Vertical Component of Propulsive & Breaking Forces

• The Vertical Component: Single Step:

  • Sections to look at that give indication of gait & functioning of lower limb
  • 4 areas to take note of:

  1. Heel Strike to First Peak (F1)
  2. First Peak (F1) to Trough (F2)
  3. Trough (F2) to Second Peak (F3)
  4. Second Peak (F3) to Toe-off
• The Vertical Component: Single Step:

  • Stance phase, half a cycle
  • Heel to heel (from one foot to another)
  • Vt component represents the vt propulsion of CoM
  • How much pushing CoM up & down
  • The curve almost the same no matter age, gender etc
  • 2 peaks & a trough
  • 1st peak = contact peak
  • 2nd peak = active/propulsion peak
  • Pretty much same for running just more compressed, may lose trough
• Heel Strike to First Peak (F1):

  • What is happening?
  • The foot strikes the ground
  • The body decelerates downwards
  • Weight is transferred from back foot to the front foot during initial double support (loading)
  • F1 magnitude should be in the order of 1.2 times the person’s body weight
• Heel Strike to First Peak (F1):

  • What does this peak signify?
  • How confident person is to load that front foot
  • The speed of walking
  • If experiencing pain or discomfort, shy to put weight on first foot, lower peak
  • Same if slowly from last step to this step as slowing movement of CoM
• Heel Strike to First Peak (F1):

  • CoM at stance phase getting ready to swing CoM drops, because as move limb out, feet split apart, goes down, force moves downward as move body, force of gravity & mass, & whatever push you putting in ground when making heel contact
• Heel Strike to First Peak (F1):

  • Fmg + Fp > BW
  • So peak = Force of mass x gravity (Fmg) +
  • Whatever push (Fp), you’re pushing in ground as making heel contact
  • Should = greater than your BW (body weight)
  • Tells you how confident they are
• First Peak (F1) to Trough (F2):

  • What is happening?
  • As body starts to progress over the lower limb, the knee extends raising the CoM
  • Progress gait over CoM, extending knee, shifting mass over top stance leg CoM moving upwards
  • As the CoM approaches its highest points, it is slowing down, or decelerating its upwards motion
  • At mid stance
• First Peak (F1) to Trough (F2):

  • F2 magnitude in the order of 0.7 times the person’s body weight (0.7 x BW) = under BW
• First Peak (F1) to Trough (F2):

  • What does the trough signify?
  • Willingness or confidence to move all their weight on top of stance limb
  • If is painful will be shy to move over stance limb, speed thru stance phase, = shallower trough, or even deeper trough (to minimise time)
• First Peak (F1) to Trough (F2):

  • Heel contact to mid stance
  • CoM rises to highest point
  • Upward force acting on body
  • Have push, gravity acting against us
  • Force of push - force of gravity
  • Reaction force will be less than BW
  • Fp - Fg < BW
• Trough (F2) to Second Peak (F3):

  • What is happening?
  • The CoM now falls as the heel lifts & the foot is pushed down & back into the ground by the action of muscle in the posterior compartment of the ankle joint
  • Propelling body forward, need muscle action in foot to cause that to happen
  • Prev using momentum, now need to create force
• Trough (F2) to Second Peak (F3):

  • What is happening?
  • Both the deceleration, downward & propulsion from the foot & ankle complex cause the second peak (F3)
  • Similar to first peak, first = braking mechanism
  • second = propulsion/pushing out of stance phase
• Trough (F2) to Second Peak (F3)

  • F3 magnitude should be in the order of 1.2 times the person’s body weight
  • Similar to braking/impact peak (1st peak)
• Trough (F2) to Second Peak (F3):

  • What does the second peak signify?
  • Typically a low peak tells us that the person has a weak or poor ability to propel themselves
  • Pathological in gastrocnemius or amputees, reduce mobility of ankle joint
  • Ability to propel CoM upwards as leave stance phase of movement
• Trough (F2) to Second Peak (F3):

  • Mid stance into end of step phase
  • Movement from high CoM to low CoM
  • Mass + our push = greater than BW
  • If at constant pace similar to first peak
  • Fmg + Fp > BW
• Second Peak (F3) to Toe Off:

  • What is happening?
  • The foot is unloaded as the load is transferred to the opposite foot
  • The time taken to off load from the back foot will relate to the speed of transfer of the weight to the front foot
  • Speed = how fast you are moving & how willing to transfer
• Second Peak (F3) to Toe Off:

  • What is happening?
  • Therefore, the longer the offloading period from the back foot, the lower the first peak during loading on the front foot
  • If take longer here, will affect first peak of next step, not as steep (longer time to offload), next peak lower (around 1 = to body weight)
• eg Amputee gait

  • Similar pattern to healthy stance
  • 2 peaks & trough
  • Magnitudes look a bit different
  • Below knee amputee
• Pathology vs Healthy:

  • Differences:

  1. Height of peaks, lower for amputee
  2. The proportion of time spend in initial loading/impact phase = 10% longer for amputee
  3. The depth of the trough, shallower for amputee
  4. Active peak is even lower than first peak for amputee, as no muscular to cause push off, affects next step
• Pathology vs Healthy:

  • 2. The proportion of time spend in initial loading/impact phase = 10% longer for amputee
  • Spend more time going into loading phase due to confidence, not as steep of a slope
  • Not as much of a push
• Pathology vs Healthy:

  • 3. The depth of the trough, shallower for amputee
  • ~ 0.8 BW, as moving slower into stance, less confident, not as much transition/vt movement of CoM as moving into that, less displacement of CoM not as high
  • 4. Active peak is even lower than first peak for amputee, as no muscular to cause push off, affects next step
  • Also seen later in hz movement