1.3a biomechanical principles

• biomechanics allows performers and coaches to:

  • analyse performance
  • maximise movement efficiency and sporting technique
  • reduce or prevent injuries
  • design and choose the correct equipment to satisfy the demands of the activity
• newton's first law of motion

  also known as the law of inertia

  a body continues in a state of rest or uniform velocity unless acted upon by an external or unbalanced force

  e.g., the 100m sprinter will remain at rest in the blocks until an external force large enough to overcome their inertial creates motion. equally, when the sprinter reaches a constant velocity, they should continue at constant velocity until an external or unbalanced force acts upon them to change
• newton's second law

  also known as the law of acceleration

  a body's rate of change in momentum is proportional to the force applied and acts in the same direction as the force applied

  e.g., the greater the force applied to the sprinter, the greater the rate of change in momentum and therefore acceleration away from the blocks. the force is applied in a forward direction and so the sprinter drives towards the line
• newton's third law

  also known as the law of reaction

  for every force applied to a body there is an equal and opposite reaction force

  e.g., when the 100m sprinter applies a down and backward action force into the blocks, the blocks provide an equal and opposite up and forward reaction force to the sprinter to drive them out of the blocks
• application of newtons 3 laws

  NL1: the rugby ball will remain at rest on the conversion tee until an external force is applied by the rugby player's foot to it

  NL2: the greater the size of the force applied by the rugby player to the ball the greater the rate of change of momentum and acceleration towards the post. the rugby ball will accelerate in the same direction the force applied

  NL3: a forward and upward action force is applied to the rugby ball from the rugby player's foot. the rugby ball will apply an equal and opposite down and backward reaction force to the player's foot
• velocity

  velocity is the rate of change in displacement (the shortest straight line route between start and finish points)

  velocity = displacement / time taken

  velocity is measured in metres per second (m/s), displacement in metres (m) and time taken in seconds (s)
• momentum

  momentum is the quantity of motion possessed by a moving body

  momentum = mass x velocity

  momentum is measured in kilogram metres per second (kgm/s), mass is measured in kilograms (kg) and velocity in metres per second (m/s)
• acceleration

  acceleration is the rate of change in velocity

  acceleration = (final velocity - initial velocity) / time taken

  acceleration is measured in metres per second per second (m/s/s), change in velocity is measured in metres per second (m/s) and time taken in seconds (s)
• force

  force is a push or pull that alters the state of motion of a body

  force = mass x acceleration

  force is measured in newtons (N), mass in kilograms (kg) and acceleration in metres per second per second (m/s/s)
• there are two types of force

  • internal force generated by the contraction of skeletal muscles. a 100m sprinter must contract the rectus femoris to extend the knee and gastrocnemius to plantar flex the ankle to generate the force required to drive away from the blocks
  • external forces comes from outside the body and acts upon it. these are weight, reaction, friction and air resistance
• force's 5 effects

  1. create motion
  2. accelerate a body
  3. decelerate a body
  4. change the direction of a body
  5. change the shape of a body
• net force

  the sum of all forces acting on a body, also termed resultant force. it is the overall force acting on a body when all individual forces have been considered
• balanced forces

  these occur when two or more forces acting on a body are equal in size and opposite in direction. net force = 0, the body will remain at rest or in motion with constant velocity
• unbalanced forces

  these occur when two forces unequal in size and opposite in direction. a net force will be presented and the body will change its state of motion, either accelerating or decelerating
• vertical forces- weight

  weight is the gravitational pull that the earth exerts on a body and is measured in newtons. weight force is always present and acts downwards from a bodys centre of mass. it can be shown on a diagram by a vertical arrow extending from the centre of mass downwards.

  weight (N) = mass (kg) x acceleration due to gravity (m/s/s)
• vertical forces- reaction

  reaction is the equal and opposite force exerted by a body in response to the action force placed upon it and is measured in newtons. it is a result of newton's third law of motion and is always present when two bodies are in contact. normal reaction can be shown on a diagram by a vertical arrow extending upwards from the point of contact with the surface
• horizontal forces- friction

  friction is the force that opposes the motion of two surfaces in contact and is measured in newtons. friction can be shown on a diagram by a horizontal arrow extending (usually) in the same direction as motion from the point of contact parallel to the sliding surface
• factors affecting friction

  • roughness of the ground surface- increasing roughness increases friction e.g., athletes run on tough, rubberised tracks
  • roughness of the contact surface- increasing roughness increases friction e.g., sprinters wear spiked shoes
  • temperature- increasing the temperature of the ground and contact surface increases friction e.g., F1 drivers have a warm up lap
  • size of the normal reaction- increasing size of normal reaction, increases friction e.g., shot putters have a high mass (NL3 creates a high reaction force) allowing greater friction, preventing over rotation
• horizontal forces- air resistance

  a force that opposes the motion of a body travelling through the air. it is a form of fluid friction and is measured in newtons. it can be shown on a diagram by a horizontal arrow extending against the direction of motion from the centre of mass
• factors affecting air resistance

