Does the Stance Leg Push or Does the Swing Leg Pull?

We have to carefully define system. If the system is the whole body then there is no net gain of energy from the swing leg. If the system is the trunk then the trunk looses energy to the swing leg in the first half of swing and then gains it back on the second half of swing.

Kevin's point was that at end of stance phase, when there is ankle plantarflexion the center of mass is at its lowest point before it climbs upward over the new single limb stance limb. So, ankle push, if it occurs, provides energy to the swing leg to lessen the backward pull of new swing leg. So, it indirectly helps in raising the center of mass. Directly, at the time the ankle plantar plantar flexion occurs, the center of mass is not raised.

That's accurate. The first half of swing the trunk looses energy to the swing leg and in the second half of swing the energy is gained back. However, the trunk looses less energy if there is ankle push to replace hip pull.

I think we need to look at the concept of "powering gait". Once gait is started, not much power is needed. The main task is keeping the body upright, which takes considerable effort. From, Newton's first law. Things will stay in motion unless acted upon by an external force.

We are basing our understanding on physiology, neurology and engineering principles. One could infer from your above statement that you are implying that you do not need engineering (or physics) to explain motion. Do you agree that you cannot just look at motion and EMG recordings to explain motion?

The reason that the body rises to its maximum height at the middle of single leg stance can be explained by the trade off between potential and kinetic energy. From the point of maximum height (middle of single limb stance) the body falls forward gaining kinetic energy and then reaches maximum velocity at its lowest point (double support). Then kinetic energy is lost to as the body climbs back up to the middle of single support. One could say this is how gait is powered.

The longer stride you take the more velocity change you would have. However, less energy is lost to the falling backward if there is some ankle push. Try walking with really long stides without using some ankle push. This is why a "propulsive" gait is faster. You can take a longer stride and have a faster swing leg with ankle joint push.

Regards,

Eric

 

Hi Stanley,

There's a difference between known sources of error and invalidation of the concept. If known sources of error or small you can accept the data with those known limits.

The co contraction issue becomes a problem with some questions. That was the interesting part of the Neptune paper. The Neptune paper was trying to sort out the contribution of individual muscles with modeling.

Inverse dynamics gives a net joint moment. So, if there is contraction of the hamstrings at the same time there contraction of the quads, the net joint moment will tell us which group is producing more moment.

A very short question with many possible answers.

The vertical and horizontal components are independent of each other so there will not be a constant proportion.

The vertical component of ground reaction force is determined by gravity and vertical accelerations. That's why you see the typical double humped curve of force with single limb stance. You have to remember that acceleration is out of phase with position and velocity. After toe off of the trailing leg, the vertical vector rises above body weight so the center of mass is accelerating upward. The upward acceleration slows as the body reaches its highest point and ground reaction force dips below body weight. And so forth

The horizontal component is going to be determined by the amount of push off. There will be push off from body lean and from muscular activity which is variable.

Some of the time the question that you hope to answer will not be addressed with inverse dynamics and sometimes they will.

If you want to know if the stance leg pushes, or the swing leg pulls then then you can use inverse dynamics to answer the question by looking at power output from the various joints. When there is positive ankle power, before toe off, the muscles of the ankle joint are providing push.

Regards,

Eric

 

Great discussion.

It would seem to be quite important that we look at the reactive longitudinal ground shear graph to understand when braking becomes accelerating, and when acceleration and braking peak.

At the precise time when single support becomes double support, braking occurs on the contacting limb while the side in pre-swing rapid decreases its rearward thrust. Therefore, the peak in ground shear is reached at the termination of single support. The peak in braking occurs just prior to the onset of single support phase. Therefore, if we are to believe that muscle action on the stance limb is creating foot/ankle motions which essentially push the body to create forward movement, it would stand to reason that the peaks in ground shear would coordinate with the peak in muscular contraction. The truth is THEY DO NOT! Inman notes as much in his text "Human Walking".

The power for motion is a very complex yet highly efficient interaction between the entire body and the foot against the support surface. Without this thrust of the foot against the ground...walking would not occur. What is of interest is how this "push" is created, and whether the power to create it is related to an active, passive, or some combined process. This power, however we come to understand it, must involve the most efficient method by which the CoM is maneuvered relative to the stance limb. The centrifical force caused by the action of the swing limb, has an effect on the CoM. Since this becomes positive in midstep, it coordinates precisely with the change in the long. shear graph from brake to thrust. The effects of triceps concentric contraction is to push the swing limb into motion....not to actively propulse the body forward. This is not to say that active pushing is not possible, but in efficient gait....it is not desireable.



