Week 2 Kinetics, Biomechanical Principles 1

linear load

force

angular/rotational loads

torque

loads on tissue

push or pull, move, fixate, deform, injure

tension

pulls in opposite directions

compression

pushes inwards in same direction

shear

force pushes across each other

torsion

combined loading

tension on tendon

generally good at resisting

compression on cartilage

generally good at resisting

internal forces on joints

muscle force

external force

gravity, weight, etc

joint reaction force

produced between the surfaces of the joint and any other periarticular structure (arthritis may cause this)

equation for forces of the arm

internal force = external force + joint reaction force

torque equation

= force x distance from axis of rotation (moment arm)

isotonic contraction

type of contraction where the muscle length changes but the contraction doesn’s (examples are concentric and eccentric)

isometric force

muscle activation with no significant change in muscle length or joint angle, internal torque = external torque

concentric force

contraction with muscle shortening in lenth, example: raising a glass of tea to mouth (flexion)

eccentric force contractions

contraction with muscle lengthening, example: squatting down to the floor (hip and knee extensors)

agonist

prime mover muscle

antagonist

muscle that does the opposite motion

synergist

muscles that work together, synergistically

force couples

special kind of synergists, two muscles that work in opposite directions to rotate a body part, example: upper trap and lower trap rotate glenoid cavity upwards

1st class lever

axis of rotation is in the middle, example: atlanto-occipital joint

2nd class lever

load is in the middle, example is ankle plantar flexion

3rd class lever

force is in the middle, example is elbow flexion, shoulder flexion, this is what our body has most of

mechanical advantage

MA = ration of y to x, where y is the internal force moment arm, x is the external force moment arm

work

W = F x D , where F is force and D is distance

MA of less than 1

larger force required because the external moment arm is greater

MA of greater than 1

smaller force requires because the internal moment arm is larger

2nd class levers trade offs

can move a BIG load but at a small ROM

3rd class levers trade offs

can move through a large ROM but require a lot more force, more joint reaction forces, but have more velocity too

Newton’s 1st Law

Law of Inertia, a body at rest will stay at rest, a body in motion will stay in motion (unless acted on by an external force)

Newton’s 2nd Law

law of acceleration, the amount of acceleration depends on the magnitude of force aplide to the object and is inversely proportional to the object’s mass (sum of F = m x a)

CoM

center of mass close to the center of gravity, assumes the mass is evenly distributed throughout the body, around S2 for the average person

mass moment of inertia

distribution of mass about an AoR, can change the torque required to produce change in movement

Newton’s 3rd Law

Law of action-reaction, for every action there is an equal and opposite reaction, a force applied will be responded to by a force of equal magnitude in the opposite direction

anthropometry

measurements: length, mass, weight volume, measurements of the physical body

free body diagram

simple sketch representing the interaction between body and environment, with forces represented by vectors

force

a push or pull, = ma, need to consider the vector quantity, spatial orientation, point of application

Torque when related to force

Torque = F x MA

force systems

classified by direction and point of application, type of systems include linear, parallel, and concurrent

force vectors

lines that represent direction, magnitude, acceleration of force, can be combined from heads-to-tail method (tip to tail)

linear force systems

2 or more forces acting on the same segment, in the same plane, and along the same line, may be in the same or opposite directions

parallel force systems

exists whenever two or more forces are applied to the same system parallel to each other (ex knee brace)

concurrent force systems

2 or more forces actign on the same segement, but act in different directions, net of 2 forces = resultant force, can result in stabilization or redirection of force (example is the patella)

requirement to produce torque

a force and the moment arm must intersect at a 90 degree angle, the rest will cause translation

vector resolution

when a single force is applied at an angle other than 90 to the axis of rotation the individual vectors are worked backwards (resolved) into the concurrent components

Ground reaction force

force that the ground exerts back against your foot when it strikes

joint reaction force .

the force that the joint exert back against forces from muscle, passive tissues, and gravity

GRF and JRF

makes the action = reaction equation work, otherwise there would be surplus force left over on one side of the equation