Biomechanical Principles for Analysis of Human Movement

Linear and Angular Concepts of Human Movement

Linear Motion

• Movement in a straight line. All body parts travel the same distance in the same direction and at the same time.
• Distance: Total length of the path traveled by an object. (scalar quantity)
• Displacement: Change in position of an object from start to finish, considering direction. (vector quantity)
• Speed: How quickly an object covers distance. $Speed = \frac{Distance}{Time}$ (scalar quantity)
• Velocity: How quickly an object changes position, including direction. $Velocity = \frac{Displacement}{Time}$ (vector quantity)
• Acceleration: Rate of change of velocity. $Acceleration = \frac{Change \ in \ Velocity}{Time}$. Can be positive (speeding up), negative (slowing down) or zero (constant velocity). Measured in $m/s^2$.

KEY TAKEAWAY: Understand the difference between scalar (magnitude only) and vector (magnitude and direction) quantities.

Angular Motion

• Movement around a central axis (rotation).
• Torque: A turning or twisting force that causes rotation. Torque = Force x Lever Arm. The lever arm is the perpendicular distance from the axis of rotation to the line of action of the force.
• Angular Distance: Total angle covered during rotation (degrees or radians).
• Angular Displacement: Change in angular position (degrees or radians).
• Angular Speed: Rate of change of angular distance.
• Angular Velocity: Rate of change of angular displacement.
• Angular Acceleration: Rate of change of angular velocity.

EXAM TIP: Be prepared to differentiate between linear and angular motion concepts and provide examples from sport.

Momentum

• Linear Momentum: Measure of the amount of motion an object has; resistance to changing its velocity. $Momentum = Mass \times Velocity$.
• Conservation of Momentum: In a closed system, the total momentum remains constant if no external forces act.
• Angular Momentum: Measure of the amount of angular motion an object has; resistance to changing its angular velocity. Angular Momentum = Moment of Inertia x Angular Velocity.

COMMON MISTAKE: Forgetting that momentum is a vector quantity and has direction.

Impulse

• Change in momentum of an object. $Impulse = Force \times Time$. Important for increasing or decreasing momentum. A larger force applied over a longer time results in a greater change in momentum.

STUDY HINT: Create flashcards with the formulas for each of these concepts.

Newton’s Three Laws of Linear Motion

1. Law of Inertia

• An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a force.

• Inertia: Resistance of an object to change its state of motion. Directly proportional to mass.

2. Law of Acceleration

• The acceleration of an object is directly proportional to the force acting on it, is in the same direction as the force, and is inversely proportional to the mass of the object.

• Formula: $F = ma$ (Force = mass x acceleration). A greater force will produce a greater acceleration. A greater mass will result in a smaller acceleration for the same force.

3. Law of Action-Reaction

• For every action, there is an equal and opposite reaction.

• When you apply a force to an object (action), that object applies an equal force back on you in the opposite direction (reaction).
• Example: When you jump, you exert a force on the ground (action), and the ground exerts an equal and opposite force back on you, propelling you upwards (reaction).

REMEMBER: “Inertia, Acceleration, Action-Reaction” - use this to recall Newton’s Laws.

Projectile Motion

• Motion of an object through the air after it is released. Affected by gravity and air resistance (often negligible in simple analyses).
• Factors Affecting Projectile Motion:
  • Height of Release: Greater the height, the longer the flight time (assuming other factors are constant).
  • Angle of Release: Angle at which the object is projected.
    • Optimal angle for maximum distance (in a vacuum) is 45 degrees.
    • If release height is above landing height, optimal angle is less than 45 degrees.
  • Speed of Release: Most important factor. Greater the speed, the greater the distance.

Description: A diagram illustrating projectile motion. The diagram shows a ball being thrown at different angles (30, 45, 60 degrees). The 45-degree angle results in the longest distance, assuming the height of release and landing are the same.

APPLICATION: Discuss the implications of projectile motion on different sports (e.g., basketball free throw, long jump, javelin throw).

Anatomical Third-Class Levers

• Lever: Rigid structure that pivots around an axis (fulcrum).
• Components of a Lever:
  • Axis (Fulcrum): Pivot point of the lever (e.g., joint).
  • Force (Effort): Force applied to the lever (e.g., muscle contraction).
  • Resistance (Load): Force that the lever is used to overcome (e.g., weight of a limb or an external object).
• Mechanical Advantage: Ratio of the force arm to the resistance arm. $Mechanical \ Advantage = \frac{Force \ Arm}{Resistance \ Arm}$.
• Third-Class Levers:
  • Force is located between the axis and the resistance.
  • Most common type of lever in the human body.
  • Mechanical advantage is always less than 1 (MA < 1).
  • Designed for speed and range of motion, rather than force.
  • Example: Bicep curl. Axis = elbow joint, Force = biceps muscle, Resistance = weight in hand.

Description: A diagram illustrating a third-class lever, clearly showing the placement of the axis, force, and resistance.

VCAA FOCUS: Questions often ask about identifying the components of a lever system in a specific sporting movement.

Equilibrium: Stability

• Equilibrium: State of balance where there is no net force or torque acting on an object.
• Stability: Resistance to disruption of equilibrium.
• Factors Affecting Stability:
  • Centre of Gravity (COG): Point around which an object’s weight is equally balanced. Lower COG increases stability.
  • Base of Support (BOS): Area within the points of contact between an object and its supporting surface. Larger BOS increases stability.
  • Line of Gravity (LOG): Imaginary vertical line passing through the COG to the ground. Stability is greatest when the LOG falls within the BOS.

Description: A diagram illustrating the centre of gravity, base of support, and line of gravity for a person standing.

EXAM TIP: Apply these principles to sporting examples such as a wrestler trying to maintain balance.

Qualitative Movement Analysis Stages (Biomechanical Principles)

• Qualitative Movement Analysis: Systematic observation and evaluation of the quality of human movement, without relying on numerical measurements.
• Stages of Qualitative Movement Analysis:
  1. Preparation:
     • Identify the purpose of the analysis (e.g., skill improvement, injury prevention).
     • Determine the key phases of the movement.
     • Understand the characteristics of optimal technique.
     • Develop an observation strategy.
  2. Observation:
     • Observe the performance of the skill, either live or recorded.
     • Observe from multiple angles.
     • Focus on key aspects of the performance.
  3. Evaluation:
     • Identify strengths and weaknesses in the performance.
     • Compare the observed performance to the optimal technique.
     • Prioritize areas for improvement.
  4. Error Correction:
     • Provide feedback to the performer.
     • Suggest drills or exercises to correct errors.
     • Re-observe the performance to assess improvement.

VCAA FOCUS: Be prepared to apply the stages of qualitative movement analysis to a specific sporting skill and suggest appropriate interventions.