Kinetics

Question 1

In regards to humans, give some examples of internal forces and some examples of external forces.

Answer 1

Internal forces: muscle, ligament pull, bone on bone, friction.
External forces: gravity, inertia, GRF, water resistance, wind, friction, impact

Question 2

In the human body, where is the center of gravity located? What does the line of gravity represent and where does it pass through?

Answer 2

Normally just anterior to S2. The LOG represents to force of gravity on an object and passes through the COG toward the center of the Earth.

Question 3

What are the potential consequences of forces acting on an object?

Answer 3

Acceleration or Deformation

Question 4

Stress

Answer 4

force per unit area. It develops on the cross section of a structure in response to externally applied loads.

Question 5

For a given force, how does the area over which the force is applied affect stress?

Answer 5

Decrease the area, stress increases. Increase the area, stress decreases.

Question 6

Strain

Answer 6

deformation that develops within a structure in response to externally applied force.

Question 7

Stiffness:

Answer 7

resistance offered by tissue or material as it deforms from an external force.

Question 8

Elasticity:

Answer 8

ability to return to original shape after load is removed.

Question 9

Hysteresis

Answer 9

Work done to deform is greater than work to return to original shape.

Question 10

Where does the lost energy go from hysteresis?

Answer 10

It is lost as heat.

Question 11

Creep:

Answer 11

increase in strain over time with a constant load.

Question 12

In what kind of material does creep occur?

Answer 12

Viscoelastic materials.

Question 13

The yield point:

Answer 13

load exceeds the material’s ability to regain original shape, and permanent deformity results.

Question 14

The failure point: the failure of the material

Answer 14

the failure of the material

Question 15

Physiological region:

Answer 15

area under the stress/strain curve before the yield point. The region in which tissue will deform permanently.

Question 16

Plastic region:

Answer 16

after yield point, material continues to deform until the failure point.

Question 17

On a stress strain curve, what is indicated by the slope?

Answer 17

Stiffness of the material.

Question 18

What characteristic would you assign to a material with a steep stress strain slope?

Answer 18

Very steep slope means very stiff material (ex. Tendon)

Question 19

What happens if the fatigue of repetitive loading outpaces the repair processes of a tissue?

Answer 19

Tissue damage.

Question 20

Macrotrauma:

Answer 20

Acute stress that occurs when a single force is large enough to cause injury of biological tissues

Question 21

Microtrauma:

Answer 21

A repetitive stress that by itself is insufficient to damage a tissue, but causes injury when repeated over time

Question 22

What kind of forces can cause rotation?

Answer 22

Parallel translational forces (two men moving a boat example)

Question 23

What is a moment arm?

Answer 23

Distance is perpendicular between line of force and axis of rotation.

Question 24

Which class of level always presents a mechanical advantage? Why does it present a mechanical advantage?

Answer 24

Second class always presents a mechanical advantage, because the load is between the axis of rotation and the effort (muscle). In other words, the moment arm is always longer than the effort. The MA is always greater than 1.

Question 25

Provide an example of each class of lever in the human body:

Answer 25

1st: atlantooccipital joint
2nd: lower leg when standing on tip toes
3rd: biceps brachii

Question 26

How is torque calculated?

Answer 26

Torque= force x distance. Distance (aka moment arm) is perpendicular between line of force and axis of rotation.

Question 27

What effect does a large moment arm have on torque?

Answer 27

b. The larger the moment arm, the larger the torque (an increase in force application would also increase torque)

Question 28

When does a force have no moment? What are the implications of this?

Answer 28

a. A force has no moment when the force passes through the axis of rotation
b. A force passing through the axis of rotation is all linear, it has no rotary (torque) component

Question 29

As one does a squat, why does the force production of the quadriceps need to increase?

Answer 29

When one does a squat, the moment arm for gravity (external moment arm) increases as you move deeper into the squat and the distance between the axis of rotation (your knee joint) and the line of gravity increases. Therefore, your quadriceps must increase their force output to counter the greater gravitational torque.