Biomechanical movement

Welcome to Biomechanical Movement!

Ever wondered why a sprinter uses starting blocks, or why a gymnast tucks their body tightly during a somersault? That is biomechanics in action! Think of biomechanics as the "physics of sport." We are going to look at how forces act on our bodies and how we can use these laws of science to perform better and stay safe. Don't worry if physics isn't usually your favorite subject—we’ll break it down using examples you see on the pitch and in the gym every day.

3.2.2.1 Biomechanical Principles: The Basics of Motion

Before we can run or jump, we need to understand Force. A force is simply a push or a pull that can change the state of motion of a body. In sport, we use Newton’s Three Laws of Linear Motion to explain how things move.

Newton’s Three Laws

1. The Law of Inertia (First Law): An object will stay still or keep moving at a constant speed in a straight line unless a force acts on it.
Example: A football will stay on the penalty spot until the player kicks it.
2. The Law of Acceleration (Second Law): The rate of change of momentum is proportional to the force applied and takes place in the direction the force is applied. We often use the formula: \(F = m \times a\) (Force = Mass \times Acceleration).
Example: If you hit a hockey ball harder (more force), it will accelerate faster than if you hit it softly.
3. The Law of Action and Reaction (Third Law): For every action, there is an equal and opposite reaction.
Example: When a sprinter pushes back against the starting blocks, the blocks push forward on the sprinter with the same amount of force.

Centre of Mass and Stability

Your Centre of Mass (CoM) is the point at which your body is balanced in all directions. It’s not a fixed point; it moves as you move!
Stability is how difficult it is to disturb your balance. To be more stable, you need:
• Low Centre of Mass: Bending your knees (like a defender in basketball).
• Wide Base of Support: Spreading your feet apart.
• Line of Gravity: This is an imaginary vertical line from your CoM to the floor. It must stay inside your base of support for you to stay balanced.
• More Body Mass: Generally, a heavier athlete (like a rugby prop) is harder to move than a lighter one.

Quick Review: Newton's Laws explain why we move (Inertia, Acceleration, Reaction), and stability explains how we stay upright.
Key Takeaway: To stay stable, stay low and keep your feet wide!

3.2.2.2 Levers: Our Body’s Mechanical Tools

Your bones and muscles work together as levers. Every lever has three parts:
1. Fulcrum (F): The pivot point (usually the joint).
2. Effort (E): The force used to move the load (muscle contraction).
3. Resistance (R): The load or weight being moved.

The Three Classes of Levers

Use the mnemonic 1-2-3 / F-R-E to remember which part is in the middle:
• First Class (Fulcrum in middle): Example: Extending the neck. (Fulcrum = neck joint, Resistance = head weight, Effort = neck muscles).
• Second Class (Resistance in middle): Example: Going up on your tiptoes (plantar-flexion). These provide a mechanical advantage because they can move a heavy load with little effort.
• Third Class (Effort in middle): Example: A bicep curl. These are the most common in the body. They have a mechanical disadvantage because you need more effort to move a load, but they allow for great speed and range of motion.

Key Takeaway: 2nd class levers = Power; 3rd class levers = Speed.

3.2.2.3 Linear Motion: Moving in a Line

Linear motion happens when everything moves in the same direction at the same speed. To understand this, we need to know the difference between Scalars and Vectors.

Scalars vs. Vectors

Scalars only have a size (magnitude).
• Distance: How far you traveled (e.g., 400m around a track).
• Speed: How fast you went (\(Speed = \frac{Distance}{Time}\)).
• Mass: The amount of matter in your body (measured in kg).

Vectors have size and a specific direction.
• Displacement: The shortest straight-line route from start to finish. (In a 400m race, your displacement is 0 because you finished where you started!).
• Velocity: Speed in a direction (\(Velocity = \frac{Displacement}{Time}\)).
• Acceleration: How quickly your velocity is changing.
• Weight: The force of gravity on your mass (measured in Newtons).
• Momentum: How hard it is to stop a moving object (\(Momentum = Mass \times Velocity\)).

Impulse: Force x Time

Impulse is the time a force is applied to an object. In sprinting, we look at force-time graphs.
• To increase momentum (speed up), you want a large positive impulse (pushing hard against the ground).
• To decrease momentum (slow down), you use negative impulse (braking forces when your foot lands in front of you).

Quick Review: Scalars are just numbers. Vectors tell you which way you're going!
Key Takeaway: In a sprint, you want to maximize your positive impulse to build momentum quickly.

3.2.2.4 Angular Motion: Spinning and Rotating

Angular motion is movement around a fixed point (an axis).
• Angular Displacement: The angle (in radians) through which a body rotates.
• Angular Velocity: How fast something is spinning.
• Angular Acceleration: The rate of change of angular velocity.

Moment of Inertia (MI)

This is how much an object resists spinning. It depends on mass and how far that mass is from the axis.
The Ice Skater Example:
• When a skater tucks their arms in, the mass is close to the axis. MI decreases, so Angular Velocity increases (they spin faster!).
• When they spread their arms out, MI increases, so they spin slower.

Key Takeaway: To spin faster, get small! To slow down, spread out.

3.2.2.5 Projectile Motion: Things in Flight

A projectile is any object (or human) thrown or jumped into the air. Once it leaves the ground, only gravity and air resistance act on it.

Factors Affecting Flight

1. Height of Release: Higher release usually means further distance.
2. Angle of Release: Theoretically, 45 degrees is best, but in sport, it’s often lower (around 35-42 for shot put) because of how our muscles work.
3. Speed of Release: This is the most important factor! The faster you throw it, the further it goes.

Flight Paths:
• Heavy objects (like a shot put) follow a parabolic path (a perfect curve) because gravity is the main force.
• Light objects (like a badminton shuttlecock) follow a non-parabolic path because air resistance slows them down quickly, making them drop steeply.

Key Takeaway: Release speed is king for distance!

3.2.2.6 Fluid Mechanics: Drag and Lift

When you move through air or water (fluids), two forces act on you: Drag and Lift.

Drag

Drag is a force that acts against you, slowing you down.
• To reduce drag: Use streamlined positions (the "tuck" in cycling), smooth clothing (skinsuits), or follow someone closely (drafting).
• Factors: Drag increases as your velocity increases. If you double your speed, your drag quadruples!

The Bernoulli Principle: Creating Lift

This explains how a discus or a ski jumper stays in the air longer.
Bernoulli discovered that air moving faster has lower pressure.
• If an object (like a discus) is shaped or tilted so air moves faster over the top, the pressure on top is lower than the pressure underneath.
• This pressure difference creates Upward Lift.
• Interesting Fact: Formula 1 cars use this in reverse! Their wings create Downward Lift to "glue" the car to the track so they can take corners at high speeds.

Key Takeaway: Use streamlining to beat drag, and use the Bernoulli principle (angles/shape) to gain lift.

Final Encouragement

Biomechanical movement can seem like a lot of definitions, but remember that it's all about how athletes interact with the physical world. If you get stuck, try to picture the movement in your head—why does a cyclist crouch? Why does a diver tuck? The science is usually just explaining what athletes do naturally to win!