Kinetics - Pt 2

force composition

addition of the force vectors representing 2+ forces into a single resultant force

• A + B = C (but with vectors)

what is the force composition of multiple internal and/or external forces

• normal or typical motion

• poor movement patters/poor motor control motion

can you break a single resultant

yes → you can break it into vertical and horizontal resultants

point of application - force compositions

they all have the same point of application

parallel component

parallel to the lever or segment on which it is acting will b the tangential component with a primary function of compression or distraction

perpendicular component

perpendicular to the lever or segment on which it is acting will be the normal component with a primary function of providing rotary movement

what force can act as a stabilizing force for the joint

tangential component

levers

principle of a rigid bar, rod, or segment rotating about its pivot point or fulcrum when acted upon by forces that produce rotation

what do levers require

2 linear forces each acting at a distance from the pivot point or axis

simple levers: 4 variables

1. muscle force (Effort = EF in drawings)

2. moment arm for the muscle force (EA)

3. Resistance force in the form of gravity/external force pulling on the segment/system (RF)

4. moment arm for the resistance (RA)

first class lever

• forces are applied on opposite sides of the fulcrum/axis

• attempts to cause rotation in opposite directions

• ex) see saw

second class lever

• the external resistance force/load is in between effort force and axis

• both forces act at a distance from the same side of the axis

• strength lever

third class lever

• the effort force is between the resistance force/load and axis

• speed lever (fast)

mechanical advantage (MAd)

MAd = effort arm (EA) / resistance arm (RA)

MAd > 1.0

the internal (muscle) force has the advantage → a small amount of force can overcome a larger amount of resistance

MAD < 1.0

a larger amount of internal force is needed to overcome a smaller resistance - OR - a small amount of external resistance can overcome a larger amount of internal force

force couples

forces acting together to move an object around a pivot point

what type of lever are most joints

third class lever → resistance has advantage

• can really only change EF (muscle force) and RA (moment arm for resistance force)

how to make muscle work harder

to create more force, increase the RA (moment arm for resistance force)

how to make muscle work less

to decrease force, decrease RA (moment arm for resistance force)