1. chapter 3
Principles
of Motion
and Stability

2. Define:
Biomechanics
Developmental biomechanics

3. What Changes From A to B? A B Click image to view video

4. Changes Are Predictable
Based on optimizing principles of motion and stability
Can be seen across variety of motor skills
Often produce more force, velocity, or accuracy

5. Newton’s First Law
An object at rest stays at rest and an object in motion stays in motion until acted upon by a force.
Inertia is resistance to motion related to mass.
Momentum is the product of mass and velocity.

6. Newton’s First Law, Simplified
We must exert force to
move objects
move ourselves
More inertia means that
it is harder to move
more force application is required

7. Newton’s First Law: Child Learns to Swing a Bat
What must the child learn about inertia?
What must the child learn about momentum?

8. Moving an Object Farther or Faster
Increase force.
Increase distance over which force is applied.

9. Adding Distance to Improve a Kick
Increase step length (linear distance).
Increase range of motion (rotational distance).
Click image to view video

10. Newton’s Second Law
Object’s force is related to mass and acceleration (F = m  a).
Object’s acceleration is related to force applied and inversely related to mass
(a = F/m).

11. Newton’s Second Law
A person can throw only as hard as he or she can throw.
Given a constant force level, how could you increase acceleration when throwing a ball?

12. Newton’s Third Law
To every action, there is an equal and opposite reaction.
When you push on something, it pushes back on you!

13. Using Newton’s Third Law
Oppositional movements and directional force

14. Force Generation Aided by Oppositional Movements
Click image to view video
Click image to view video

15. Force Generation Aided by Planer Movements
Use force in the plane of motion where you want to move yourself or an object.
Avoid rotational movements that reduce force in the desired plane.
\insert video clip 3.6, ID# (from 4E CD-ROM, Auxiliary, Standing Long Jump, example 6)\
Click image to view video

16. Increasing Velocity: Rotating Limbs and Projected Objects
Increase rotational velocity (swing it faster).
Increase relative length (fully extend it at release or contact).

17. Why Not Keep Limb Extended Throughout?
The leg would have too much rotational inertia.
Click image to view video
Click image to view video

18. Force and Time
To make an object move, increase force application for a given time.
Example: karate chop to bricks.
To make an object stop, increase time over which a given force is applied
Example: soft landing in gymnastics.

19. Question:
What developmental skills involve learning to absorb force?

20. Stability and Balance Stability: ability to resist movement
Balance: ability to maintain equilibrium
Stability–mobility trade-off

21. Increasing Stability Increase base of support.
Lower the center of gravity.

22. Increasing Balance Increase stability.
Improve strength, coordination, and proprioception.

23. Take-Home Messages
The principles of motion and stability apply to all actions and objects.
Be aware of changing individual constraints.
Manipulate task and environment to aid in optimal skill performance.
Not everyone will become proficient.