  • velocity- higher the velocity the higher the AR e.g., the greater the velocity of a cyclist, the greater the AR opposing their motion
  • shape- the more aerodynamic, the lower the AR. many sports use a aerofoil shape to minimise air resistance (streamlining) e.g., the shape of a cyclists helmet
  • frontal cross sectional area- decreasing frontal cross sectional area reduces AR e.g., low crouched position of skiers
  • smoothness of surface- increasing smoothness reduces AR e.g., smooth lycra suits of sprinters and cyclists
• streamlining

  the creation of smooth air flow around an aerodynamic shape to minimise air resistance
• free body diagrams

  a clearly labelled sketch showing all the forces acting on a body at a particular instant in time
• if weight is equal in size to reaction, net force is zero. forces are balanced (equal in size but opposite in direction), and so the body will remain at rest e.g., a basketballer travelling in constant vertical velocity

  if reaction force is greater than weight, net force is positive. forces are unbalanced and acceleration in an upward direction will occur e.g., a basketballer leaving the ground in the take off phase in a lay up shot
• if F = AR, net force is zero. forces are balanced in size and opposite in direction and so the body will move at constant velocity e.g., a sprint cyclist who has reached maximum velocity on the track

  if F > AR net force is positive. forces are unbalanced and acceleration in a forward direction will occur e.g., a sprint cyclist accelerating away from the starting line

  if AR > F net force is negative. forces are unbalanced and horizontal deceleration will occur e.g., a sprint cyclist crossing the finishing line sitting up and decelerating
• limb kinematics

  study of movement in relation to time and space

  • use- 3D or optical motion analysis records an athlete performing a sporting action, allowing the efficiency of movement
  • optimising performance- data produced can be used by coaches to improve performance / specific techniques of athletes
• force plates

  ground reaction forces are measured in laboratory conditions using force plates

  • uses- athletes balance, run and jump on a force plate, which assesses the size and direction of forces acting on the athlete, acceleration rates, work and power output
  • optimising performance- used for sports biomechanics assessments, gait analysis, balance rehabilitation and physical therapy
• wind tunnels

  steel frame building containing wide fans, where artificial wind is produced

  • use- technology is used to develop the drag reduction system. objects such as cycle helmets and F1 cars can be tested for aerodynamic efficiency
  • optimising performance- engineers study the flow of air around the object. the aim is to improve the flow of air around an object, streamlining its path through the oncoming air and potentially increasing lift of decreasing drag
• centre of mass

  the point at which a body is balanced in all directions; the point from which weight appears to act
• the centre of mass can move outside of the body and act as a point of rotation- for example a gymnast performing a tucked somersault moves their centre of mass in front of the body and rotates around it
• the fosbury flop technique

  • uses a j-curve to allow greater velocity in the approach
  • plants the outside foot to allow the inside leg to lift, along with arms, at take off to raise centre of mass as high as possible
  • fully extends the spine to rotate around the bar moving centre of mass outside the body and below the bar. only one section of the body has to be above the bar at any one time (compared to the scissor kick where the whole body has to be above the bar at the same time)

  as the centre of mass is below the bar, fosbury flop requires less force at take off to clear the same heights
• stability

  the ability of a body to resist motion and remain at rest. stability is also the ability of a body to withstand a force applied and return to its original position without damage (remain in a balanced position)
• factors affecting stability: mass of the body

  the greater the mass of a body the greater its inertia, and therefore the greater its stability: for example, sumo wrestlers typically have a high mass to withstand great applied forces
• factors affecting stability: height of the centre of mass

  the lower the centre of mass, the greater the stability. e.g., when a gymnast lands a jump, they flex at the hip and knee to lower their centre of mass and have a stable landing, as do marital arts performers in defensive situations who sit low into a wide stance
• factors affecting stability: base of support

  the greater the size of the base of support the greater the stability. this can be moving two points of contact (feet) wider apart to create a larger surface area or it could be increasing the number of points of contact: for example, the two feet and two hands down in the gymnastic bridge position. a table tennis player stands with feet wider than shoulder width to increase stability
• factors affecting stability: line of gravity


  the line of gravity is an imaginary line which extends from the centre of mass downwards to the floor. the more central the line of gravity to the base of support the greater the stability e.g., a netball shooters line of gravity falls within her base of support whereas a goalkeeper who is marking the shot's centre of mass falls outside and is in front of her base of support, leading to over rotation
• maximising stability

  a sprinter preparing in the blocks has maximum stability. the crouched position gives a low centre of mass. the base of support is large, with five points of contact. the line of gravity falls within the base of support and sprinters typically have a high mass due to their high proportion of muscle mass
• minimising stability

  • when "set" is called, sprinter lifts their hips raising their CoM, lifts one knee reducing points of contact, and leans forward shifting the line of gravity to the edge of base of support, reducing stability ready for movement
  • when the gun is fired, instability is maximised to aid performance. chest lifts raising the CoM, hands come off the track minimising base of support and points of contact, and the line of gravity falls in front of the base of support causing the body to fall forwards, minimising movement time and gives a good start to drive forwards from
• lever systems are the co-ordination of our bones and muscles, primarily to create human movement. they have two main functions:

  1. to generate muscular effort to overcome a given load
  2. to increase the speed of a given movement
• the component parts of a lever system are:

  • lever (bone)
  • fulcrum (joint)
  • effort (muscular force)
  • load (weight or resistance)
• bones act as levers, rigid structures which rotate around a fixed joint. this fixed point is the fulcrum and in the human body are joints. the muscles that surround a joint create internal forces that move the bones they are attached to. when a muscle contracts, an effort is created. if this effort is large enough to overcome any load placed upon a lever, such as a weight in the hand, it will pull on the lever to create movement