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Sagittal Plane Hiccup?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by trophikas, Jun 10, 2008.

  1. trophikas

    trophikas Active Member

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    Gday all

    Just a quick queery about the sagital plain theory. It is stressed that windlass mechansim/1st MPJ ROM is imperative, allowing your foot to 'auto support', and act as a solid propulsive lever. HOWEVER, its proponents also state that the stance leg DOESNT push/aid in forward propulsion, as the swing leg 'supposedly' pulls/drags the bodys' COM of mass forward via a pendulum type mechanism.

    Why does the stance leg/foot need to be an efficient propulsive lever if it doesnt have to propel anything?????:wacko::craig:


  2. Admin2

    Admin2 Administrator Staff Member

  3. Dananberg

    Dananberg Active Member

    Mr T,

    You've got this all wrong. It not that the stance leg doesn't push...it's that the "push" is passive rather than active. Its not the muscles of the stance limb that push, but rather the momentum of the CoM (pulled by the swing leg) that creates the power for the push. As such...the importance of the foot to provide a sagittal plane pivot should be evident. It allows for the efficient advancement of the body over the planted foot, thus creating a low energy cost thrust for forward motion. Hope that this clears this up.

  4. Howard:

    Sorry, can't agree with you on this one. Numerous research studies show that the stance limb does undergo concentric ankle joint plantarflexion and pushes in a posterior direction against the weightbearing surface. The body simply can't be accelarated forward without this push from the stance phase limb.

    Can the swing limb pull the body forward if the body is not touching the ground....as in a weightless environment?...NO. The body needs ground contact and the frictional force from that ground contact to move itself forward.

  5. Mr. T:

    I totally agree with your analysis of one of the inconsistencies of sagittal plane theory. The body is pushed forward by the stance phase limb, there is no question about this. The plantar fascia helps make the foot a more "efficient propulsive lever" by increasing the dorsiflexion stiffness of the forefoot on the rearfoot (i.e. increasing the forefoot dorsiflexion stiffness).

    If the foot does not "push the body forward" as is hypothesized in sagittal plane theory, then why would it be, in the sagittal plane theory, to being concerned about making the foot a more "efficient propulsive lever". If the stance phase foot does not push, then it really shouldn't matter how efficient the stance phase foot is during the propulsive phase of gait.:confused:
  6. Adrian Misseri

    Adrian Misseri Active Member

    Just a different thought......
    Heard a suggestion once that walking and gait is just controlled falling forward. It's as economic and efficient as possible. what the legs and feet do is to keep us falling forward without actually hitting the ground.
    Maybe that suggests that the shift of center of gravity forward is just as important, and that the swing leg helps to move it forward, with the stance leg forming the support, and the concentric ankle action of the support leg just assists in the final propulsion and stability of the ankle at this point?
  7. Adrian:

    Walking is much more than just a "controlled falling forward". The trajectory of the center of mass (CoM) upwards "a controlled falling upward?" and downwards is critical to the exhange of potential and kinetic energy (Novacheck, Tom F.: The biomechanics of running. Gait and Posture, 7:77-95, 1998). In fact, recent research does not support the concept of "controlled falling" (Orendurff MS, Bernatz GC, Schoen JA, Klute GK: Kinetic mechanisms to alter walking speed. Gait and Posture, 27:603-610, 2008). Walking speeds have been shown to affect the amount of concentric ankle joint plantarflexion during propulsion.

    To say that the "concentric ankle action of the support leg just assists in the final propulsion and stability of the ankle" is also not supported by research. Rick Neptune's forward dynamics research clearly shows that the gastrocnemius and soleus muscles have separate and distinct functions for supporting and accelerating the CoM and accelerating the swing leg forward in early swing phase (Neptune RR, Kautz SA, Zajac FE: Contributions of the individual ankle plantar flexors to support, forward progression and swing initiation during walking. Journal of Biomechanics, 34:11 : 1387-1398, 2001).

