Biomechanics Section 1: Forces and Levers. Biomechanics “Describe how functional anatomy and...
-
Upload
james-mckinney -
Category
Documents
-
view
231 -
download
5
Transcript of Biomechanics Section 1: Forces and Levers. Biomechanics “Describe how functional anatomy and...
Biomechanics
Section 1: Forces and Levers
Biomechanics
• “Describe how functional anatomy and biomechanical principals relate to performing a physical activity”
• Coaching movement• Understanding movement• Optimising movement• Russian Gynastics
1.2 What is a force?
A force is defined as a push or a pull
Movement Term
Description Example
Translation Moves from A → B
A golf shot. It starts on the tee and finishes on
the fairway
Rotation If the force applied is not
through the COG the object will
rotate
Putting spin on a volleyball
serve
Deformation The object loses shape on impact
A squash ball being hit
1.3 Centre of GravityThe main force acting on all parts of the body
is gravity.
We can define the COG as follows:The point in the body at which all parts of the
body are in balance OR the point at which gravity is centred.
When we stand in the anatomical position, our COG is located around the hip region.
Why is COG lower in women than men?More mass is concentrated around the hips
and below in women. This gives advantages ion sports requiring balance e.g. beam work in gymnastics.
The Axes of Rotation
Axes of Rotation Sporting Examples
Longitudinal Pirouette in danceTwist in diving
Spinning in ice-skating
Transverse Forward/backward somersault
Forward/backward roll
Sagittal CartwheelCricket delivery
Barani or free cartwheel
1.4 LeversA lever consists of three parts1. Resistance2. Effort3. Fulcrum or pivot
Levers perform two main functions:1. To increase the resistance that can
be moved with a given effort, e.g. a crowbar.
2. To increase the velocity at which an object will move with a given force.
Class Illustration Definition Examples
First The fulcrum lies between the effort and
the resistance
See sawCrowbarHammer
pulling out a nail
Second
The resistance
lies between the fulcrum
and the point of effort
Wheelbarrow
Opening a door by the
handleRowing a
boat
Third The effort lies between
the resistance
and the fulcrum
Biceps curlMost limbs of the body
Revision
Sporting Movement Axis of rotation
One handed cart wheel Sagital
Backward somersault Transverse
Rotation phase in discus Logitudinal
A twist in a dive Longitudinal
Backward roll Transverse
Biomechanics
Section 2: Motion
3.2 Newton’s Laws of Motion
[A] Law 1An object at rest tends to remain at rest unless acted
upon by some external forceThis is known as inertia
Which has more inertia? Why?The 175kg weights have more inertia because it has a
greater mass
Having a great deal of inertia can be advantageous in some sporting situations. How?
If you have a lot of inertia you can be difficult to move e.g. in a rugby scrum, wrestling, judo
Of course having a lot of inertia has disadvantages as well in sporting situations. How?
If you have a lot of inertia, you require a lot of force or effort to get you moving. It can also mean a decrease in agility.
[B] Law 2If a table tennis ball, tennis ball and bowling ball are each hit
or bowled with the same amount of force, which one accelerates the most? Why?
The table tennis ball because it is lighter.
If the tennis ball is hit with gradually increasing force, what happens to its acceleration?
The acceleration will increase with increasing force.In summary, we can say:A. The greater the force, the greater the accelerationB. The smaller the mass, the greater the acceleration when a
constant force is applied.C. The mass will accelerate in the direction the force is
applied.
These statements can be summarised by an equation:Force = mass X acceleration
F = ma
How could you apply Newton’s 2nd Law to sporting situations?The harder you hit the ball, the faster and possibly further it
will travel. For example, swinging a golf club slowly with force gives the golf ball less force hence acceleration than if you swung the golf club forcefully.
[C] Law 3When we apply a force to something it is known as action
forceThe object we apply a force to, applies a force back, a
reaction force
What other example can you think of? When a ball is bounced, it bounces back in a direction
opposite to that in which it was dropped.If these forces are equal, why is earth not pushed backward
when we drive out of the starting blocks in a 100 metre race?
