7 Principles of Biomechanics

• STABILITY

• MAXIMUM EFFORT

CATEGORY 2

• MAXIMUM VELOCITY

ACTIVITY: Perform the following kicks 

1. stand near the ball, put your kicking foot on the ball and push it as far as you can
2. stand near the ball, draw your kicking leg back and strike the ball with no follow through
3. stand near the ball, draw your kicking leg back and strike the ball with a follow through
4. stand back from the ball, take a step towards it, drawing your kicking leg back and striking the ball with a follow through

Answer the questions that follow:

1. What can you observe from your results?
2. How could you use joints other than in your legs to help produce maximum force in a soccer kick?

CATEGORY 3

• LINEAR MOTION

The production of maximum velocity requires that the joints be used in sequence from largest to smallest.

ACTIVITY: Choose a SPORT ACTION and consider the order of joints used. 

Examples: 

• Hockey Slap Shot
• Golf Swing
• Soccer Throw-in
• Spike in Volleyball
• Vault in Gymnastics

Draw a series of stick figures to show the performance of the skill you chose, using the larger, slower joints first, followed by the smaller faster joints.

Attempt to Perform the Skill without the larger joints being involved.

Add the use of shoulders, hips and torso in proper sequence to improve the performance of the skill.

Explain how and why the use of all joints from largest to smallest increases the velocity achieved in the performance of these types of skills.

• LINEAR MOTION

The greater the applied impulse, the greater the increase in velocity. Works in reverse too!

ACTIVITY: Choose a SPORT ACTION and consider the IMPULSE used. 

• Volleyball overhead serve
• Smash in Badminton
• Soccer Kick
• Dunk in Basketball
• Hit in Baseball
• Bounce on a Trampoline

Perform the Skill without a lot of Impulse

Gradually Add the use of greater impulse.

Explain how and why the more impulse will lead to increased velocity

HOW WOULD THIS WORK IN REVERSE? (hint: egg toss)

Movement usually occurs in the direction that is opposite the applied force 

CATEGORY 4

• ANGULAR MOTION

If something has a LARGE MOMENT OF INERTIA (Large MI) it is going to be hard to get it going, but is the MI is SMALL it should be easy to get it going.

The moment of inertia is the resistance of an object to rotate and is rotational equivalent of mass. The moment of inertia depends upon;

• the mass of the object, the more mass the larger the moment of inertia,
• the distribution of the mass away from the axis of rotation, the further away the mass is from axis of rotation the greater the moment of inertia.

A playground roundabout spinning will have more angular momentum if it is spinning faster (greater angular velocity) and if there are more people on the roundabout (greater mass and moment of inertia). The more people on the roundabout the more force is required to start the roundabout because there is a larger moment of inertia. 

Based on the equation the greater the moment of inertia the greater the angular momentum thus it would take more force to stop the roundabout if there is greater angular momentum.

Examples of Moment of Inertia in Track & Field

RUNNING: By bending the leg on recovery, it is easier for the quadriceps to lift a bent leg due to a lower moment of inertia.

HURDLING: By bending the trailing leg lifting towards the trunk, it can be moved much faster over the hurdle.

POLE VAULTING: Flexing the legs on the swing phase toward the bar enables the body to rotate much quicker and easier.

Athletes are concerned with THREE types of TORQUE:

1. Rotation of Entire Body- Axis = off centre force
2. Rotation of Individual Body Part= muscles produce torque
3. Rotation of a Projectile= impart spin on an object

Athletic Injuries are common when applying Principle #6- WHY?

ACTIVITY: Choose one of the SPORT ACTION below:

• Figure skating, Acrobatics, Gymnastics
• Freestyle swimming
• Swinging of a cricket or baseball bat
• Swinging of a badminton or tennis racket
• Running or racing on a circular track
• Leveraging on a hockey stick
• Swinging
• Paddling a bicycle
• Rowing a boat

• ANGULAR MOMENTUM

Angular momentum is constant when an athlete or object is free in the air

Once an athlete or object is in the air- it will travel with a constant angular momentum- meaning, 

Angular momentum enables us to explain why the rate of spin (angular velocity) changes when the moment of inertia changes. This is because angular momentum of a system remains constant throughout a movement provided nothing outside the system acts with a turning moment on it. 

This phenomenon is the Law of Conservation of Angular Momentum.  

The conservation of angular momentum is very important in airborne sports. 

Close attention must be paid to how athletes can manipulate their moment of inertia by altering the positions of their body segments to increase and decrease angular velocity, i.e., increase and decrease the rate at which they spin. 

1. Competitive divers pull their limbs in and curl up their bodies when they do flips. Just before entering the water, they fully extend their limbs to enter straight down. Explain the effect of both actions on their angular velocities. 

Also explain the effect on their angular momentum. 

2.     Suppose a child gets off a rotating merry-go-round. 

Does the angular velocity of the merry-go-round increase, decrease, or remain the same if?. .

(a) He jumps off radially.

(b) He jumps backward to land motionless.

(c) He jumps straight up and hangs onto an overhead tree branch?

(d) He jumps off forward, tangential to the edge? Explain your answers. (Refer to Figure).

ACTIVITY: Look at the image below ad try to explain what is happening at each A, B & C in relation to the questions posed below?