    The research is out there that clearly demonstrates that the ankle plantarflexors are much more than just passive mechanisms in the stance phase foot during propulsion as is suggested in sagittal plane theory. All one needs to do is read the research to see the reality of walking mechanics.
  8. trophikas

    trophikas Active Member

    Gday kEVIN

    Great to have the input of both you and Howard on this one. I have read through previous posts on this forum and it strikes me as bizare that as a proffession specialising in lower limb mechanics, we are yet to reach a consensus on how we propell ourselves!!! Still I guess its exciting in a way as it boggles the mind how much it will improve us as clinicians and with our treatments once we have nutted out these fundamentals(and I cant see how these opposing theories could = same treatment options, they are diametrically opposed and therefore must alter how you look at pathomechanics and consequently treat.)
    Another argument that seems to make sense in regards to swing leg 'pulling through' COM is that they have built simple robots that have propelled themselves forward using nothing other than a 'pendulum' type motion! Sorry to be vague on this but it was an example that Dr Danenberg raised so perhaps he could elaborate on it?

    Once again thankyou both for furthering I understanding of biomechanics and taking the time to reply to these posts


  9. CraigT

    CraigT Well-Known Member

    I probably have to take half a day off and read the full thread regarding pull vs push, but why can it not be a combination of both?:confused:
    Surely when both mechanisms are working together in unison, this would likely be most efficient.
  10. Scorpio622

    Scorpio622 Active Member

    Hi Kevin,

    After reading through the previous discussion of "Does the Stance Leg Push or Does the Swing Leg Pull?" , I realize that we are now beating the ghost of the dead horse.

    I think that both limbs participate with the forward movement but tend to lean towards the swing leg as the greater generator of force. Answer me one question:

    Why do many BK amputees walk so well with a standard prosthesis? After rehab, many look very close to normal upon gait observation. Doesn't this suggest that the stance leg is passive and primarily needs internal stability and plantar friction moreso than intrinsic muscle energy (of which the prosthesis has absolutely none)???

  11. David Smith

    David Smith Well-Known Member

    Craig T

    Yes well spotted, it cannot be denied that moving the swing leg forward does tend to move the total CoM forward of the suports leg and cause a moment with respect to the support foot that causes angular acceleration of the CoM towards the ground. The external force or energy to cause this rotation this comes from gravity. The swing leg then becomes the supports leg also and we have double support. And that is where the cycle would end without some extra force to continue forward motion. While it is true that there are mechanisms that can 'walk' by the described method above, they can only do this down a slope. So the energy required to continue forward motion is supplied by gravity. Not that much different to a rolling ball.

    However for us to have bipedal continuous forward motion we require some extra energy and that comes from the contraction of the Gastroc soleous complex which gives us a little lift. Why do we require the lift? Well consider the legs two side of a triangle, when the legs are together the hight of the triangle is at maximum. As we make a stride the base becomes longer and the hieght reduces. At this point it would be impossible for the rear leg to swing thru unless there was a groove in the ground to allow this. Of course what happens is we are lifted by the ankle plantarflexion action and so then we can swing thru and fall down the hill like the mechanism does. IE from potential to kinetic energy. (the mechanism has it's momentum and the slope angle to lift over the arc of the stance leg).

    Now if the CoM is forward of the supporting foot then for the angular acceleration of the CoM to occur there must an element of linear acceleration, which = inertial force. Therefore there needs to be another force opposite to and balancing this inertial force. This is the friction between foot and ground, wihtout which the foot would just slide backwards (relative to the forward progression).

    Therefore we have a combination of angular acceleration, and linear acceleration in the vertical and horizontal saggital planes. This results in a basic forward propulsive gait.

    Now to regulate the gait speed in terms of CoM forward velocity we can push harder? But in terms of the GSC, this action can only lift with respect to the local reference frame. However when the CoM is forward of the supporting foot, then the GSC can also push relative to the global reference frame. Therefore it is required that we do not put our swing leg down so early and this enable us to push for longer (we'll ignore the moments from the hip action for now). A longer push or applied force will equal greater terminal velocity. To leave our leg in the air for longer means we must do something like elongate our stride, which is what we usually do when we walk faster. Sprinters lift the knee as high as possible this gives them much greater falling time and therefore much increased propulsive time. The major propulsion in this example is now coming from more proximal muscle groups however.

    Other methods of increasing the CoM velocity might be to shorten the stride and increase the number of pushes per unit time but this generally uses more proximal muscle groups than for the efficient walking gait that uses the potential energy properties of the achilles tendon.

