👋 Welcome to Biomechanics: Generating Movement! (B.1)

Welcome to the start of the Biomechanics section! Don't worry if physics seems intimidating—we are going to simplify it.
This chapter, Generating Movement in the Body, is all about understanding the fundamental mechanical principles that allow your body to move, run, jump, and lift.
We'll focus on how your bones and muscles work together as simple machines called levers, and how you maintain your balance and stability. Think of this as the physics manual for your athletic performance!

1. The Body as a System of Levers

Movement in the human body is achieved when muscles pull on bones, using joints as pivot points. This system mimics a simple mechanical device known as a lever.

1.1 Components of a Lever System

Every lever has three essential components. The relative position of these components determines the lever's class and its mechanical function.

  • Fulcrum (F): The fixed point or pivot around which the lever rotates.
    (In the body: This is typically the joint—like the elbow or knee.)
  • Effort (E): The force applied to move the lever.
    (In the body: This is the muscle contraction.)
  • Load / Resistance (R): The weight or resistance that needs to be moved.
    (In the body: This is the weight of the limb, or an external object like a dumbbell.)

🧠 Memory Trick for Components: Remember F.E.R. (Fulcrum, Effort, Resistance/Load).

1.2 Classifying Levers

Levers are categorized based on which component lies in the middle of the other two.

The Mnemonic: 123 - RFE

  • 1st Class Lever: The Fulcrum (F) is in the middle. (R-F-E)
  • 2nd Class Lever: The Resistance/Load (R) is in the middle. (F-R-E)
  • 3rd Class Lever: The Effort (E) is in the middle. (R-E-F or F-E-R)

Class 1 Lever: F is in the middle (R-F-E)
  • Analogy: A seesaw or scissors.
  • Function: Can produce both balanced movement (speed) or generate significant force, depending on the position of the fulcrum.
  • Example in the Body: The neck joint (atlanto-occipital joint) when nodding your head.
    R = weight of the head (front), F = neck joint, E = posterior neck muscles (trapezius).
Class 2 Lever: R is in the middle (F-R-E)
  • Analogy: A wheelbarrow.
  • Function: Always maximizes force advantage (Mechanical Advantage > 1). You can lift a large load with relatively little effort, but you sacrifice speed and range of motion.
  • Example in the Body: Standing on your tiptoes (plantar flexion).
    F = ball of the foot (toes), R = weight of the body (ankle/mid-foot), E = calf muscles (gastrocnemius).
Class 3 Lever: E is in the middle (F-E-R)
  • Analogy: Fishing rod or bicep curl.
  • Function: Always maximizes speed and range of motion (Mechanical Advantage < 1). This requires a large effort force, but allows the hand or foot to move very quickly over a large distance.
  • Example in the Body: Biceps during a forearm curl.
    F = elbow joint, E = biceps insertion point, R = weight in the hand.
  • Did you know? The vast majority of levers in the human body are 3rd class. This is because sport generally prioritizes rapid movement and large ranges of motion (speed) over maximizing force (which 2nd class levers do).

Quick Takeaway: Levers help us understand the trade-off in movement: do we want more force (Class 2) or more speed/range (Class 3)? The body predominantly chooses speed.


2. Stability and Equilibrium

If movement is about getting from point A to point B, stability is about staying balanced (or choosing when to become unbalanced, like starting a sprint!).

2.1 Defining Key Concepts
  • Centre of Gravity (COG): This is the imaginary point where the total weight of the body is concentrated, and where the body is perfectly balanced.
    (For a human, the COG is usually located around the hips/navel area when standing anatomically, but it moves constantly as you change position.)
  • Line of Gravity (LOG): An imaginary vertical line passing downwards through the COG to the ground.
    (The LOG represents the direction gravity is pulling your body.)
  • Base of Support (BOS): The area enclosed by the outermost points of contact with the supporting surface.
    (If you are standing, the BOS is the area between your feet.)
  • Equilibrium: The state where all opposing forces (and torques) are balanced, resulting in no acceleration.
2.2 Factors Affecting Stability

For any athlete—whether a wrestler or a gymnast—maximizing stability means making it harder to be moved or knocked over. Conversely, minimizing stability allows for easier initiation of movement (e.g., jumping).

The four key rules for maximizing stability are:

  1. Lower the COG: The lower the COG is relative to the ground, the more stable the object.
    (Example: A sumo wrestler adopts a low stance to increase stability.)
  2. Increase the Mass: An object with greater mass requires a greater force to disturb it.
    (Example: A heavy basketball player is often harder to shift than a light one.)
  3. Increase the BOS: A larger base of support provides greater stability, particularly in the direction of the greatest force.
    (Example: Taking a wide stance when lifting a heavy weight.)
  4. Ensure LOG is within the BOS: As long as the LOG falls within the base of support, the body will remain stable. Once the LOG moves outside the BOS, you must move or fall.
    (Example: When you lean too far over to pick something up, your LOG moves outside your feet, and you must step out to prevent a fall.)

Common Mistake Alert! Students often confuse COG and LOG. Remember, the COG is a point in the body, while the LOG is the vertical line extending from that point to the ground.

2.3 The Role of Stability in Sport
A. Maximizing Stability (Static Sports)

In sports requiring resistance to force (like wrestling, rugby, or sailing), athletes aim to:

  • Keep the COG as low as possible (bending knees, crouching).
  • Widen the BOS (spreading feet, maximizing contact area).

B. Minimizing Stability (Dynamic Sports)

To initiate movement quickly (like starting a sprint, jumping, or changing direction), athletes intentionally decrease stability to make it easier to accelerate:

  • Raise the COG slightly.
  • Shift the LOG close to the edge of the BOS (leaning forward at the start of a race).

Did you know? In high jump, athletes using the Fosbury Flop technique intentionally move their COG outside their body (below the bar) at the peak of the jump, which allows them to clear a higher bar for the same amount of effort!

Quick Takeaway: Stability is crucial for control. Athletes manipulate their COG, LOG, and BOS to optimize either balance (e.g., lifting) or mobility (e.g., sprinting).


3. The Role of Joints in Movement Generation

While levers provide the mechanical structure, the joints determine the type and range of movement possible. Different joints provide different degrees of freedom for the levers.

3.1 Joint Classification Review (Prerequisite Knowledge)

Joints are the fulcrum points of our lever systems. They are generally classified by their structure and the movement they permit:

  • Ball and Socket: Allows for movement in all planes (tri-axial). Example: Hip, Shoulder.
  • Hinge: Allows movement in one plane (flexion/extension; uni-axial). Example: Elbow, Knee.
  • Pivot: Allows rotation around a long axis (uni-axial). Example: Neck (atlas/axis), Radioulnar joint.

The structure of the joint dictates the length and direction of the lever arms, directly impacting the mechanical advantage and efficiency of movement generation.

3.2 Generating Force: Muscle Contractions

The Effort (E) in our lever system is produced by muscle contractions. Movement is initiated when the muscle generates enough force (tension) to overcome the Load/Resistance (R).

  • Concentric Contraction: Muscle shortens while producing tension. (Lifting the weight during a bicep curl.)
  • Eccentric Contraction: Muscle lengthens while producing tension. (Lowering the weight during a bicep curl; crucial for deceleration and absorbing force.)
  • Isometric Contraction: Muscle produces tension but does not change length. (Holding a plank position.)

Encouragement: Understanding how the three components (F, E, R) interact through these contractions is the absolute core of B.1. Once you know where the Fulcrum is relative to the Effort and Load, you can explain any movement in sport!