kinetics Flashcards
kinematics vs kinetics
kinematics = describes motion w/o regard for mass/ forces
kinetics = forces that produce/ stop/ modify motion
5 variables that determine and describe motion:
- type
- location
- magnitude
- direction
- rate of motion or rate of change
2 types of motion:
- translatory/ linear = all parts travel same disrance in the same period of time in the same direction
- rotary = every point moves about a pivot/ axis of rotation
how is the magnitude of motion measured for each type?
- linear distance = meters/ feet
- rotary distance = degrees
–> ROM
distance:
how far a force moves a body
how is location of motion described?
arounds axes (AP, ML, SI aka longitudinal)
how is direction of motion described for rotary motion?
- about medial-lateral axis = sagittal plane - flexion/extension
- about anterior-posterior axis = frontal plane - abduction/adduction
- about superior-inferior axis = transverse plane - medial/lateral rotation
**except thumb, ankle, clavicle
how is direction of motion described for translatory/linear motion?
according to axis AND + or -
how can rate and change of motion be described (3)?
- velocity = rate at which motion occurs
- acceleration = rate at which velocity changes
- torque = motion occurring about an axis
4 types of forces that affect body motion:
- gravity - 9.81m/s^2
- muscles - contraction, stretching
- external resistance - muscles work against
- friction - opposes contact force, primarily responsible for human movement
mass vs weight
mass = amount of matter something contains, measured w balance comparing known matter to unknown amount of matter
weight = pull of gravity on mass (mg) measure on a scale
moment:
force acting at a distance from axis
M = d x F
newtons 1st law:
law of inertia - body at rest/motion will stay at rest/motion until acted on by external net force
sumF = 0
newtons 2nd law:
law of mass/ acceleration
F = ma
newtons 3rd law:
law of action/ reaction - for every action force there is an equal and opposite reaction force
3 elements of a lever:
- axis/ fixed point
- resistance force (R)
- moving/ effort force (F)
resistance arm:
perpendicular distance from axis to line of action of resistance
force arm:
perpendicular distance from axis to moving force
first class lever:
- seesaw (axis b/n force and resistance arm)
ex: C1 = skull sitting on first vertebra - poor mechanical advantage as muscle must generate a lot of force to overcome R
second class lever:
- wheelbarrow (axis at end, force arm > resistance arm)
ex: achilles tendon relative to toes - mechanical advantage bc long F arm
third class lever:
- bicep curl w dumbbell (most common in body)
- poor mechanical advantage = force arm is smaller than resistance arm
- can move small weight long distance
mechanical advantage:
equation:
ratio b/n length of F and R arms
MA = (force arm length)/(resistance arm length)
what can MA ratios be?
> 1
<1
= 1 (no advantage, F arm length=R arm length)
how can you increase MA?
practical example:
increase length of the force lever
ex: closer to person = shorter resistance arm
torque:
force applied around a joint/axis
T= F x d
(so smaller moment arm, the less torque generated)
what happens when the angle b/n forces increases?
what does it mean when angle = 0?
resultant force decreases
= forces are in line w eachother
COG or COM:
- point about which mass of object is balanced/ concentrated
- a few finger widths below belly button
***exceptions: pregnancy, wider pelvis
stable equilibrium
body returns t0 former position after light perturbation
unstable equilibrium
body seeks new position after light perturbation
neutral equilibrium
COG displaced but remains at same level
ex: person in wheelchair
when in a form of equilibrium, what does the degree of stability depend on (4) ?
- height of center of gravity above base of support
- size of base of support
- location of gravity line w/n base of support
- body weight
base of support (BOS):
when body is stable, line of gravity passes through center of BOS
larger BOS = more stable the object is
can small changes in muscle length create angular displacements?
yes
what does the amount of force a muscle must exert to overcome external resistance depend on?
lever arm length
are the body’s levers designed more for speed or strength?
speed (most lever systems in body are third class)
when does max resistance torque occur?
when segment is horizontal/ perpendicular distance to axis is greatest
single fixed pulley:
mechanical advantage?
- dont provide a mechanical advantage
- change direction of a force
ex: cable machine
moveable pulley:
mechanical advantage?
- provide mechanical advantage
- pulley rope must be pulled twice the distance that weight is raised
ex: leg traction system
anatomical pulleys:
mechanical advantage?
- change course of tendon
- increase mechanical advantage = move tendon further from axis of rotation
ex: patella
leverage factor:
- use principle of leverage to increase effect on an applied force; mechanical advantage = magnifies force + move object w less effort
ex: therapist position relative to client when doing muscle testing - increase effective speed of movement w a given force
ex: golf swing
stretching vs joint mobilization:
stretching a joint = external forces applied distally resulting in high stresses to articular tissues
joint mobilization = external force applied proximally, recreate arthrokinematics while minimizing stress to articular tissues