    So in my view when talking about this type of efficient walking the propulsion comes from the planted foot. This is evident from force diagrams and force plate data. However for the foot to have the ability to be propulsive the CoM must be forward to the foot and this can be enabled by the swing thru of the contralateral leg. Therefore there is a synergistic action here and one cannot happen without the other.

    These are my thoughts and you can all try and slay me now if you like ;)

    All the best Dave
    Last edited: Jun 11, 2008
  12. David Smith

    David Smith Well-Known Member

    Here's some diagrams that might help. I left out some of the less relevant forces and inertial torques and accelerations so its not to busy.


  13. David Smith

    David Smith Well-Known Member


    'Oranges are not the only fruit' and GSC is not the only muscle group that can produce forward propulsion. The real physiological ankle is not mechanicaly fixed. Therefore if it is fixed it is muscle contraction that keeps the foot plantarflexed and non compliant against GRF that is trying to dorsiflex the foot. The plantarflexed foot causes the CoM to rise but the amputee can use hip moments to increase forward progress and can move the CoM forward of the foot by greater hip extension, which = higher moments for propulsion. This would have a similar effect to normal gait as described in my last post and with short slow stride may not look to different from normal.

    When they try to increase the gait velocity do you see major changes then? Can they run normally?

    Cheers Dave
  14. Dananberg

    Dananberg Active Member


    I can’t believe that we continue to argue about this concept. As you know all too well, I ABSOLUTELY agree that there is a push against the support surface that create reactive ground shear and causes the body to advance forward. Our disagreement involves the source of power for this thrust….not whether or not there is a thrusting taking place. I don’t mind debating….but do find misstatements of what I have said annoying and misguided.

    It is well established that the gluteals (primary hip extenders), for instance, fire at heel strike while the hip is flexing, and then by mid step, shut off as the hip extends through the end of single support phase. Hip extension, therefore, is caused by something other than the muscle groups that are capable of extending the hip joint. Peak thrust (ie, longitudinal ground shear) is reached at the end of single support phase. Thrust rapidly DECREASES as concentric contraction of the gastrox takes place in the terminal double support phase. Inman in “Human Walking” notes that when evaluating muscle contraction as the prime mover for gait, there is a 6% differential between when peak contraction is achieved and when peak thrust develops. Keeping these thoughts in mind, having the body use the swing limb to “destabilize” the CoM in the forward direction, so that body weight, acted upon by gravity, falls to the ground and drives the stance limb through the end of single support. This creates a very energy conserving model to describe the propulsion process. It also matches the timing of the peak in thrust with the end of single support phase and fits precisely with the concept that the stance limb as a “passive” rather than “active” thruster against the support surface.

    I also read Dave's comments regarding amputee gait and found them most interesting and well written. I would, however, ask the following question. If the body uses plantarflexion of the ankle to raise the CoM as you describe, why then does peak elevation of the CoM occur in the middle of single support (when the foot is flat on the ground), and not at the termination of toeoff (which is actually to low point of CoM elevation)? Doesn't it make more sense that the power supplied by the GSC thrusts the pre-swing limb into swing phase, rather than pushing the entire body forward while the opposite limb is striking the support surface and creating a rather hard braking moment?

    Efficient gait is all about conservation of energy. Having muscle power drive the limb pushing air seems far more effective than having those same muscles fire out of phase and push the ground.

  15. Howard:

    I was thinking the same thing.....why are we going around this stump again?? However, you did write earlier "You've got this all wrong. It not that the stance leg doesn't push...it's that the "push" is passive rather than active."

    I don't understand how you can say that the "push" is passive when the following are known research-supported facts of human walking:

    1. Kinematics and kinetics studies clearly show that the ankle joint undergoes concentric ankle joint plantarflexion during propulsion.

    2. The gastrocnemius and soleus muscles are firing during propulsion when the ankle joint is plantarflexing.

    In order to meet your criteria that the push is passive from the foot, then 1) the gastrocnemius/soleus should have no contractile activity during propulsion, which we know is not the case, and 2) the kinematic/kinetic studies would need to show that there is no concentric ankle joint plantarflexion occuring during propulsion, which we also know is not the case.

