chapter 3 Principles of Motion and Stability

Define: Biomechanics Developmental biomechanics

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

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

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.

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

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

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

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

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).

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?

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

Using Newton’s Third Law Oppositional movements and directional force

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

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# 323946 (from 4E CD-ROM, Auxiliary, Standing Long Jump, example 6)\ Click image to view video

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

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

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.

Question: What developmental skills involve learning to absorb force?

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

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

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

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.