Howard

 

Howard:

While I agree with your statement "Without this thrust of the foot against the ground...walking would not occur", I cannot agree with your statement "The effects of triceps concentric contraction is to push the swing limb into motion....not to actively propulse the body forward."

Regardless of when the peak anterior shear force from ground reaction force (GRF) occurs, the fact remains that throughout the concentric ankle joint plantarflexion stage of walking gait (i.e. propulsion), there remains a definite anterior shear force from the ground onto the plantar foot, acting to push the body forward. In addition, to say that active pushing of the foot on the ground is not desirable, does not make sense when you stated earlier that "Without this thrust of the foot against the ground...walking would not occur."

Active pushing of the foot on the ground is essential for normal walking, whether it is slow or fast walking. In addition, this active pushing of the foot on the ground does push the body forward, since the swing limb, by itself, can not pull the body forward without the anterior shear forces from the ground pushing on the plantar foot during the latter half of the stance phase of walking gait.

Great discussion....just like old times.:drinks

 

Let’s define system as the entire body

Eric, where does the potential energy go when you lift the leg using the gastrocnemius at the beginning of swing? If you can burn calories to lift a leg, then how can you say there is no net gain of energy?
Also, if you are using the hip flexors at the second half of swing when the stance leg is on the ground resisting the action of the swing leg, you are again burning calories, this time to provide kinetic energy to the swing leg. This is like having two people pulling a swing-one in the front (the hip flexors) and one in the back (the gastrocnemius).

Kevin said “In regards to the center of mass lowering during propulsion, I like to think that ankle joint plantarflexion helps smooth or decelerate the fall of the center of mass during propulsion.” So you are disagreeing with Kevin also.

Eric, as I said before, the energy to maintain gait is equal to walking on a 4° grade. The point about not requiring much energy does not support either argument. The energy required for gait is used to accelerate the center of mass upwards.

Eric, if a theory is to hold water, it has to make sense whether you look at it from an engineering basis, a neurologic basis or a physiologic basis. After reading the Neptune article, I realized that what I said did not make complete sense from an engineering basis, I modified what I originally said, and used the concept of the energy is stored in the fascia around the muscle cells, the ligaments, and tendons. On the other hand, your theory makes no sense on a neurologic or muscle physiologic basis, and you have not modified your theory to make sense using these as a basis.

I would like to think that the powering of gait comes from the muscles that raise the center of mass. Energy cannot be created or destroyed, and gait is not perpetual motion, so powering the gait being explained by the trade off between potential and kinetic energy seems to fall short.

You are describing an inefficient gait. I agree that in this case you may choose to use the triceps surae in a way that it was not intended.

Regards,

Stanley

 

Kevin,

So you agree that the force of gravity is causing the leg to swing forward. I wonder if you also agree that lifting the leg requires energy to produce the potential energy. Since energy cannot be created or destroyed, and you feel that the swinging of the hip is unable to put energy into gait, (as the negative effect on the inverted pendulum when the center of mass is posterior to the center of gravity is equal to the positive effect on the inverted pendulum when the center of mass is anterior to the center of gravity), then where does this energy go?

Regards,

Stanley

 

Let's examine "Therefore, if we are to believe that muscle action on the stance limb is creating foot/ankle motions which essentially push the body to create forward movement, it would stand to reason that the peaks in ground shear would coordinate with the peak in muscular contraction. "

We both agree that at the end of swing phase energy is transferred from the swing leg to the body. The muscular contraction that pushes the swing leg occurs, if it happens, at the initiation of swing. That impulse, right before toe off, adds energy to the swing leg. The stored energy in the swing leg is later added to the trunk at the end of swing. This is what is missing in the "stand to reason" argument. The peak muslce push does not have to come at the same time as peak ground reaction shear.

The most efficient mechanism by which the center of mass is moved relative to the stance limb is momentum of the body. When the center of mass of the whole body is behind the stance leg the velocity of the body is slowed. When the center of mass is anterior to the stance leg the velocity of the body is increased. The swing leg is just part of the center of mass. So the goal of rapid gait is to slow the body the least. Ankle push, at toe off lessens the rearward pull of the swing leg applied to the body. Therefore, for a more rapid gait, ankle push is desireable.

