Biomechanics

• Mechanics

  The study of the effect of forces on rigid bodies during static and dynamic situations
• Rigid body

  A body which cannot be deformed, stretched or compressed
• Newton's first law of motion

  The law of inertia: An object at rest or travelling in uniform motion will remain at rest or travelling in uniform motion unless acted upon by a net force; The more mass a body has (not weight) the more resistant it is to change its motion
• To overcome the net inertia of an object a net external force greater than the object's inertial force must be applied
• Application of Newton's first law

  • Sit to stand and upper body inertia is overcome by the trunk muscles
  • Use of a lap belt in a wheelchair if the patient has weak trunk muscles and is stopped suddenly
• Momentum

  A body will remain in a state of uniform motion/rest until affected by a force acting on it; An object can only have momentum when it is moving. The larger its mass the more momentum it has
• Momentum

  Momentum = mass X linear velocity
• Momentum

  • Whiplash injuries
  • Head injuries
• Newton's second law of motion

  The law of acceleration: A force acting on a body will produce acceleration proportional to the applied force (Force= Mass x acceleration; F=ma); Acceleration - the rate of change in velocity
• Application of Newton's second law

  • Exercise prescription: To attain speed in moving the body, a strong muscular force is generally necessary
• Newton's third law of motion

  The law of action / reaction: For every action there is an equal and opposite reaction
• Application of Newton's third law


  • The Ground Reaction Force
  • Joint compression in weight bearing
• Ground reaction force


  Length represents size of force (Bodyweight = hip height in standing); Direction of arrow = direction of force; Centre of pressure (bottom or arrow) shows where force acts
• Force


  An interaction that tends to change the motion of an object with a mass - unit = Newton
• Mass


  Property of matter causing a resistance to change in motion - unit kg
• Matter


  Substance that occupies space
• Body


  Collection of matter bounded by a closed space
• Force


  • It is a vector quantity: Has both magnitude and direction
• Forces acting on the body

  • Internal forces: Muscular
  • External forces: Push/ pull, Gravity, Friction, Resistance, Ground reaction force
• Muscular force


  Composition/Resultant forces: Resultant force = Forces acting parallel in the same direction; Composition force = Forces acting at an angle
• The force of attraction between objects and the earth is called the law of gravitation or law of attraction
• Weight


  Weight = gravity x mass (w=gm)
• Centre of mass


  In the human body in the anatomical position, it is located near S2; It changes with position
• Statics


  The study of systems that are in a constant state of motion whether at rest with no motion or moving at a constant velocity
• Balance, equilibrium and stability

  Equilibrium: the resultant forces and moments acting on the body are equal to zero (all inertial and dynamic forces are in balance); Stability: A stable body returns to its starting position after displacement; An unstable body continues to displace under the force of gravity once a force of short duration has been applied; Neutral stability
• Factors affecting stability during standing


  • Body mass
  • Friction between the body and the contact surface
  • The base of support
  • Location of the centre of gravity with the base of support
• Line of gravity and base of support


  Line of gravity: a vertical line passing through the centre of gravity/MASS; Base of support: represented by a line drawn around the surface area of the body part that is resting on a supporting surface, and the space between
• Lever


  A rigid bar that rotates around a fixed point called a fulcrum
• Classes of levers


  • Class I lever: fulcrum between effort (force) and load (resistance)
  • Class II lever: load (resistance) between effort (force) and fulcrum
  • Class III lever: effort (force) between fulcrum and load (resistance)
• Moment or moment of force or torque

  The tendency of a force to produce rotation about an axis
• Moment arm


  The perpendicular distance between the point of application of the force and the fulcrum
• Load or resistance arm


  The moment arm between the fulcrum and the load or resistance to be moved
• Effort or force arm

  The moment arm between the fulcrum and point of application of the force that counters the load