Welcome to Biomechanical Principles!
Ever wondered why a sprinter uses starting blocks, or why a high jumper arches their back? That is biomechanics in action! Think of biomechanics as the "physics of sport." It’s all about how forces interact with our bodies to create movement.
Don't worry if physics wasn't your favorite subject at GCSE—we are going to break this down into simple, sporting steps. By the end of these notes, you’ll be able to "see" the forces acting on every athlete you watch!
1. Newton’s Laws of Motion
Sir Isaac Newton came up with three laws that explain how everything moves. In PE, we apply these to athletes and equipment.
Newton’s First Law: Inertia
Definition: An object will remain at rest or continue at a constant velocity unless acted upon by an external unbalanced force.
Analogy: Imagine a football sitting on the penalty spot. It won't move until you kick it. Once it's moving, it would technically fly forever if air resistance and gravity didn't pull it down!
Newton’s Second Law: Acceleration
Definition: The rate of change of momentum of an object is directly proportional to the force causing it and takes place in the direction in which the force is applied.
The Simple Version: If you want something to speed up (accelerate), you need to hit/push it harder. Also, a heavier object (more mass) needs more force to get moving than a light one.
The Formula: \(Force = mass \times acceleration\) or \(F = ma\)
Newton’s Third Law: Reaction
Definition: For every action, there is an equal and opposite reaction.
Sporting Example: When a basketball player jumps, they push down on the floor (Action). The floor pushes up on the player with the same amount of force (Reaction), which is what actually lifts them into the air!
Quick Review: The Three Laws
1. Inertia: Objects are lazy; they keep doing what they are doing.
2. Acceleration: Force = Mass x Acceleration. Push harder to go faster.
3. Reaction: You get back what you give. Push the ground, it pushes you.
2. Forces in Sport
A force is simply a push or a pull that alters the state of motion of a body. In Biomechanics, we look at several specific types.
Types of Force
• Net Force: The overall force acting on an object when all individual forces are added together.
• Balanced Forces: When two forces are equal in size but opposite in direction. The object stays still or stays at the same speed.
• Unbalanced Forces: When one force is bigger than the others, causing the object to change speed or direction.
The "Big Five" Sporting Forces
1. Weight: The force of gravity pulling you down. Calculated as \(Weight = mass \times acceleration \space due \space to \space gravity\).
2. Reaction Force: The upward force from the ground (linked to Newton's 3rd Law).
3. Friction: The force that occurs when two surfaces rub together. In sport, we usually want more friction (like spikes on running shoes) or less friction (like wax on skis).
4. Air Resistance: The "drag" force that slows you down as you move through the air.
5. Internal Force: Generated by our own muscles contracting.
Manipulating Friction and Air Resistance
Athletes try to "cheat" these forces to win:
• To Increase Friction: Use chalk on hands (gymnastics) or rubber soles on basketball shoes for grip.
• To Decrease Air Resistance: Wear skin-tight "skinsuits" (cycling), use aero-helmets, or "draft" (tuck in) behind another runner.
Key Takeaway: Free Body Diagrams
In your exam, you might have to draw or label a Free Body Diagram. This is just a simple sketch using arrows to show forces. Remember: The length of the arrow shows how big the force is, and the direction shows where it's pushing!
3. Biomechanical Calculations
Time for some quick maths! Don't panic—the formulas are straightforward.
1. Force (Newtons - N):
\(Force = mass \times acceleration\)
If a 10kg shotput is accelerated at \(5m/s^2\), the Force is \(50N\).
2. Momentum (\(kg \cdot m/s\)):
\(Momentum = mass \times velocity\)
Think of momentum as "how hard it is to stop." A rugby prop has a lot of momentum because they have a high mass!
3. Acceleration (\(m/s^2\)):
\(Acceleration = \frac{final \space velocity - initial \space velocity}{time}\)
4. Weight (Newtons - N):
\(Weight = mass \times 9.81\) (where 9.81 is gravity on Earth).
Common Mistake: In everyday life, we say "I weigh 70kg." In PE, 70kg is your mass. Your weight is a force measured in Newtons!
4. Centre of Mass and Stability
The Centre of Mass (CM) is the unique point at which the body is balanced in all directions.
Factors Affecting the Position of CM
Your CM isn't fixed! It moves when you move.
• If you lift your arms up, your CM moves higher.
• If you bend your knees, your CM moves lower.
• Interestingly, the CM can even be outside the body, like when a high jumper performs the Fosbury Flop and arches their back over the bar!
Stability: How to Stay Balanced
To be more stable (like a rugby player in a scrum), you should:
1. Lower your Centre of Mass: Bend your knees.
2. Widen your Base of Support: Spread your feet apart.
3. Line of Gravity: Ensure the imaginary line falling from your CM stays inside your base of support.
5. Levers: The Body’s Machines
Our bones and muscles act as levers to move our limbs. Every lever has three parts:
• Fulcrum (F): The pivot point (the joint).
• Effort (E): The force used to move (the muscle contraction).
• Load (L): The weight being moved (the limb or equipment).
The Three Classes of Levers
Use the mnemonic F-L-E 1-2-3 to remember which part is in the middle!
• First Class (F in middle): Example: Extending the neck to head a football. The joint (Fulcrum) is between the muscles (Effort) and the head (Load).
• Second Class (L in middle): Example: Going up on your tiptoes (plantar flexion) at the ankle. The weight of the body (Load) is in the middle. Note: These have a high Mechanical Advantage—they can move heavy loads with little effort!
• Third Class (E in middle): Example: A bicep curl. The muscle (Effort) pulls between the elbow (Fulcrum) and the hand (Load). This is the most common lever in the body!
Quick Review: Lever Components
• Effort Arm: The distance from the Fulcrum to the Effort.
• Load Arm: The distance from the Fulcrum to the Load.
6. Analysis Through Technology
Biomechanical analysis isn't just done by eye anymore. Coaches use high-tech tools to find tiny improvements.
Key Technologies
• Limb Kinematics: Using high-speed cameras and sensors to track body movement in 3D. It helps identify errors in technique (e.g., a golfer's swing).
• Force Plates: Measuring the ground reaction forces when an athlete steps or jumps on them. Great for analyzing a sprinter’s start or a long jumper’s take-off.
• Wind Tunnels: Giant fans that blow air at an athlete. Used by cyclists and skiers to test their body position and equipment (like helmets) to see how aerodynamic they are.
Summary of Technology Benefits
• Optimise Performance: Find the "perfect" technique.
• Injury Prevention: Spot movements that might put too much stress on a joint.
• Equipment Design: Create faster bikes or more supportive shoes.
Final Key Takeaways
• Newton's Laws are the foundation of all movement.
• Friction and Air Resistance can be manipulated to help or hinder an athlete.
• Stability depends on a low CM and a wide base.
• Levers (FLE 123) turn muscle contractions into movement.
• Technology provides the data needed to reach elite levels.