Stability & Levers

• Centre of Mass (CoM)

  the point at which a body is balanced in all directions/ the point from which the weight of a body appears to act
• Stability

  the ability of a body to resist motion & remain at rest/ the ability of a body to withstand a force applied & return to its original position without damage
• Centre of Mass

  • location is dependent on the distribution of body mass & can be manipulated to improve sporting technique by altering body shape
  • can move outside the body & acts as a point of rotation
  • density & position are difficulties when locating the CoM

  e.g Fosbury Flop high jump technique - uses a j-curve to allow greater velocity in approach, athlete CoM is moving outside of body & below the bar when jumping, requires less force at take-off to clear the same height
• Stability

  • more stable = harder to move
  • it an object is stable, it wants to stay still
• Factors Affecting Stability

  • mass of the body - greater mass of body = greater stability (sumo wrestlers typically have a high mass)
  • height of CoM - lower CoM = greater stability (gymnasts flex at knee & hip to lower CoM when landing jumps)
  • base of support - greater size of base of support = greater stability, can be moving 2 points of contact wider or increasing the number of points of contact (2 feet 2 hands in a bridge position)
  • line of gravity - more central line of gravity is to base of support = greater stability (netball goal shooters line of gravity falls within her base of support)
• Maximising Stability

  • sprinter preparing in the blocks
  • crouched position gives low CoM
  • base of support is large, 5 points of contact
  • line of gravity falls within base of support
  • high mass due to high proportion of muscle mass
• Minimising Stability

  • when set is called sprinter lifts their hips, raising their CoM
  • one knee is lifted, reducing points of contact
  • leans forwards, shifting line of gravity to edge of base of support
  • reduces stability ready for movement
• Lever Systems

  the coordination of our bones & muscles to create human movement
• 2 Main Functions of Lever Systems

  1. to generate muscular effort to overcome a given load
  2. to increase the speed of a given movement
• Components of a Lever System

  • lever (bone)
  • fulcrum (joint)
  • effort (muscular force)
  • load (weight/resistance)
• Lever

  rigid structure which rotates around a fixed point
• Fulcrum

  the fixed point about which the lever rotates
• Effort

  the force applied by the user of the lever system
• Load

  the force applied by the lever system
• 1st Class Lever

  • extension of the neck when preparing to head a football
  • the effort is the muscles in the neck contracting, the fulcrum is the neck joint, & the load is the weight of the head
  • has a mechanical advantage

  
  

  load-fulcrum-effort
• 2nd Class Lever

  • calf raise
  • the fulcrum is the toes/ball of the foot (ankle), the load is the weight of the performers body weight & gravity, & the effort is the gastrocnemius contracting
  • have a mechanical advantage

  
  

  fulcrum-load-effort
• 3rd Class Lever

  • flexion of the elbow during a bicep curl
  • the fulcrum is the elbow joint, the effort is the bicep brachii contracting, & the load is the performer weight & the weight their holding
  • most common lever in the body

  
  

  load-effort-fulcrum
• Way to Remember Levers

  1-F-N

  2-L-A

  3-E-E
• Effort Arm

  the distance from the fulcrum to the effort
• Load Arm

  the distance from the load to the fulcrum
• Mechanical Advantage

  • when a levers effort arm is longer than its load arm
  • means a large load can be moved with relatively little effort
  • occurs in second class levers
• Mechanical Disadvantage

  • when a levers load arm is longer than its effort arm
  • means a large effort is required to life a relatively small load
  • occurs in third class levers