Biomechanics

Principles of Force

Newton’s first law - Inertia

• An object at rest tends to remain at rest unless acted upon by some external force

• An object in motion tends to remain in motion and to travel in a straight line with constant velocity unless acted upon by an external force

• Definitions:Object – this may be anything from an item such as a volleyball to a human bodyForce – a push or a pull (measured in newtons)Mass – the amount of substance in a body (measured in kgs)External – from outside the object

Forces which:

• Slow objects downGround and air friction, gravity and contact with other players

• Speed up objectsimplements for hitting, such as racquets and body parts that apply force

Overcoming inertia

• For movement to happen, the force applied must be enough to overcome inertia.

• The bigger an object’s mass and the bigger any resisting force (eg. friction) the bigger the force needed to move it.

Applying force to a moving object

• When a new force is applied to an object that is moving, the direction in which the object moves may be changed.

• For the direction of the object to change completely to the direction of the new force applied, the new force must be much greater than the original force. Otherwise the movement becomes a deflection.

The ball is tossed up into the air…

Inertia and the Volleyball Serve

Direction of Force

In order for the ball to be tossed up we need to move the arm (overcome the inertia of the arm) and apply a force to the volleyball (overcome the inertia of the volleyball) so that the ball is tossed up into the air. The force is produced by contraction of the arm muscles: Biceps and Deltoids

The ball is struck to go over the net

The ball is currently going straight up. In order for it to go over the net we need to change the direction of the ball. To do this a second force is applied (using the hand). The greater the force applied the greater the change in direction.

Direction of Force

Point of Application

Newton’s third law – action / reaction

• For every action there is an equal an opposite reaction

• By applying a force backwards on the ground, the ground pushes the runner forwards

DIRECTION OF MOVEMENT (REACTION

FORCE (ACTION)

• The swimmer applies a force backwards on the water and the water pushes the swimmer forwards

FORCE (ACTION)

DIRECTION OF MOVEMENT (REACTION)

FORCE (ACTION)

MOVEMENT(REACTION)

Projectile Motion• What is a projectile?

Any body released into the air is a projectile.

• Once it is in the air a projectile can gain no extra propulsion.

• In the air it comes under the influence of gravity and air resistance.

• A body can be released into the air by either – Throwing – such as a Discus Striking – such as a tennis ballProjection of the body itself – such as in High jump

Vertical and Horizontal Components of Projectile Motion

• VerticalIf you throw a ball straight up in the air its motion is only vertical. The force of gravity acts on the ball to stop its upward movement and pull it back to earth.

• HorizontalA throw from the boundary to the wicket keeper in cricket has a horizontal as well as vertical component ie. it moves along as well as up. Air resistance slows the ball down and gravity pulls it back to earth.

Factors affecting flight:• Speed of release - The

greater the speed of release, the greater the horizontal distance

• Height of release – the greater the height of release, the greater the horizontal distance

• Angle of release – There is an optimum angle of release for each object, but that will depend on a number of factors including aerodynamics of the object and height of release

Trajectory – flight path of projectiles.

All projectiles have a parabolic flight path

Angle of Release – requirements for different activities

Activity• Tennis Serve• Volleyball float

serve• Long jump• Discus• High jump• Standing back

somersault

Angle (degrees)

• -3 to 15• 13 to 20• 17 to 22• 35 to 39• 40 to 50

• 75

What are the different objectives for our projectiles? And, what do we need to do to maximise our chances

of success?

• Height such as high jump, pole vaultMaximum possible speed of releaseMaximum possible angle of release

• Distance such as discus, long jumpMaximum possible speed of releaseCorrect angle of release for activityApplication of spin in some cases

• Speed such as rugby passMaximum possible speed of releaseLowest possible angle of release

• Accuracy such as archery, netball

FORCE SUMMATION• Any desired movement is a combination of a number of

forces. A sequence of movement is used to produce optimal velocity (best speed).

• Force Summation = the sum of all forces.

• We know that when we want to throw a ball a long distance we cannot just rely on the strength of our arm alone. We include our torso (upper body) and legs, which have the strongest muscles in the body, as well.

• The greater the force required the more body parts must be used.

• Timing of the movement is essential and follows a unique pattern. Each segment should be moved at the instant the previous segment begins to slow down. Correct sequencing is especially important when maximal force is desired.

• In humans, the slowest, strongest and heaviest parts (with the most inertia) are moved first, followed by the next strongest/heaviest ie. the torso and thighs, followed by the shoulder, arm and then the fingers which are the fastest, weakest and lightest part.

• Use the Javelin as an example. Can you identify the first, second, etc…body parts to start moving?

• Speed of release is important as the faster the speed of release the greater the force imparted (given) to the object/body. That is why in most throwing events you see the performer building up faster and faster until the desired speed is hit for the release.

• In addition to this the longer you are in contact with the object the longer the forces have to impart to that object. That is why in Javelin we take a run up, large step and then pull the arm through from an outstretched position.

• To gain maximal force summation you need stabilisation of body parts.

For example: a stable lead leg in the volley ball serve- opposite to the arm executing the serve.

Stabilised Leg

Levers• A lever is a length of solid material (eg a

bone) that is used to apply force to another object.

• Within the lever system there are 3 parts:

Fulcrum – a pivot point

Load (resistance) – weight that needs to be moved

Effort – the force applied to move the load

First Class Lever System

EF

• This is perhaps the most easily understood lever system. The load and effort are applied either side of a central fulcrum. The best example of common use in training is probably the ‘tricep curl’

 

 

L

• The resistance as in the loaded barrow is between the fulcrum of the wheel and the gardener’s effort at the handles.

 

• There are few obvious such levers in the body. The closest accepted example is a heel raise.

 • In a heel raise the principle effort is

from the muscles of the lower leg and one would expect the centre of mass to be in front of this. The fulcrum near the ends of the toe joint would be outside the centre of mass.

The Second Class Lever System

• The effort is applied between the resistance and the fulcrum

   

• Most of the lever systems in the body are third class, they are not particularly good at producing force but are rather designed to operate at speed.

The Third Class Lever System

Force Arm & Lever Length

• The longer the lever, the more effective it is in producing velocity (Force/Time)

• A Longer lever travels a greater distance, when moving the same amount of degrees

• “Merry-Go-Round/Roundabout example

Force Arm & Lever Length

• Tennis players can hit a tennis ball harder with a straight arm drive rather than with a bent elbow because the lever (including the racket) is longer and moves at a faster speed

Force Arm & Lever Length

• In baseball, hockey, golf, field hockey, and other sports, long levers produce more shear force and thus better performance

• For quickness of movement, it is sometimes desirable to have a short lever arm,– Catcher shortens the windup,

by only bringing the ball next to the ear to make a quick throw to second

EFFORT

LOAD

FULCRUM

What class of lever is this?