The earth has a huge mass and therefore huge inertia. We cannot generate enough force to overcome this inertia.
Action
Reaction
Action
Reaction
Action
Reaction
3.5 MomentumWe can rewrite Newton’s First Law to include
momentum: An object that is moving will continue to move in the direction the force was applied until another force is applied.
[A] Linear MomentumMomentum can be calculated via an equation:
Momentum = mass x velocity(kgms-1) (kg) (ms-1)
Use this equation to calculate which athlete has the greatest momentum. Mo (A)= 75 kg x 6.5 ms-1 Mo (B) = 80 kg x 5.5 ms-1
= 487.5 kgms-1 = 440 kgms-1 Player A has the greatest momentum.
[B] Angular MomentumConsider the situation of teaching young
children how to swing a softball bat. Is it easier for them to swing using a normal grip or a choke grip? Why?
The choke grip is easier because the COG of the mass is now closer to the axis of
rotation (the hands).
Consider an ice skater spinning. How so they speed themselves up in the spin? How do they slow themselves down?
To speed up, they bring their body parts closer to the axis of rotation. Then move them out again to slow down.
Delete the incorrect option in each case.
When the mass is moved closer to the axis of rotation, inertia increases / decreases, and angular velocity increases / decreases. As a consequence the body spins faster.
When the mass is moved further away from the axis of rotation, inertia increases / decreases, and angular velocity increases / decreases. As a consequence the body spins slower.
Other sporting examples of this principle in action include:
1. Pirouettes in ballet2. Somersaults in tumbling3. Springboard diving movesExplain in biochemical terms the relationship that
occurs between moment of inertia and angular velocity as the gymnast performs the headstand to forward roll.As the gymnast tucks A – C, the mass is brought closer to the axis of rotation. AS a consequence inertia decreases and angular velocity (speed of roll) increases. As the gymnast opens out from C - E, the mass is moved further from the axis of rotation. Inertia increases, angular velocity decreases.
Let us assume that the gymnast is unable to stand at the end of performing the skill, instead they fall backwards. Explain in biomechanical detail what may have happened for this to occur.They opened out too early (inertia increases, angular velocity decreases,) i.e. they don’t have enough speed of rotation to get to the standing position.
Plot the change in inertia and angular velocity as the gymnast performs a headstand to forward roll
An
gula
r V
eloc
ity
Iner
tia
High
Low
A B C D E
Key: = INERTIA
= ANGULAR VELOCITY
3.6 Conservation of Momentum
The Law of Conservation of Momentum states:
When objects collide, momentum is conserved throughout.
The total momentum before impact (before the ball is hit) is:
Equal to the momentum of the bat AND the momentum of the ball
Basically:Momentum before impact = momentum
after impact
Using this concept, explain why:a. The white ball in pool slows down
after impacting the black.Some of the momentum of the white ball is passed on to the black ball.
b. When suddenly braking in a car, your body moves forward (thankfully you are wearing a seatbelt).The momentum of the car is transferred to your body. This is why objects in a car are propelled forward on braking.
Remember, the total momentum of the club and ball before impact must equal…
The total momentum of the club and ball AFTER impact
Step 1: Calculate the momentum of the club and ball before impact.
Step 2: Calculate the momentum of the club and ball after impact.
Club momentum
Mo = m x v
= 0.35kg x 30m/s
= 10.5 kgm/s
Ball momentum
Mo = m x v
= 0.04kg x 0m/s
= 0 kgm/s-1
(because the ball is
stationary)
Total momentum
Club + Ball
= 10.5 kgm/s + 0kgm/s
= 10.5 kgm/s
+ +
Club momentum
Mo = m x v
= 0.35kg x 25m/s
= 8.75 kgm/s
Ball momentum
If total Mo must be 10.5 kgm/s and the club is 8.75 kgm/s the ball must be:
10.5 kgm/s – 8.75 kgm/s
= 1.75 kgm/s
Total momentum
= 10.5 kgm/s+ +
Step 3: Since we now know the momentum of the ball after impact, we can calculate the velocity of the ball by rearranging the momentum equation.
a. An ice hockey player with a mass of 85 kg propels themselves forward with a velocity of 5 m/s. What is their momentum?
