
Interval of time from heel strike of one foot to heel strike of same foot. consists of 2 phases: stance and swing.
the distance covered on one foot during one complete gait cycle
the distance covered on one foot during one-half of the gait cycle
number of steps/minute when a person walks normally
Stance Phase of Gait Cycle
weightbearing portion of gait cycle. starts at heel strike of one foot to toe-off of opposite foot.
Periods of stance phase: contact period, midstance period, propulsive period.
Swing Phase of Gait Cycle
Non-weight bearing portion. Toe off of one foot to heel strike of same foot. 40% of gait cycle.
Contact period of stance phase
heel strike to forefoot loading (or opposite toe-off)
Midstance period of stance phase
forefoot loading (or opposite toe-off) to heel lift
Propulsive Period of stance phase
heel lift to toe-off
limits inversion and eversion
lateral talocalcaneal ligament
medial talocalcaneal ligament
Lateral ankle ligaments
- Anterior talo-fibular
- Calcaneofibular - limits STJ supination
- Posterior talo-fibular
Medial ankle ligaments
- superficial deltoid
- deep deltoid
strongest portion of superficial deltoid, limits eversion of STJ and ankle
Subtalar Joint 3:1 ratio
For every degree of sagittal plane motion, there are three degrees of transverse and frontal plane motion
purpose of pronation (STJ motion)
shock absorption and adaptation to uneven terrain
purpose ot supination (STJ motion)
Creates rigid lever for propulsion and prepares foot for heel contact
(foot doesn't propel well from a pronated position)
Open kinetic chain
non-weight bearing (swing phase), occurs in gait, calcaneus does all the work (moves in all 3 planes), tibia has little influence
closed kinetic chain
weight bearing (stance phase), occurs in gait, both calcaneus and talus do the work, tibia has great influence
calcaneus goes back to only moving in frontal plane
STJ in Open kinetic chain
- Pronation (dorsiflexion, abduction, eversion)
- Supination (plantarflexion, adduction, imversion)
STJ in closed kinetic chain
talus does equal and opposite motion as calcaneus. tibia does same transverse plane motion as talus!
- pronation: calcaneus everts, talus plantarflexes and adducts
- supination: calcaneus inverts, talus dorsiflexes and abducts
tibia in closed kinetic chain pronation
internally rotated (because talus is adducted)
tibia in closed kinetic chain supinated
externally rotated (because talus is abducted)
STJ during contact period
Heel strike to forefoot loading
at heel strike, STJ is supinated then undergoes rapid pronation throughout contact period. Neutral position just after heel strike.
STJ during midstance period
forefoot loading to heel lift
STJ is slowly supinating, still pronated just before heel lift. Neutral position around heel lift.
STJ during Propulsive Period
Heel lift to toe off
STJ is supinated and still supinating even more (a rigid lever)
STJ during swing phase (open kinetic chain)
- 1st half: STJ pronates
- 2nd Half: STJ supinates
Remember in OKP, calcaneus does all the work! In pronation, it everts, abducts and dorsiflexes. In supination, it inverts, adducts and plantarflexes.
STJ Neutral position
STJ neither pronated nor supinated. Measure non-weight bearing.
Calculating STJ Neutral position
- Amount of max STJ inversion and max STJ eversion. Then use NP = (total ROM/3) - eversion
- where ROM = measured calcaneal inversion + measured calcaneal eversion. If negative, NP is everted. If positive, NP is inverted. Normal is 0-2 degrees inverted.
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