Howard, Kevin, Is it now 10 years that we've been having this discussion. I've always enjoyed it and have learned much from it.

regards,
Eric

 

Hi Eric,

So how much error is there as a result of co-contraction?

Eric, Co-contraction has to do with antagonists contracting at the same time. Neptune was looking at the Gastroc and Soleus. At the ankle the Gastroc and Soleus do not qualify as antagonists.

Wouldn’t the vertical vector be a function of the cosine of the angle formed by the top of the talus, the first metatarsal and the ground?

I would like to read the Winter article, so I can see what he says.

Regards,

Stanley

 

Eric, Kevin and all,

The best part of these discussions is how much we all learn.

I wrote in relation to ankle push off "but in efficient gait....it is not desireable." And you replied "that is rapid gait, ankle push of is desireable".

I agree...but these are two different situations. I am talking about efficiency...and as such, the most refined method of propulsion is required.

The other part of this discussion involves the time of peaking of reactive ground shear and peak muscular contraction. You suggested that they do not have to be timed together.

I completely agree...but given the topic of discussion, does the stance leg push or the swing limb pull, I think this matters. If the peak muscular contraction occurs at a time when propulsive thrust is decreasing, it is not peak muscular contraction that provides the bulk of the power to the step. All aspects are important....its just how they coordinate that makes this important.

To my way of thinking, the most EFFICIENT method involves the least amount of concentric muscular contraction. As these various methods combine to advance the CoM beyond the planted foot...the planted foot must permit the body to advance over it. This IS is the basis of understanding sagittal plane facilitation. When the foot fails to permit this motion, at the time when the body is EFFICIENTLY creating the power for motion....we have a rock and hard place analogy. There are several solutions. 1) Switch to a more inefficient style, ie, muscular contraction (but this eventually leads to overuse) or 2) Dissipate this power though the body in ways other than as propulsive influence. When this occurs at the foot level...we call it excessive midstance pronation. In most cases, both of these mechanisms occur simultaneously, leading to a host of symptoms we all see daily.

Howard

 

Stanley:

You are putting words in my mouth. Show me, anywhere in all my responses on this subject on this forum or in any of my extensive writings on foot and lower extremity biomechanics, where I made the statement "the swinging of the hip is unable to put energy into gait". If you can find anywhere that I made that statement, then I will answer your question. Otherwise, you are misrepresenting what I have been discussing on this thread over the past five weeks.

 

I certainly agree, Howard.

I also would agree that the stance phase limb does not create all the power for the progression of the body forward during walking. However, using the timing of peak electrical activity in the muscle via EMG is somewhat deceiving regarding the timing of peak muscle-tendon force because of the known physiological phenomenon of electromechanical delay which is estimated at between 30 ms to 100 ms in human skeletal muscle. In other words, if one only used EMG data to try and estimate when the forces are occurring during the stance phase of gait, then one needs to advance the EMG graph about 30-100 msecs forward, which puts the gastrocnemius-soleus complex peak force output at just about the time the maximum posterior shear force from the foot is also occurring.

I agree completely here.

This again brings us to the chicken and egg scenario of whether the late midstance pronation comes first, causing a limitation of hallux dorsiflexion, or whether a limitation of hallux dorsiflexion causes late midstance pronation. I prefer the former explanation. However, we certainly have been around this stump many times......haven't we?

 

Two situations, toe off with ankle plantar flexion, toe off without ankle plantar flexion. No plantar flexion = no added energy from the ankle.

With no ankle plantar flexion there is no energy added to the swing leg from ankle push and all the energy added to the leg has to come from hip pull. So the above is true with no ankle push.

With ankle push, which is how this thread started, energy is added to the swing pahse. So the above is no true, becaues energy is added to gait with ankle push.

Which hip flexors are you referring to?

What I said was that the center of mass is not raised by ankle plantar flexion. This is consistant with your and Kevin's position.

Stanley, I don't understand how walking on level ground can be equal to walking on a 4 degree grade. Do you have a site for that?

Could you remind me where my theory does not hold water?