    Therefore, I also don't understand why we are still discussing this matter. It seems very clear to me that the push from the foot on the ground during the propulsive phase of human walking is active, not passive.:drinks
  16. Nick:

    No, I think we are beating the dust from the bones of a long-dead horse.;)

    I have never said that I think the swing limb is unimportant in walking gait. In fact, I think that my discussions with Howard over the years has given me much better appreciation of the importance of the swing phase limb. In fact, for those just now tuning into this long-standing discussion, let me say it again: the swing limb and its proper mechanical actions are vital to the normal kinetics and normal kinematics of walking gait.

    I agree that a normal below-knee (BK) amputee can function quite well during gait with a BK prosthesis. I would also agree in the specific case of a BK amputee with a BK prosthesis, that the prosthetic does push against the ground in a posterior direction in the second half of stance phase but that this is more of a passive push than in normal walking gait. In fact, the posterior push from a BK prosthetic foot has been clearly shown in gait studies.

    You must remember, though, that a BK amputee can still generate quite significant moments about the hip and knee joint which can actively generate propulsive force between the prosthetic foot and the ground. Does this mean that the BK amputee has an "active push"? I'll let you decide......depending on how you define "active push" versus "passive push".

    By the way, I do not consider a gait that lacks active ankle joint plantarflexion during propulsion to be "normal".....we call that type of gait either an apropulsive gait pattern or we consider that to be the normal way that a BK amputee will walk with a BK prosthesis. In addition, all of these examples of abnormal bipedal locomotor gait patterns are still called "walking", no matter how they are performed.
    Last edited: Jun 11, 2008
  17. Dananberg

    Dananberg Active Member


    I think the problem we are having is in the concept of what is the "propulsive phase". As single support phase ends, the reactive longitudinal ground shear peaks. The current podiatric definition of the propulsive phase is "heel off to toe off", but I find this erroneous. Once peak is reached, propulsion (ie, ground shear) rapidly DECREASES thru the end of toeoff!

    The real issue to me is that the double support phase is really not propulsive (the opposite side is actually braking during this same period), so the fact that the GSC is concentrically contracting during this phase does not denote that the bearing limb is actively propulsing the CoM. If is were, then ground shear would be increasing. Instead, it is entering pre-swing....and a phase that promotes toeoff of the swing limb...and not a push directly to the CoM which would have to occur while the other side is simultaneously braking. The active propulsive arguement is too wasteful to create efficient gait and energy conservation. Sorry you can't see this....

  18. Howard:

    The ground shear does not need to be increasing to be propulsive, it only has to be positive, which it is. Also, do you have any references to back up your claim that "The active propulsive arguement is too wasteful to create efficient gait and energy conservation."? Or is that just your opinion?

    Good to be beating the horse cadaver again with you.:deadhorse::drinks
  19. Dananberg

    Dananberg Active Member


    I know that as long as propulsion is positive, it is propulsive. But don't you think it a bit odd that this is shear force is rapidly decreasing at the time when propulsion, according to the direct muscle action arguement, should be peaking?

    As far as references for this statement, try Inman muscle firing studies in Human Walking. How does the hip joint extend when the gluteals are off? If one were to believe the model you represent, then hip joint extension must occur in the presence of hip extensor muscle activity....but it clearly does not. So...its not a very far leap of faith to assume that if the grand design shuts off the hip extensors during hip extension, that turning them on to create this same motion would be wasteful.

    As a final comment, the passive nature of the weight bearing limb occurs during the single support phase and coordinates with eccentric action of the leg/thigh musculature. Once double support begins, I agree that concentric contraction of the GSC occurs....but, with the knee and hip simultaneously and rapidly flexing as a condition for pre-swing, I find it very hard to believe that the CoM can be "propulsed" from two joints away via a plantarflexing ankle.

    I know how much stake you put into physical models. I just don't want to make a biologic structure fit engineering terms for the sake of simplicity. Understanding, even if it means departing from the comfort of our usual thought processes, is what my ultimate goal has become. To paraphrase Einstein, I don't care about how people think it works, I want to know how our creator thinks it works.

