Joint forces and moments

force is an action that can…

cause object to change velocity, shape and resist other forces

COM velocity

rate of change of COM position, vector

COM acceleration

rate of change of COM velocity, vector

force formula

mass x acceleration

inertia

objects resistance to being moved

what does more mass/inertia mean

same force can accelerate object less, more force needed to accelerate the same amount

angular velocity

rate of loading

angular acceleration

rate of change of velocity

moment of inertia

depends on axis rotation and distribution of mass, controlled by changes in body configuration

why does MOI matter

mass distributed more distally, changes with body’s COM location

newtons first law (rotational)

body’s angular momentum constant unless acted on by net movement

lower MOI

higher rotational velocity

higher MOI

lower rotational velocity

role of forces

act to change linear momentum/motion

role of moments

act to change rotational momentum/motion

when do moments occur?

when forces are applied at a distance and angle away from reference point

moment formula

force(distance)(sing(angle))

moment distance vector

reference point to the point of application

what make the angle?

area between mforce vector and distance factor

moment arm calculation

distance x sin(angle)

moment arm distance vector

perpendicular distance from pivot to force’s line of action

external movement

moments from external forces

internal moment

moments from internal forces

moments during isometric contraction

internal moments equals external moments

moments during concentric contraction

internal moments greater than external moments

moments during eccentric contraction

internal moments less than external moments

muscle insertion angle

orientation of muscle insertion relative to the long axis of the segment

first class levers

EMA and IMA about equal, IMA/EMA equals 1

second class levers

IMA longer than EMA, IMA/EMA equals more than 1

third class levers

EMA longer than IMA, IMA/EMA equals less than one

muscle action

potential for a muscle to create a moment in a particular motion

agonist muscle

muscles directly contribute to movement objective

antagonist muscle

muscles perform action opposite of agonist