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To continue our discussion on whether the stance phase limb pushes to allow normal walking to occur (and how this may be affected by a below knee prosthesis), here is some research showing how similar the braking and propulsive shearing forces are during walking gait between the normal and prosthetic foot (Zmitrewicz RJ, Neptune RR, Walden JG, Rogers WE, Bosker GW: The effect of foot and ankle prosthetic components on braking and propulsive impulses during transtibial amputee gait. Arch Phys Med Rehabil, 87:1334-1339, 2006).

By the way, without friction between the weightbearing surface and foot, normal walking could not take place since these shearing forces could also not occur.

Attached Files

Effect of Foot and Ankle Prosthetic Components.pdf (218.9 KB, 51 views)

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Sincerely,

Kevin

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Kevin A. Kirby, DPM
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California School of Podiatric Medicine at Samuel Merritt College

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I seem to remember that one of the reasons why it was thought the swing leg pulls the trunk forward, is that during mid stance the triceps surae, particularly soleus, act eccentrically to decelerate the tibia and absorb energy, so it was believed that the stance leg could not be contributing to forward propulsion at this time. It is only after heel lift when the knee is flexed that the triceps surae contract concentrically to propel the leg into swing, probably to late to propel the body forward, as at this time the leg is no longer a rigid lever, the analogy used was this would be like pushing rope.


I recall reading some bio mechanical research from the U SA using dynamical simulations, that indicated when soleus decelerates the tibia, it causes a knee extension moment that also accelerates the hip into extension in mid stance, propelling the trunk upward, and forward, in this way energy is transferred to the trunk by the eccentric action of the soleus. This makes a lot of sense to me.


Most active amputees seem to prefer some kind of energy return and storage foot for various reasons that have been put forward, such as a decreased reliance on the hip flexors to power the leg into swing. I believe that an ESAR foot would replicate the action of the soleus muscle in mid stance, by allowing more forward progression of the tibia and its timely deceleration, where as a SACH foot may not. This may explain some of the preference for this type of foot.

Yes, I've heard that explanation many times. Basically it is all wrong. First of all, answer me this question: how can the swing leg pull the trunk forward in walking if it can't also can't pull the body forward on a frictionless surface or in mid-air? Do the laws of physics suddenly change as the body touches a surface with friction? Second, even though the knee is flexing after heel off, the ankle is plantarflexing also so that the distance from the plantar forefoot to the hip is not shortening but probably lengthening. Third, the pushing rope analogy is very weak and innaccurate since the lower extremity is more accurately modelled as three rigid segments, the thigh, leg and foot with two articulations, the knee and ankle. How is this three rigid-segment, two articulation system anything like a rope?? Fourth, the triceps surae is active throughout the latter half of stance phase, not just during propulsion.

Root et al talked about the soleus decelarating the tibia back in 1977 (Root ML, Orien WP, Weed JH: Normal and Abnormal Function of the Foot. Clinical Biomechanics Corp., Los Angeles, CA, 1977.) Rick Neptune has done a lot of work in this area and you can check out his papers at his website. He has clearly demonstrated that "forward propulsion (defined as horizontal acceleration of the center of mass) in walking is provided by the hip and knee extensors in early stance and the plantar flexors in late stance (Neptune, R.R., Zajac, F.E., Kautz, S.A., 2004a. Muscle forcere distributes segmental power for body progression during walking. Gait Posture 19, 194–205.)

The dynamics of the ankle joint plantarflexors is more complicated than whether the push the body forward or not. Here is another one of Rick Neptune's papers that shows that your assertion that the stance phase limb does not push the body forward is false and should be forgotten along with the other popular podiatric biomechanics myths:

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Kevin

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Kevin A. Kirby, DPM
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Kevin;

You state that if the swing limb cannot pull the body foward when walking on ice that this proves that the swing limb cannot pull the body forward on regular ground.

The forward swinging limb is not effective if the stance limb is not stable. For it to be stable in your ice exmple, the stance limb would need crampons.

I do not think your example adds anything at all to this discussion, so please quit using it. Coefficients of friction add a whole new dimension to newtonian phsyics discussions. Lets keep it simple. Please just limit your examples to stuff we can all walk on effectively? ;-)

The swing limb does assist gravity in pulling the body forward, when the stance limb is stable enough to resist the backward forces driving the body forward.

The stance limb also propels the swing limb into stance somewhat. I think that is not possible for the propulsion of the achilles complex to do this alone for very long.