  20. It makes sense to me that the posteriorly-directed shear force from the foot on the ground is continually decreasing in magnitude in propulsion since the vertically-directed force from the foot is also decreasing at this time in gait due to the center of mass rotating further anterior, relative to the foot, as propulsion progresses.

    As far as a "direct muscle action argument", I don't think I ever said that concentric ankle joint plantarflexion was the only action that allowed walking to progress. There is obviously both passive and active factors that occur during walking with the active factors being more prominent during faster walking speeds than during slower walking speeds. The passive factors are just as important as the active factors, if not more important in some cases. Therefore, to make the statement: "the foot does not push the body forward actively", is plainly wrong. However, if you made the statement: "the active push from the foot is not the only factor that pushes the body forward since passive factors may be as important or even more important to the progression of gait in walking", then I would certainly agree with that.

    I was not speaking of hip extension when I asked you to provide references for your following statement: "The active propulsive arguement is too wasteful to create efficient gait and energy conservation."? I don't remember Inman making that statement in Human Walking, did he? Active propulsion has certainly been shown to be present, especially at faster walking speeds. In addition, the gastroc-soleus has been shown to be electrically active and, with the known electromechanical delay that occurs within the neuromuscular system of animals, one would expect the peak of electrical activity for the gastroc-soleus to be approximately 50 ms prior to the peak of ankle joint plantarflexion moment. This is what we see in the research that has been performed so far.

    I find no difficulty in understanding that force can be transmitted to the ground from the plantarflexing foot when the knee joint is flexing and hip joint is flexing. As long as the length from the metatarsophalangeal joints (MPJs) to the center of mass (CoM) that is gained from ankle joint plantarflexion increases a larger distance than the flexion of the knee and flexion of the hip shortens the distance from the MPJs to the CoM, then, effectively, the MPJ to CoM distance is lengthened.

    Just as a child on a swing needs very little active pushing force from the adult pushing the child in order for the child/swing to maintain constant swing amplitude and swing frequency, the pendulum mechanism of the preswing limb does not need a great amount of pushing force from the foot in order to maintain a constant velocity of walking. This is why walking is so efficient, the pendulum mechanisms are inherently passive in nature so that these pendulum mechanisms just need to be primed with each step by an small active push from the ankle joint plantarflexors in order to maintain constant walking velocities. Research seems to indicate that this active pushing effect from the ankle joint plantarflexors and foot becomes more important as walking speed increases.

    I find great comfort in mathematical models and engineering models since I know that the cars I drive, the buildings I work and live in, the tools I use, and the bridges I drive over have all been designed by using physical models that are based on engineering and mathematics principles. Even though I have never taken a course in engineering and never took a course in biomechanics before entering podiatry school, I was always very good in mathematics. Possibly because of my comfort with mathematics, I inherently understood, early on in my podiatry education, that if the mechanical properties of the foot and lower extremity could be quantified mathematically with the same modelling approaches that were already in common use in the cars, tools, machines, buildings and bridges that I was already familiar with, then the certainty of these mechanical properties of the foot and lower extremity would be on much more solid ground. I do prefer to be on solid ground when I am discussing topics with other professionals. As a result, I have never been too comfortable with "ideas and feelings" of clinicians who do not share my strong reliance on the principles of Newtonian mechanics and are therefore not firmly grounded in the bedrock of mathematics.

    “I was like a boy playing on the sea-shore, and diverting myself now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.” Sir Isaac Newton

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Thanks for the entertaining and enlightening discussion, Howard. As usual, I am enjoying it greatly.:drinks
    Last edited: Jun 12, 2008
  21. Stanley

    Stanley Well-Known Member


    After analyzing this with Eric by critically evaluating many excellent papers on the subject, :drinks at this thread:

    I think we came to the conclusion that energy is stored in the Achilles and other tendons, and the plantar fascia via eccentric contraction. When the COM advances anteriorly, the energy is released allowing for an active concentric propulsion.


  22. Stanley

    Stanley Well-Known Member


    After analyzing this with Eric by critically evaluating many excellent papers on the subject, :drinks at this thread:

    I think we came to the conclusion that energy is stored in the Achilles and other tendons, and the plantar fascia via eccentric contraction. When the COM advances anteriorly, the energy is released allowing for an active concentric propulsion.


