Horizontal Component of Propulsive & Breaking Forces

• Ground Reaction Force Fluctuation During Gait:

  • Horizontal (Hz) component
  • Graph tells us forwards or backwards (direction)
  • Talking about reaction forces (+ opposite to push)
• The Anterior-Posterior Force Component:

  • The anterior-posterior component of the ground reaction force during walking may be split into 4 sections

  1. Heel Strike to Posterior Peak (F4)
  2. Posterior Peak (F4) to Crossover
  3. (0 = where 0 force acting)
  4. Crossover to Anterior Peak (F5)
  5. Anterior Peak (F5) to Toe Off
• Heel Strike to Posterior Peak:

  • What is happening?
  • Claw back = little bit of pos before go into neg (may or may not be present)
  • After initial claw back (if present), the heel is in contact with the ground & the body decelerates causing a posterior shear force
  • As make heel contact, CoM moving forward & downward
  • Because of these forces of these forces get a reaction force that is acting downwards against our CoM (why is called a braking force)
• Heel Strike to Posterior Peak:

  • F4 magnitude should be in the order of 0.2 times the person’s BW
• Heel Strike to Posterior Peak:

  • What does the posterior peak signify?
  • Acting against forward movement = braking force
  • Confidence in loading front foot
  • Reduction in loading force = not confident
  • Could be related to surface eg ice
  • Lot less forward braking component as push more up & down less forward & backward component
• Braking force (neg) = reaction force of forward downward push = neg reaction force
• What is Claw Back? = Pos bit at start

  • If have slippery surface or unconfident about surface
  • They land heel back/initial contact then pull foot backwards
  • Leaves foot down then pulls it back = reaction force backward & downward
  • Only seen if surface is slippery
  • Force plate is slippery so can often happen on them
• What happens when we aren’t confident on our surface?

  • Pushing down, taking small steps
  • More up & down less forward & backward
  • Lower peak & more claw back
• Posterior Peak (F4) to Crossover:

  • What is happening?
  • The body begins to move over the stance limb (into mid stance = 0), reducing the horizontal component of the resultant GRF
  • Reducing the braking component
  • Moving into propulsion
  • Forward directory becomes more vertical, less horizontal (reducing), why peak neg moves up to become more pos
• Posterior Peak (F4) to Crossover:

  • What is happening?
  • At the crossover point the horizontal force is 0 (causing moving towards pos)
  • Therefore, the only force acting is that of the vertical GRF
  • Mid stance = basically weight force almost completely vertical, CoM directly over CoP, all forces now acting downward, even for just a moment = 0 at that point in time
• Posterior Peak (F4) to Crossover

  • What is happening?
  • Crossover should occur around 55% of stance phase
  • Dependent on confidence may be longer
  • If move quickly less time (lower % of time)
• Crossover to Anterior Peak (F5) - Propulsion Peak:

  • What is happening?
  • The heel lifts & the foot is pushed down into the ground by the action of muscles in the posterior compartment of the ankle joint
  • To propel body forward
  • Mid stance to heel raise
  • CoM forward ahead of stance limb
• Crossover to Anterior Peak (F5) - Propulsion Peak

  • What is happening?
  • This has the effect of producing an anterior compartment of the ground reaction force, which propels the body forwards
• Crossover to Anterior Peak (F5) - Propulsion Peak:

  • F5 magnitude should be in the order of 0.2 times the person’s body weight
  • Equivalent to braking peak = constant pace/velocity
• Crossover to Anterior Peak (F5) - Propulsion Peak:

  • What does the anterior peak signify?
  • Tells us about strength of muscle to perform propulsion
  • Reduced peak = poor ability to propel body forward
  • Due to muscle activity or reduction in ankle movement
  • Backwards hz force, CoM ahead of stance limb, downward force, now have backward directed mass force in pos direction (forward)
• Anterior Peak (F5) to Toe Off (heel raise to toe off):

  • What is happening?
  • This is now the period of terminal double support, where the force is now being transferred to the front foot & the anterior force, therefore, reduces
  • The length of time the force takes to reduce & offload can affect the loading during the next foot contact
  • Steeper slope = moving more quickly into next phase
  • Shallower = more time