Finally, just because there is activity in a muscle during most aspects of stance does not mean that it is active the way you claim. Peter's statements the timing of eccentric and concentric accelerations are very accurate. The achilles complex is firing primarily to resist backward momentum from the forward swinging limb. I will not argue that it pushes off somewhat. But, that complex alone is not enough to get us walking, let alone running all the time.
Bruce

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Bruce E. Williams, D.P.M.
Breakthrough Podiatry

I have changed my user name to peter96 from peter64@ hotkey because of a lost password.



Kevin and Bruce sorry if i have not made myself clear, i was just pointing out the rationale behind the theory rather than trying to support it. I agree that the stance leg does propel the body forward during mid and late stance.



Good point i had not considered this.


Thank you for the link Kevin there are some interesting papers.

Actually, Bruce, I think my examples of a person on ice or a person at the middle of a trampoline jump trying to pull them themselves forward by swinging their leg forward are excellent examples of showing how the swing leg, by itself, does not, and cannot pull the body forward. Without understanding that one needs a frictional force between the foot and ground in order to push the body forward is not complicated, this is basic high school level physics. I don't understand why, when trying to comprehend a complex task like human walking, that we don't try to break down the requirements for walking into easy to understand concepts.

I agree that the swing limb and the stance limbs work in unison to drive the ground reaction force patterns seen between the lower limb and ground during walking. However, the notion that the swing phase limb "pulls the body forward" is misleading and innaccurate since without the stance phase limb pushing backward on the ground, the swing phase limb can pull all it wants and the body's center of mass will not accelerate forward as a result.

So, why would anyone still teach that the "swing limb pulls the body forward, the stance phase limb does not push the body forward"?

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Kevin

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Adjunct Associate Professor
Department of Applied Biomechanics
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In addition to Kevin's suggested reading I would like to add the work of David Winter on joint power. Joint power is used to answer where increase in the energy of the body comes from. Body energy can be potential energy (increased height) or kinetic energy (increased velocity). The muscles convert chemical energy into mechanical (kinetic or potential) energy. Joint power is calculated using inverse dynamics. Joint power is equal to joint moment x joint angular velocity. When the product is positive energy is added and when the product is negative mechanical energy is lost to heat in the muscles. So, in your example above, during stance phase, the ankle joint motion is dorsiflexion at a time when the ankle moment is plantar flexion. In this situation energy is lost. Later in the step when there is ankle joint plantar flexion and there is a plantar flexion moment energy is added to the soon to be swing leg. So, when there is positive power the rope is being pushed. The rope is stiff enough that it can be pushed.

In gait, the swing leg has to gain some energy so that it can pass the trunk. It can gain energy from hip pulling power or ankle push power. (The swing leg can only gain energy from either end. This power has been measured, so this question of does the swing leg pull the body forward has been answered. It should be noted that the total power added to the swing limb can be some hip push and some hip pull and the proportions can vary with each step. You can choose to add more ankle push when you walk. However, more ankle push will put more strain on the foot.

In looking at the power curves, over time, when there is hip pulll acting on the swing leg, there is a power flow from the trunk to the soon to be swing leg. That is, at the beginning of swing, when the hip (trunk) pulls the swing leg forward, there is an equal and opposite reaction of the leg pulling the trunk backward. At the end of swing phase, the leg is slowed and energy is transferred from the swing leg back to the trunk. So, in the first half of swing, if hip pull is used, the leg pulls the trunk backward. In the second half of swing phase the leg pulls the trunk forward. So the net energy gain of the trunk can be zero from the swing leg. It will be a net of zero if there is no ankle push. When there is ankle push there will be a net energy gain of the trunk from the swing leg.

So, when people try to teach the concept of the swing leg pulling the body forward they are ignoring where the swing leg gets its energy. If you want to understand gait you have to look at the whole picture and not just the second half of swing phase.

Regards,

Eric Fuller

Kevin;

my problem with your ice or outer space examples is that it proves that neither of our ideas on propulsion would be accurate. On ice and in outerspace without a proper stable pushoff point neither the foot at toe-off or the active swing limb will move the person forward. As you state, ground friction is very important here.

I find that the notion that we propulse forward purely from the foot as equally misleading as you feel about the swing limb. The function of the propulsive musculature of the lower extremity is primarily for stabilization from my point of view. In light of that, it makes no sense to me that we could launch ourselves forward from step to step constantly due to the firing of the Achilles complex. It is not strong enough to do this for any length of time. Does it fire to start the swing limb? To that I will definitely agree.

Finally you last point, "why would anyone still teach that the "swing limb pulls the body forward, the stance phase limb does not push the body forward"?
Because it makes no sense in the reverse. Not in my mind.