Mo = m x v = 85 kg x 5 m/s = 425 kgm/s
Ball Velocity (m/s)
V = Mo ÷ m
= 1.75 kgm/s ÷ 0.04 kg
= 43.75 m/s
Mo
mass
velocity
b. Calculate the velocity of the puck after impact.
Using this information, the velocity of the puck is…
Stick momentum
(before)
Mo = m x v
= 2kg x 25m/s
= 50 kgm/s
Puck momentum (before)
Mo = m x v
= 0.5kg x 2m/s
= 1.0 kgm/s-1
(because the ball is stationary)
Total momentum
(before)
= 51 kgm/s
+ +
Stick momentum
(after)
Mo = m x v
= 2kg x 17m/s
= 34 kgm/s
Puck momentum (after)
51kgm/s – 34kgm/s
= 17kgm/s
Total momentum
(after)
= 51 kgm/s
+ +
V = Mo ÷ m
= 17 kgm/s ÷ 0.5 kg
= 34 m/s
c. Explain using the principle of conservation of momentum, why the hockey puck moves toward the goal faster than the 2m/s it was originally travelling at.
Because some of the momentum of the player and stick has been passed to the puck on impact.
d. (Extension) Use this information to calculate the hockey player’s velocity after striking the puck.
Player Mo before = 425 kgm/s but puck after impact = 17 kgms-1
Change in Mo of player = 425 khms-1 = 408 kgms-1
V = Mo ÷ m= 408 kgms-1 ÷ 85 kg
= 4.80 m/s
3.7 Generating Momentum[A] Generating Linear Momentum Consider the hockey player. How do they give
maximum momentum to the ball when it is flicked? What are they doing?Get down low, step into the shot, strong grip on lever (stick), use whole body in shot, follow through, good range of motion.
[1] Using body SegmentsWe should look to se as many body segments as possible when trying to give an object maximum momentum. Why?
Because we can maximise the muscular force that each muscle group associated with each segment can generate.
In the hockey illustration, what body segments are being used?
Legs, hip, trunk, shoulder, arms and wrist.
[2] Stretch OutBefore we begin the sequence of movements, such as the throwing action, we should stretch muscles out to their optimal length. Why?It allows the muscle to be contracted with optimum force.
In the hockey illustration, how do we see this principle being applied?Large step forward. Extension of arms around the stick.
[3] Sequencing of Body SegmentsIn effect we use the body like a giant whip. What are the benefits of this?The momentum generated by larger segments is passed onto smaller ones until we male contact/release etc.
In the hockey illustration, how do we see this principle being applied?Legs – trunk – arms – wrist – stick - ball
[4] Timing of Body SegmentsWhat could happen if the timing of body segments is “out of order”?Not only does the movement lack co-ordination but maximum force generated can be lessened.
How does correct timing ensure maximum momentum?It means we use those larger muscle groups first and the smaller muscle groups last.
[5] Full Range of MotionWhat are the benefits of this?The greater the range of motion, the higher the speed of the extremities on release/contact.
Using your knowledge of generating momentum, explain how the athlete generates maximum momentum to the javelin upon release.
1. They use the large muscle groups of the leg and trunk to generate force initially. This is then passed to the shoulder, arm and finally hand upon release of the javelin.
2. They fully extend the arm (at shoulder) prior to the throwing action.
3. The timing is legs – trunk – shoulder – arm – hand – javelin4. The arm moves through its full range of motion to maximise
lever length and force summation.
[B] Generating Angular MomentumAn eccentric force is…A force applied to one side of the
COGDraw in the position of the force
that needs to be applied in order to generate topspin and backspin on the balls below.
