Principles of Motion and Stability

Principles of Motion and Stability

Center of Gravity

• Younger children
• People with certain Disabilities
• older adults
• people learning knew skills

...Often attempt to improve balance by increasing stability.

In locomotor tasks, they will increase their base of support by planting their feet wide or out-toeing their feet. They keep their center of gravity well within their base of support by avoiding excessive trunk rotation or limb movement.

Single Support (One-footed)

vs

Double Support (two-footed)

Principles of

Stability & Balance

A Stable object is one that resists movement

Highly Stable

Moderately Stable

• In many activities, maximal performance of the skill requires performers to minimize stability in order to increase mobility.
• In locomotor skills, a person momentarily sacrifices stability (two-foot support base) in order to move by alternately losing and gaining balance (one-footed support base)
• The body's weight is pushed forward ahead of the base of support and the person moves the leg forward to regain balance

Balance is your ability to control your body without movement against gravity.

Stability is your ability to control your body during movement.

Not Very Stable

Open/Closed Kinetic Chain

Balance relates to the ability of an object to maintain equilibrium

Force Absorption

A healthy body is often described as a well-oiled machine. Like a machine, it’s made up of otherwise fixed segments given mobility by joints.

A kinetic chain is the notion that these joints and segments have an effect on one another during movement. When one is in motion, it creates a chain of events that affects the movement of neighboring joints and segments.

Our body absorbs force by transferring the force to our muscles, where contractions in the opposite direction absorb the force.

In order to absorb large forces safely, our body seeks to absorb the force by

1. increasing the time of absorption, increasing the movement length used to absorb the force, or

2. increasing the area in which the force is absorbed. The body will also apply a force in the opposite direction, usually using an eccentric contraction.

increased stability = balance

Maintaining balance = does not = stability

There are two kinds of kinetic chain exercises: open and closed.

In open kinetic chain exercises, the segment furthest away from the body — known as the distal aspect, usually the hand or foot — is free and not fixed to an object.

In a closed chain exercise, it is fixed, or stationary.

The athlete bends at the knees, hips and ankles, whilst creating an eccentric contraction of the major muscle groups (quads, hamstrings, glutes, and calves) against the downward force. This contraction absorbs the force by creating an equal and opposite force canceling out the applied force.

In order to increase the safety of the landing, the athlete can make the eccentric contraction go for longer by allowing a larger bend at the joints.

GRAVITY

What are example of force absorption can you think of in other sports?

Simplest way to think about it...

All objects are attracted to each other and that the amount of attraction depends on the objects' masses (amount of material).

Using Principles of Motion to detect and Correct Errors

Movements occur in a context governed by certain principles of motion and stability

Our weight as the force of attraction of the Earth (gravitational pull)

Because the Earth is so large, we are drawn toward its center. If you jump up, you don't continue to rise...You come back down.

1. Observe the complete skill

THAT IS...

Newton's Third Law of Motion

To every action there is an equal and opposite reaction.

Because gravity acts on the long jumper equally and at all times, his flight path resembles a semicircle

Body parts action and reaction

Certain physical laws of motion LIMIT your movements .

2. Analyze Each Phase and its key elements

Newton's Second Law of Motion

Think of it in the following way. If you push on some else’s hand, they feel your force.

But you feel the force too. You push on them; they push back on you.

The same thing works with gravity. The Earth pulls on you; you pull on the Earth.

Acceleration of a person or object is proportional to the force applied to it and inversely proportional to its mass.

3. Use Knowledge of mechanics in your analysis

• Does the mover have optimal stability when applying force?
• Is the mover using all muscles that can contribute to the skill?
• Is the mover applying force with the muscles in the correct sequence?
• Is the mover applying the right amount of force over the appropriate frame of time?
• Is the mover applying force in the correct direction?
• Is the mover correctly applying torque and momentum transfer?
• Is the mover manipulating any linear or rotary inertia properly?

4. Select Errors to be Corrected

Now you try it...

Application of Force (for any movement to occur)

• Focus on the major errors
• Ignore minor problems
• Work on one aspect of performance at a time (improvement in one area may trigger improvement in other areas)

Newton's First Law of Motion

5. Decide on Appropriate methods for the correction of errors.

An object at rest stays at rest, or an object in motion stays in motion, until acted on by a force (McGinnis, 2005)

1. Keep safety in mind

2. Use language that your student understands over mechanical terms

3. Use outside sources such as the internet or textbooks (videos) to teach movement skills like www.csuchico.edu/isbs

With a partner or on your own, find a video of someone performing a skill and use the last 5 steps to analyze the skill