Toe off, when it infrequently happens in patients, starts things off. The stance limb will then store and return energy for elevation of the CoM to allow the swing limb to clear provided the upper leg adn back muscles continue to provide stabilization of the process. At this point gravity takes over and the swing limb finishes and the process repeats itself.

I just do not see how your statement can be any more valid than mine. There is a cross over point. I imagine that only time will tell whether one process is more necessary than the other. You feel your way and I feel mine.

The one difference I see is that I, like Howard and others, have in-shoe data that shows me F/T changes that correlate to greater step length when the orthotic is configured to eliminate blockages of pivoting motion in the stance limb.

The swing limb timing does not change in general, only the stance limb timing reprsentative to step length which Howard has correlated to increased hip extension I think. To me this just reinforces that gravity is working so much more in stance and swing because when there is little to no stoppage of forward foot pivot motion, the foot is able to transition from toe-off into swing with a lengthened sep length.

If the foot truly was propulsed by the achilles complex and other muscles then failure of pivoting thru stance would not be a problem in my mind.


Bruce
PS, maybe we could debate this at PFOLA some time? Might make for an interesting time. :)

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Bruce E. Williams, D.P.M.
Breakthrough Podiatry

We don't have to launch ourselves forward every step. There is momentum. Winter has shown that there is a trade off of potential and kinetic energy so not much has to be added. Starting at the high point on the stance limb the potential energy (height) is highest and the kinetic energy (velocity) is lowest. Then as the body falls forward height is lost and the the velocity increases until the lowest point (double support) is reached. Then the kinetic energy is converted back to potential energy as velocity decreases as the body climbs up on top of the new stance leg. So the whole body energy does not change much.

However, the swing leg energy does change. It has to be accelerate horizontally, which is much easier than lifting it. Sagittal plane proponents have said that the gastroc soleus is not capable of lifting the leg for thousands of steps in a day. It doe not have to be lifted, it just has to be swung. It's like the difference between pushing someone sitting on a swing and lifting a person sitting on a swing. It does not take that much force to make the leg swing forward.


I am not saying that you don't have data. What I am saying is that I disagree with your interpretation of the data. You have to separate the observation from the explanation. You may have made the observation that certain force/time data corelates with happier patients. However, that does not mean the "blockade" explanation correctly explains what occurs.


Count me in.

Regards,

Eric

Bruce:

No, what my examples show is that the often lectured-upon statement that the swing leg can "pull the body forward" by itself, without the pushing action of a stance phase limb, is false. A person can push themselves forward with just the stance phase limb. A person with only one leg can hop on one foot, moving forward with the pushing action of that foot only. Is the one legged hopping man pulling himself forward by the swing leg also?? No, he is pushing himself forward with the stance phase foot.

A person in outer space can not, no matter how hard they try, swing their lower extremity forward to move their body forward. Since the swing phase limb during normal walking is swinging in space, just like the person trying to move their body forward by swinging their limb forward in outer space, why would one think that this swinging action would now work to pull the body forward in a normal weightbearing environment?

My problem is not with the idea that the actions of the swing phase limb are important. I agree that the swing phase limb is important. However, to continually lecture that the "swing limb pulls the body forward, the stance phase limb does not push the body forward" is what I have a big problem with because it is false, misleading and does not acknowledge the considerable research data that proves the stance phase limb does push the body forward during the latter phases of stance phase of walking.

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Kevin

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Kevin A. Kirby, DPM
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I'm not sure I understand why there is even a debate, Bruce is correct. A diver can can his/her position in space after leaving the board by altering body position. The same does for and ice skater....why do the rotate faster with arms close than wide. Finally, try sprinting withour having both off-side arms and legs crossing the midline. Angular momentum crated by the non-weighted body segment is vital to linear motion. Without it, the person would move ina circle. Simple physics writen in every bioechanics book that does not need psuedoanalysis that question the laws of pysics. End of conversation.

Kevin Miller

Great quote, Kevin. Keep up the good work.

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Kevin

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In the instance of the diver ,and the skater , they can rotate their body in space, but unless they have velocity in a horizontal direction, when they leave the ground i cannot see how they can move there center of mass in a horizontal direction. Another example is a long jumper who seemingly writhes their body around during their time in the air, to advance their body forward, i suspect that they may be doing this to advance their feet, and head ,and arms forward in a jack knife position ,so there feet strike the sand as far away as possible from the take off board , the forward position of their arms and head stops them from falling back into the sand.