  23. trophikas

    trophikas Active Member

    Dear all

    Your replies to this thread have been absolutley brilliant and the proceeding discussion is exactly what I was hoping to stimulate. To Kevin and howard, thanyou both so much for taking the time to 'thrash it out'. I have learnt far more from your respective thrust and counter parry (is that a lancing term?) than from learning your respective theories in isolation.

    Thanyou both for your valuable time and expertise

  24. stevewells

    stevewells Active Member

    To All,

    Do the ankle flexors fire to produce plantaflexion or to prevent dorsiflexion of the ankle? may be a questoin worth thinking about. Is the plantaflexion motion of the ankle seen whilst the foot is still weightbearing or as the toe lifts off (due to the sudden disappearance of the GRF)
  25. Dananberg

    Dananberg Active Member

    Pendulums in a coupled system (reciprocally swinging arms and legs) do supply energy to each other. With the energy storage (potential) and return (kinetic) that this, coupled with the spinal engine inpart on the entire body, gait is a highly efficient process. This is the real point I have been trying to make. A passive weight bearing limb periodically acted upon by short bursts of muscle action combine to make walking a near perpetual motion process.

    Certainly seems like we as close to agreement as we can get.

  26. For now....agreed.:drinks
  27. The gastrocnemius-soleus complex (GSC) produces an ankle joint plantarflexion moment during the latter half of stance phase. Any moment acting across a joint axis can cause either one of three conditions:

    1. Acceleration of motion in direction of moment.
    2. Deceleration of motion in direction opposite of moment.
    3. Stabilization of motion to a constant or zero velocity (i.e. rotational equilibrium).
    (Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989.)

    At the ankle joint, during late midstance phase, the ankle joint plantarflexion moment from contractile activity of the GSC decelerates the ankle joint dorsiflexion motion caused by the action of ground reaction force (GRF) acting on the plantar forefoot. This is called eccentric ankle joint dorsiflexion.

    At the ankle joint, during propulsion (after heel off), the ankle joint plantarflexion moment from contractile activity of the GSC accelerates ankle joint plantarflexion motion since the GSC-caused ankle joint plantarflexion moment is now greater in magnitude than the ankle joint dorsiflexion moment from the actions of GRF acting on the plantar forefoot. This is called concentric ankle joint plantarflexion.

    Therefore, in order to more fully understand the interplay between external forces acting on the foot (i.e. GRF) and the internal forces acting on and within the foot (e.g. from muscle contractile activity), one must understand the physics and biomechanics concepts of rotational equilibrium and joint moments.

    Here is an excellent book for those of you who want to increase your knowledge in this regard:

    Fundamental of Biomechanics: Equilibrium, Motion and Deformation, 2nd Edition
  28. David Smith

    David Smith Well-Known Member

    Dear Howard Danenberg

    you wrote

    This is essentially true however my explaination and diagrams were somewhat simplified to suit the format. It would take a book to explain in detail and precisely all the interactions of force and sacceleration of segment body masses.


    If you look at this diagram below:

    If as in 1) the swing leg was braking and causing an inertial force to act to pull the body forward then there must also be an opposite reaction force applied to the foot by horizontal GRF as shown.

    Force plate data indicates that horisontal reaction force in the saggitalk plane IE Fx is in fact positive in direction and is applied as shown. This would indicate that the net acceleration of the individual segment masses must be in the direction of +fx also and their net inertial force is in the direction -Fx (where +Fx = forward direction and is from left to right in the diagram.)


    Would this not imply that there is another force acting to accelerate the CoM which is the muscles of the leg and probably the force of GSC contraction and Achilles elastic energy.

    All the best Dave
  29. David Smith

    David Smith Well-Known Member



    At heel strike there is a large braking force -Fx, as mid stance approaches this -Fx drops to zero. Also the rear leg is at late propusion - toe off and + Fx is also small. The CoM has reached its peak height as the leg swings thru. The push from the rear foot assited the CoM to reach its peak. As the swingleg comes forward gravity reverses the net acceleration and it accelerates untill the braking foot starts to slow it at this point again the rear foot assist the CoM to its peak. It would be impossible to move forward without this extra push from the rear foot / leg. If it was there would be perpetual motion. This is only apparently able to exist when the walk is down hill and gravity can take the place of the rearleg push.

