• What movement (or what position) is occurring in the right knee?

• What surface term can be used to describe what you can see of the hand in this photo?

• When performing the movement in the picture the head has moved:

• A.) Anterior• B.) Medial• C.) Posterior

Kinetics

Study of Forces Associated with Movement

Scalar Quantities• Can be measured by an instrument of scale;

they are static (no direction). Has magnitude only.– Time – Space– Mass– Speed – Temperature

Vector Quantities

• Indicate movement; can be measured only at a specific point in time as it is constantly changing

– 1) magnitude

– 2) direction

Tug of War

500 N 200 N 100 N 400 N 300 N

Factors to Describe Vectors• Magnitude• Direction-

• Vertical=Y• Horizontal=X

• Sense/Polarity– Internal force=positive Y– External force=negative Y

• Point of Application- Where muscle inserts to origin

Types of Motion

• Translation or Linear - object moves in a straight line - all points of the object move in the same direction at the same speed, distance, and time

• Rotation (Axial, or angular) - One point of the object is fixed and the remainder of the object moves in an arc around a fixed point.

Resolution of Forces

• May have two forces acting in different directions in the same plane - can determine force by using the Pythagorean theorem.– a(squared) + b(squared) = c(squared)

Resolution of Forces:Clinical Application

3 N

4 N

Lever System

• Lever - rigid bar or structure that turns around a fixed point; representing the bones of the body

• Fulcrum – fixed point around which rest of bar moves; representing a joint axis

• Force - usually representative of a muscle

• Resistance - force that must be overcome by a muscle

Lever Arms

• Force - labeled F

• Resistance - labeled R

• Force Arm - distance between the application of force and the fulcrum - labeled FA

• Resistance Arm - distance between the point of resistance and the fulcrum - labeled RA

Torque

• Torque considered a rotary equivalent to a force

• Produced by levers - moves around a fixed point - the rotation makes an arc or circle

• Torque = Force x Force Arm or Resistance x Resistance Arm– measured in newton-meters or foot-pounds

First Class Lever

• Fulcrum is between the force and the resistance (e.g. see saw)– Example is atlanto-occipital joint (head and neck)– Articulation is of the occipital condyles and superior facets of

C1–

First Class Lever

1st Class Lever

1st Class Leverhttp://video.google.com/videoplay?docid=-

1013712987391348587&ei=y0KUSuyeM5qIqQLfvsy7Dg&q=atlanto+occipital+joint&hl=en

Second Class Lever

• Force and resistance are on the same side of the fulcrum with the resistance between the fulcrum and the force (e.g. bottle opener)– Example is foot (standing on toes)

Second Class Lever

2nd Class Lever

2nd Class Lever

Third Class Lever

• Force and resistance are on the same side of the fulcrum with the force between the fulcrum and the resistance (most levers in the body are third class levers).

Third Class Lever

3rd Class Lever

Match

• 1st class lever• 2nd class lever• 3rd class lever

• Fulcrum in the middle• Force in the middle• Fulcrum in the middle

Mechanical Advantage

• Efficiency of a lever - efficient when a small force required to overcome a large resistance.– MA=FA divided by RA– Force Arm is greater than resistance arm then

MA is greater than 1– In third class levers, MA is always less than 1

Mechanical Advantage

• If mechanical advantage is greater than one, then lever was built for force

• If mechanical advantage is less than one, then lever was built for speed.

Law of the Lever

• Lever is in equilibrium and will not move if: F x FA = R x RA

What class lever is this? What downward force must be exerted at F to balance the downward torques of R1 and R2?

R1F R2

What class lever is this? What downward force must be exerted at F to balance the downward torques of R1 and R2?

F R1 R2

What class lever is this? What upward force must be exerted at F to balance the downward torque of R? What is the mechanical advantage of

this lever?

FR

What class lever is this? What upward force must be exerted at F to balance the downward torque of R? What is the mechanical advantage of

this lever?

FR

What class lever is this? What upward force must be exerted at F to balance the downward torque of R? What is the mechanical advantage of

this lever?

F R

What class lever is this? What upward force must be exerted at F to balance the downward torque of R? What is the mechanical advantage of

this lever?

F R

Two Types of Musculoskeletal Torque

• External – Forces outside the body

• Internal – Produced by muscles, tendons, and other soft

tissue in the body

Types of Musculoskeletal Torque

Moment Arm

• Is the radius of the specific arc of motion.

• This line runs perpendicular from the line of force to the axis of rotation

• The moment arm or torque arm changes as the muscles contract and lengthen

Changing Resistance Arms

External Torque (Barbell)

External Torque

IFSin A = a/c

Sin 30 degrees = a/.3m

Find A

• Moment Arm =

External Torque

• Here it is!

Internal Torque (Biceps)

Internal Torque

Moment Arm

Internal and external Torque

• External torque will change throughout the range of motion as the length of the moment arm changes.

• Internal torque changes throughout the range of motion as the length of the moment arm changes! – Internal torque usually operates in response to

external torque and is affected by those changes.

Muscle Contraction

• Defined - development of tension in the muscle

Contraction versus Contracture

• A contraction is an action of the muscle

• A contracture is a pathological process where soft tissue has become physically shorter and restricts movement

Isometric Contraction

• Development of tension in the muscle, however, no movement; Length of the muscle does not change and there is no movement at a joint

• Also know as a static contraction

Isotonic Contraction

• Length of the muscle changes

• There is joint movement

Types of Isotonic Contractions

• Concentric - is a shortening contraction; Length of the muscle shortens; Force is greater than the resistance

• Eccentric - length of the muscle lengthens; Also called a lengthening contraction; Resistance is greater than force

Concentric Contraction

Eccentric Contraction

Forces on Musculoskeletal System