Peter:

You are absolutely correct. Really, this is high school physics, not rocket science. The path of the center of mass (CoM) of an individual can not be altered while the body is in flight unless affected by some other external force other than gravitational acceleration or air/wind frictional forces. Altering body position while in flight does not alter the path of the CoM, it simply alters the relative positions of the body segments relative to the CoM.

In addition, an ice skater rotating within the transverse plane can choose to decrease their moment of inertia by bringing in their arms close to their body which will accelerate their rotational motion, due to conservation of angular momentum. In addition, sprinting or running and moving arms back and forth during these activites is mechanically necessary to counterbalance the angular accelerations of the legs that would otherwise tend to rotate the torso and head within the transverse plane with each lower extremity acceleration. Moving the upper extremities in the opposite directions from the lower extremities are necessary during walking, running and sprinting to keep the trunk and head pointed forward during these activities.

Now, the big question becomes....what do all of the above posted by Kevin Milller have to do with whether a leg during swing can "pull the body forward"?? Absolutely nothing! My original points still apply.

The swing leg, can not by itself, "pull the body forward" during walking as has been lectured on repeatedly by many individuals, within the little world of podiatry, over the past decade or so. To have a leg swing in space "pull the body forward" is a mechanical impossibility. It is the stance phase limb that exerts the necessary reaction force with the ground that accelerates the CoM forward during walking, running or sprinting. Continuation of conversation.

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Kevin

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Nicee syte Kevni

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Cymun: Wheel bee laffing bout dis ohfer sum beersnext weak!

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Kevin

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So using the same concepts, would you say that arm flexion, hip flexion, and/or back extension are not important in jumping?


Stanley

Jumping to conclusions that are not valid....?

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Kevin

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I see you jumped right in to answer that.
Now to throw something else out. Why do pitchers lift their lead leg up during the windup?
Don't throw another curve.

Stanley

Stanley,

The short answer is help move their whole weight toward the plate.

Lifting the leg up high, when the center of mass is on the home plate side of the leg still on the ground will cause an acceleration of the center of mass toward home plate because of the force couple of gravity and ground reaction force. The longer the time the foot is off of the ground the longer the period of time the body will accelerate toward home plate.

Stanley, the question was asked like you had an answer in mind. Did you have the same answer?

Regards,

Eric

Kevin M.,

The problem is that you don't understand why there is a debate. I set out a nice long explanation of my position above. Then, you just repeat the assertion that your position is correct. If you want people to accept your position address what others have said.

If you have ever done a sit drop on a trampoline facing one direction, bounced up and turned around in mid air and did a sit drop facing the opposite direction you would understand the physics of body rotation. What you have to do is rotate the bottom half of your body in one direction and the top half of your body in the other direction. Thus, there is no net change in angular acceleration, yet the body faces the opposite direction. The same phenomenon can be seen by holding a cat with paws up and then dropping it. The cat will land on its feet by twisting its front and back halves in opposite directions.

We are not debating the laws of physics, we are debating the interpretation and application of those laws.

Regards,

Eric Fuller

Eric:

Thanks for your postings. I can always rely on you, with your sound biomechanics and physics reasoning, to clear up the murky waters that are often created by some of the contributors to Podiatry Arena. See you in San Diego?

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Kevin

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Yep, I'll be there

Eric,

My answer is the same.
Does the same principle apply in walking?

Regards,

Stanley

There is no doubt that KK and Eric are correct on several things they say. None what so ever. And Simon did clarify exactly what I was saying about the skater, conservation of momentum, but one need only read a few pages of Gracovetsky to see the same reasoning applies to the opposition of arm and leg motion during gait and especially running, with only one addition....the occilation of the body in the gravitational field. If there is an asymetry of ground force reaction...he referes to it as an impulse that is filtered and shaped by the lower extremity to drive spinal function.... then the legs and arms must change their movement in space to create a change in momentum to keep the center of mass moving in the intended direction. Also, consider that with the body split into bilateral symetry, the sum of the ROM of the joints on one side must equal the sum on the other or you never move in a straight line without constant corrrection. Now, since KK said it best, I am talking 8th grade physical science here, it seem that the issue may be that we are not including all of the components I just mentioned. I hope I have made myself clear. It is often difficult to explain things in such short blurbs.

One last point...not really needed, but....like KK I was a college runner. I was a half-miler, 1/4 miler at Auburn. We spent HOURS working on form. I wonder why?

regards,
Kevin M

Colleagues,

Since it seems like my earlier comments on swing phase have been misintrepretated, and I would like to clarify.