Energy can be lost as heat. For example, when you land from a jump, before you land you have kinetic energy. During the time you decelerate, there is a knee extension moment at the time knee is flexing and energy is lost into the muscles and disipated as heat.

What I said that there was very little energy used to maintain the velocity of gait. A lot of energy was used to stay up right.

I was trying to show you what happens by taking an extreme. As you go from extremely long strides to uncomfortalby long strides, to somewhat long strides there is a continuum of this concept.

 

The gait analysis systems publish their error measurements. There is no additional error from co- contraction when you measure net joint moment. What is the research question?

When performing inverse dynamics the redundancy problem does not allow for separating out moment from individual muscles. I gave the example of agonists, but the redundancy problem exists for antagonists too.

No,

This is another podiatric myth. Stand on a bathroom scale with your heel just off of the ground and then maximally plantarflex your ankle. The measurement you described has changed, but your vertical component of ground reactive force has not changed.

Regards,

Eric

 

Kevin, my exact quote was “So you agree that the force of gravity is causing the leg to swing forward. I wonder if you also agree that lifting the leg requires energy to produce the potential energy. Since energy cannot be created or destroyed, and you feel that the swinging of the hip is unable to put energy into gait, (AS THE NEGATIVE EFFECT ON THE INVERTED PENDULUM WHEN THE CENTER OF MASS IS POSTERIOR TO THE CENTER OF GRAVITY IS EQUAL TO THE POSITIVE EFFECT ON THE INVERTED PENDULUM WHEN THE CENTER OF MASS IS ANTERIOR TO THE CENTER OF GRAVITY), then where does this energy go?”

If the negative effect on the inverted pendulum when the center of mass is posterior to the center of gravity is equal to the positive effect on the inverted pendulum when the center of mass is anterior to the center of gravity, then there is no energy put into the system.

You have said the following things in the numbered posts. In post number 74 you did say that “Certainly the swing phase limb is important to walking and its energy”.

Post #3
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?
Post#6
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.
Post #10
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
Post#14
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.
Post #50
The swing leg is pulled forward by the body, however this pull by the body on the swing leg does not pull the body forward, but rather, this pull by the body on the swing leg actually pulls the body backward, as Eric has stated previously.
Post#67
Therefore, to say that the swing leg somehow powers the gait forward is simply, in my opinion, a podiatric myth that needs to be ended.
Post#74
If you will read my responses to this thread carefully, not once did I say that the hip and leg muscles of the swing leg are not important to normal walking. My only contention is that I have heard lecturers many times state "the stance phase limb does not push the body forward, the swing phase limb pulls the body forward". This is wrong. Certainly the swing phase limb is important to walking and its energy.

So it looks like I misinterpreted what you said. I have no intention of putting words in your mouth. I apologize for this error.
To avoid any future misinterpretation, could you explain the following: as the negative effect on the inverted pendulum when the center of mass is posterior to the center of gravity is equal to the positive effect on the inverted pendulum when the center of mass is anterior to the center of gravity, how does the swing leg put energy into gait?

Regards,

Stanley

 

modified from post 93

Two situations, toe off with ankle plantar flexion, toe off without ankle plantar flexion. No plantar flexion = no added energy from the ankle.

With no ankle plantar flexion there is no energy added to the swing leg from ankle push and all the energy added to the leg has to come from hip pull. So, in this situation no net energy gain.

With ankle push, which is how this thread started, energy is added to the leg in swing pahse. So, the swing leg adds energy to gait because ankle push added energy to the swing leg in this situation

Regards,

Eric

 

The main distinction in this debate is very nicely summed up above from Eric. He states that if there is no AJ PF, then there is no energy added from the ankle.

I differ in my view of "energy added". Also, I think it is much more appropriate to state that the less AJ dorsiflexion there is, the less energy return there will be from plantarflexion of the AJ during terminal stance.

Some may not see the distinction here, but I think it is much more than just splitting hairs. Howard has said quite nicely that perpetual walking is best done with the minimal use of energy from the muscular system. I agree of course. I think that PFion of the AJ is much more of a return of energy into the system of gait than an active concious firing of a muscle to propel us forward.

If we do not get a DFion contraction of the AJ then you will get very little PFion of the AJ as well. I see this on video weekly. I also have the F-scan in-shoe data to show where the forces are constant, but the foot does not move.