TOP SPIN
FORCE FORCE
BACK SPIN
Draw in the body position of the athlete in order to initiate a dive roll in gymnastics. Explain this position.The COG is in front of the feet (where the force will be applied). This causes a turning force.
How can an athlete generate more angular momentum to the body, or object?Apply greater initial force or move the COF further away from the point of force application.
force
COG
4.2 Factors Affecting Projectile Motion
What is the effect on the projectile of the vertical component?To give it height
What is the effect on the projectile of the horizontal component?To give it distance
The factors affecting the flight path of a projectile are:
1. Gravity2. Air resistance3. Speed of release4. Angle of release5. Height of release6. Spin
[A] GravityWhat is the effect of gravity on a projectile?
It decreases the height the projectile can attain
[B] Air ResistanceDraw the typical flight path of a badminton shuttle.
There are several key factors that bring air resistance into play:1. The larger the surface to volume ratio, the more air resistance
will affect the object, e.g. a badminton shuttle compared to a golf ball.
2. The surface of the object. If the surface is rough then air resistance will be greater.
3. Speed. As speed increases, so does air resistance, e.g. a space shuttle (friction).
4. Mass. The smaller the mass (lighter the object), the more air resistance will affect it.
How can we make sure we still get good distance from our projectile?Have a lower angle of release. Rugby players will kick the ball low.
[C] Speed of ReleaseGenerally the greater the speed of release,
the greater the distance gained
What are the advantages of having a high initial vertical velocity?
It will result in a longer flight caused by more height.What are the advantages of having a high initial horizontal
velocity? It will result in a longer flight time and good distance.
In which sports would a high initial vertical velocity be of advantage?
Gymnastics tumbling, high jump, ski jump (tricks).In which sports would a high initial horizontal velocity be of
advantage? Long jump, ski jump (distance), vaults in gymnastics.
Initial Horizontal velocity
Initial Vertical velocity
Direction of flight
[D] Angle of ReleaseIn sporting situations the angle of release is usually always
lower, around 35˚ to 45˚. Why?Air resistance of the body. The take-off point is usually higher than the landing point, e.g. long jump
What would happen if the angle of release were too high for a given activity?Poor distance gained.
What would happen if the angle of resistance were too low for a given activity?Poor flight time and possibly poor distance.
1. Sports in which distance is important have a lower angle of release
2. Sports in which height or flight time is important have a higher angle of release
ACTIVITY ANGLE RANK
EXPLANATION
Triple Jump 2 Going for distance
High Jump 4 You need time, but also a little distance to clear bar
Standing Back Somersault
5 You need time to somersault
Javelin Throw 3 You need distance, but also flight time
Racing Dive in Swimming
1 You dive down
[E] Height of ReleaseWhy might this be?
Time in the air will be greater.Would this mean that a golfer hitting a ball off
the top of a hill would hit it further than a golfer at the bottom of the hill? Why?YES – the ball will stay in the air longer so will have a greater chance to fly further. This assumes the same club and force is used.
In what other sports is application of this principle important?Javelin. Hold the javelin up high to gain a greater height of release.
The reason behind this can be summarised as follows:
1. As the height of release increases, the angle of release decreases.
2. As the height of release decreases, the angle of release increases.
[F] SpinWhat happens to the distance achieved with a topspin
shot compared to one with backspin?A topspin shot gives poorer distance compared to backspin.
1. Range is decreased with topspin 2. Range is increased with backspinWhat will happen with a backspin shot?
A region of high pressure (H) is created under the ball and low pressure (L) above the ball. Air moves from H-L. As a consequence the ball tends to ‘stay up’ longer.
Draw this on the two balls below:TOPSPIN BACKSPIN
L H
DIRECTION OF BALL
DIRECTION OF BALL
H L
6.2 Phases of Execution
Most complex skills can be broken down into three phases of execution:
1. Preparation phase2. Execution phase3. Post action/follow through phrase
Label each diagram accordingly.(From left to right) 1,2,3
How would you describe the key features of each phase? What is its purpose?