The comment that swing phase pulls the body forward can certainly be attributed to me. However, it is the effect of the swing limb ON THE STANCE LIMB, that causes forward progression. There is no doubt that the stance sides pushes the ground, but it is the power source for this "push" that is in question.

Neptune is quite clear in his article that it is not until late single support and then pre-swing (double support) that the triceps surae begin concentric contraction. Since the reactive ground shear curves show positive propulsion by midstep, something other than calf contraction must be causing it. Since the gluts (hip extension) shut off immediately after heel strike, what is left to produce sufficent power is the motion of the swing limb and its effect on the CoM and ultimately the trailing limb.

Hope my position on this is now clear.

Best,
Howard

What Howard said was precicley what I was trying to get at through the Gracovetsky comment. Both happen. I have a very interesting antropological article here that is a study of human gait and running. Over 70 populations were looked at. In the "civilized" west, we tend to heel strike when running - except for world class runners. I wa taught never to heel strike and ran 4 second faster in the 1/4 mile. That is a huge difference. At any rate, this article speaks of primitive groups in africa, etc., and notes that they toe strike at all speeds. What is the diffference? I'm sure KK can quote the article off the top of his head, but it has been shown that running with a heel strike causes a breaking action, as it should. The COM is behind the contact point. In toe running, howerver, the body is over the center of mass and push off happens much earlier. Is it possible that we are collecting errouneus data when we measure heel strikers? I can't see how anyone who has studied kinematics can question the reason for opposite arm and leg swing, but the heel strike issue could throw a kink in the thought process.

regards,
KM

Short answer, yes.

Long answer: When the center of mass is not directly over the center of pressure there will be a force couple caused by ground reaction force and gravity that will tend to rotate the body.

So, at the initiation of single support, just after toe off, the center of mass is behind the center of pressure and the body will be accelerated backward. (It slows down, but still has a positive forward velocity). Later, when the center of mass is in front of the stance leg the opposite occurs.

The problem we seem to be having in this debate (not you, Stanley) is that people are only focusing on the period of gait after center of mass is anterior to the stance leg. If only part of the gait is examined, then one cannot see the whole picture. Unless there is push off against the ground, from ankle plantar flexion, then the only way to move the swing leg forward is to pull the trunk backward. For every action there is an equal and opposite reaction.

Regards,

Eric

Howard:

Welcome to Podiatry Arena. I knew if I hammered hard enough on Bruce that you would probably come onto Podiatry Arena to defend your position. Good to have you on board and I miss hearing your views on these subjects. Will we be seeing you at the PFOLA meeting this week in San Diego??

Hope all is well with you and your family.

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Sincerely,

Kevin

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Hi Howard, welcom aboard.

The mechanism of the position of the center of mass of the whole body to the center of pressure on the ground can explain the forward push from the ground on the foot (or the rearward push from the foot on the ground depending on your frame of reference) in the absence of muscular contraction. When the center of mass is anterior to the center of pressure (after middle of stance phase) a force couple is created that will cause the whole body to rotate forward. If this were on a frictionless surface the foot would move backward as the head moves forward. However, we normally walk on surfaces that have friction and the tendency to rotate will cause the propulsive push seen.

It's not just the swing limb that is causing the rearward force from the foot on the ground. It is the position of the entire body, of which the swing limb is a part. I would predict that you would see this rearward force in a one legged individual walking with crutches or hopping.

Yes the position of the swing limb is important in determining the position of the whole body center of mass, but there is not anything special beyond its position that adds to shear force on the ground.

Now, to get a little deeper we can look at what moves the swing leg. What I have been saying is totaly consistant with Neptunes work that Howard referred to. (Concentric contraction of the gastroc-soleus does not happen till later in double suport.) The concept of joint power is really helpful here in understanding what is going on. Joint power is joint moment x joint angular velocity. When the moment is in the same direction power is positive and energy is added. So when there is concentric contraction, energy (either potential [height] or kinetic [velocity] ) is added to the soon to be swing leg. If there is no angular velocity then there is no energy added from ankle plantar flexors.

So, you can have either some energy or no energy added to the soon to be swing leg. If no energy is added to the swing leg from the ankle, then it all must come from somewhere. Energy is added because it goes from near zero velocity to catch up and pass the body. The only other location energy can come from is the trunk (including the contralateral leg) As the trailing swing leg gains energy from the trunk, the trunk looses energy to the swing leg. Yes, it will gain this energy back at the end of swing. But, the point is where does the leg get the energy that it gives back at the end of swing.

Cheers,

Eric Fuller