    As I explained before if the placement of the front braking foot is delayed then the rearfoot is allowed to push for longer and the CoM terminal velocity will be faster. Even though the Ankle is plantarflexing and increasing the height of the CoM in terms of the local refrence frame in terms of the global referenve frame the Com is at about the same height or lower sinve it is also experiencing an angular acceleration toward the ground. When the brakning foot comes into play this allows the CoM to climb high again due to the force couple between -HRFx and + IFx. The push from GSC add enough inertia to enable CoM progression.

    Thats how I interpret it anyway.

    Cheers Dave
  30. efuller

    efuller MVP

    It certainly be a combination of both. However, if you want efficiency and speed then you should have some muscular push. We can look at whole body energy and swing leg energy. The whole body energy does not change much as it rises and falls and as it accellerates and declerates with walking.

    Energy has to be added to the swing leg to move it forward. There is a tradeoff between hip pull and ankle push. The more hip pull the less ankle push is needed. The energy, to move the swing leg, has to come from something touching it. As the trunk pulls the trailing leg forward there is an equal and opposite pull from the leg on the trunk, pulling the trunk backward. So, if all the energy for moving the pre-swing leg into swing comes from the trunk then gait will be slowed. Yes, this energy can be recovered as the swing leg slows before heel contact, but gait is slowed at the start of swing and at heel strike. If there is ankle push off gait can be faster because there is less slowing of the trunk at the initiation of swing.

    Amputee's with prosthetic feet don't have ankle push, therefore they have to use all hip pull. The exception is energy storing prostheses that are tuned to release their energy before "toe off". There is more than one way to swing a leg.


  31. David Smith

    David Smith Well-Known Member





    Sort of Agree

    Doesn't it come from gravity? assuming no friction in the hip joint and only gravity accelerating the leg toward the ground then there are no moments at the hip to be balanced by the trunk and so no energy is required from the trunk to balance the leg swing until it in on the upswing and then preheel strike the leg drops to the ground by gravity again.

    , Eric I don't understand this statement. If the swing leg is slowing then there is inertial force opposite to the acceleration ie in the direction of forward progression of the walking. This would require an opposite force for equilibrium otherwise the leg could not slow down. This comes from the friction between foot and ground on the stance foot. Or from the inertial force of the body CoM in the opposite direction caused by the body mass accelerating in the direction of walking. Therefore energy is added by the pushing leg and cannot be subtracted until the front leg strikes ground.

    Agreed, because the strike foot is accelerating the earth (and therefore accelerating the body in the opposite direction to forward progression) and the push foot has stopped pushing ie stopped applying force to accelerate the CoM. It would be pointless and a waste of energy if the front foot was decelerating the body while the rearfoot was trying to accelerat it.

    True, without some extra energy, which can come from the ankle plantarflexors, gait will just stop

    Cheers Dave
  32. efuller

    efuller MVP

    Position at initiation of swing. Trailing leg is up in air and behind trunk. Gravity pulls down and hip pulls up. (Leg is not accelerating that fast downward.) There is a moment applied to the leg by this force couple. If the trunk were weightless the leg would pivot in the same place and would not accelerate forward.

    Another way of looking at it. There is a forward acceleration of the center of mass of the leg. From Newton's 2nd law there must have been a force acting upon it in the horizontal direction. The only thing that could apply the force is the hip.

    3rd way of looking at it. Hang a pendulum from a flimsy support. When the pendulum swings it will move the support just as the swing leg will move the hip if there is no ankle push.

    Be careful with the labeling of your free body diagrams. Energy is force times time. The swing leg's horizontal velocity is slowed by a force from the hip pulling the swing leg backward. The equal and opposite reaction of the leg pulling on the hip will accelerate the trunk forward(at the end of swing phase) . Think back to the flimsy support and the pendulum. The pendulum pulls the support in both directions. The direction of the pull depends upon where the pendulum is during the swing.

    You're right it is pointless, but it happens. If you look at force horizontal ground reaction force curves at the beginning of gait the force from the ground applied to the foot at heel contact is backward and at the same time the trailing leg has a forward force from the ground applied to it.



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