Phase 1: This is where the athlete sets themselves up to execute the skill. Mental preparation. Addressing the hockey ball (as in example above)
Phase 2: This is the force producing movements required to perform skill. In this phase the principles of force summation, timing etc are used.
Phase 3: This allows forces to be controlled or dissipate to prevent loss of control or injury.
The greater the force that can be applied to the ball, the greater the acceleration of the ball.By using correct force summation (timing, sequencing, body segments, stretch and range of motion), large forces can be generated that will be passed on to the ball.
Acceleration & Force
(2nd Law)
The muscles have to work hard now to decrease the forces generated during preparation and execution to maintain control. The greater the forces that were generated, the greater the forces required to slow down and control the movement in the follow through.
Body is positioned ready to overcome the inertia of the ball and change its direction.Body is being prepared to move.Greater forces will be required to overcome the inertia of the body compared to the ball because the player weighs more.
Inertia (1st
Law)
Newton’s Laws of Motion
Follow-through
Execution Phase
Preparation Phase
To be consider
ed
Key biomechanical principle
The sequence and timing of body segments is legs then hips then trunk then shoulders then arms and finally hands.The stick head will move quickly through the range of motion generating large forces on contact with ball.The extension of the arms and stepping forward means muscles will be at optimum force.This also allows for the greatest possible range of motion allowing for greater force to be generated.
The player may be going through the movements in their head to make sure the correct body part adds the movement at the correct time.Body is positioned to increase the chances of this happening.
Body segments Stretchin
g outSequenci
ng of segmentsTiming
Range of motion
The ball will travel in the direction that the force is applied, Therefore, if the stick follow through is toward s they target, that is the direction the ball will travel in.
For every action there is an equal and opposite reaction. If a lot of force is given to the ball it will react and travel a good distance.The ball will travel in the direction that the force is applied.
Action &
Reaction
(3rd Law)
Newton’s Laws of Motion
Generating
Momentum
(Force Summatio
n)
Follow-throughExecution PhasePreparation Phase
To be considered
Key biomechanical principle
The rear leg stays back as the stick travels in front of the body because, as one part of a body moves away from the centre of gravity, another body part must move in the opposite direction to ensure balance is maintained.
The large step towards the target increases the size of the abase of support meaning that as the player executes the shot, their centre of gravity remains within the base of support, keeping them stable and balanced throughout.
The stable body position ensures the body is balanced and controlled before executing the shot. The centre of gravity (COG) lies within the margins of the base of support (BOS)
Centre of gravity
Base of support
Line of gravity motion
The follow through is designed to reduce the momentum in the system so the player remains stable and ready to move on to the next skill.
As they move through the execution phase, momentum generated by the legs and hips moves to the trunk, then shoulder, arms and finally to the stick.Following contact, some of the momentum is transferred to the ball and it begins its flight path.Momentum is conserved throughout the movement (ball and body)
The player positions the body such that momentum can be transferred through the body to the ball.By ensuring correct sequencing and timing of body segments, they will enhance the transfer of momentum
Transfer &
Conservation of
momentum
Stability & Balance
Follow-throughExecution PhasePreparation Phase
To be considered
Key biomechanical principle
The follow through will help determine the final path. Of the player ‘hooks; the stick, the shot too will be hooked.
With force summation, the ball will be given a certain amount of speed upon release. The greater the force given to it, the greater the speed of release.
If the player drops the right shoulder and lowers their body behind the ball, it will be given more height rather than distance at take-off (increased angle)
The player positions the stick on the bottom half of the ball in order to get ‘under’ it.The angle at which the stick contacts the ball will help determine the angle of release and therefore how high and how far it will go.
Speed of release
Height of release
Angle of release
Spin
Projectile Motion
Follow-throughExecution PhasePreparation Phase
To be considered
Key biomechanical principle