This is not another "round the stump" issue as Kevin alluded to in reply to Howard. This is the very basis for differentiating between so called propulsive and apropulsive gait!

For our system of gait to work as efficiently as possible we must convert / store as much kinetic energy as possible with every movement of the limbs. To me that means utilizing gravity to assist in swing limb motion, using the motion of teh swing limb as well to "pull us forward" and utilizing concentric and eccentric contractures at and about the ankle to resist the foot from sliding backwards as the swing limb moves forward and finally in an energy return from the achilles complex thru late midstance PFion of the ankle.

Have a wonderful holiday season everyone. Be thankful that we can have civil debate from 'round the world, create new acuaintainces / friendships, and that our loved ones are there for us.
Sincerely;
Bruce

 

Hi Eric,

A good discussion is one where everyone learns something. I have learned and had to learn a lot, so this has been a great experience for me. Thank you. So let's continue.

Iliopsoas, and the Rectus femoris

It’s nice when we can all agree.

I think it was something from Inman and Saunders. The point was there would be no energy required to walk down a 4° grade.

From a muscle physiology point of view, the most efficient contractions are eccentric. From a neurologic point of view two major MRCP (electroencephalography [EEG]-derived movement-related cortical potential ) components—one related to movement planning and execution and the other associated with feedback signals from the peripheral systems—was significantly greater for eccentric than for concentric actions

Eric, losing energy as heat does not power gait. Why does it cost so much energy to stay up right.

I don’t know if you can do that. For instance, if you take really long strides, you will tear tendons and ligaments. Using your reasoning, you tear ligaments and tendons somewhat with each step. So if you walk far enough, you will tear ligaments and tendons.

Regards,

Stanley

 

Eric, from what you have written, I understand the vertical component, but the horizontal component escapes me. In regards, to the bathroom scale and proportion of horizontal components of the vector, you would need to measure the horizontal components of ground reactive force. My bathroom scale does not measure horizontal components of ground reactive force. Does yours?

Regards,

Stanley

 

The question was: how does the swing leg put energy into gait, if according to you the negative effect on the inverted pendulum when the center of mass is posterior to the center of gravity is equal to the positive effect on the inverted pendulum when the center of mass is anterior to the center of gravity? You answered that the hip can put energy into the system. We agree on this point. :drinksHoward feels that the action of the forward swinging hip has its reaction as the shearing forces of the stance leg. I don’t want to put words in Kevin’s mouth, but he has said that the swing leg cannot put energy into walking without the assistance of the stance leg (posts 6, 10, 14). It sounds like they are in agreement :drinkseven though the title of this thread is “Does the Stance leg Push or the Swing leg Pull”.

Regards,

Stanley

 

Hi Stanley,

Actually, I was asking stance leg or swing leg. In the second half of swing the swing leg has reached its maximum velocity and no additional force from the hip flexors is needed. If there was activity of the flexors at this point you would see acceleration of the swing leg.

Stanley, I understand your point about energy cost and production of force and eccentric versus concentric contractions. Gait is not just about producing force it is about producing motion. Eccentric contraction reduce motion. Concentric contractions produce motion. You cannot move with just Eccentric contractions. You have to have some concentric contractions. This is the value of joint power analysis. From that analysis we can evaluate what groups of muscles are contracting concentrically and eccentrically. Even if there is a small error the joint powers will still be positive with concentric contraction and negative with eccentric contraction.

I agree that losing energy as heat does not power gait, but it happens. At heel contact there is knee flexion motion with a knee extension moment. If there wasn't you would fall flat on your face. Gait has been called a controlled fall. You need to use muscular effort to prevent that fall.

Stanley, Don't hurt yourself. Take a long enough stride that you are getting near the point of hurting and you will see what I'm talking about.

Regards,

Eric

 

There are commercial force platforms that measure horizontal shear. Howard referred to these measurements in his post. The horizontal shear is produced by 1) the force couple of the force applied by the ground at the center of pressue and the force of gravity applied at the center of mass and 2) muscular push which is variable between steps and activities. Think about trying to start a sprint on grass. If there is sufficient friction, there will be a horizontal force from the ground applied to your foot and your body will accelerate in the direction of the force. If there is insufficient friction and you tear up a big divot, there will be a force from the foot on the divot pushing it away from you. The same happens in walking, but on a smaller scale.

Does this answer your question about horizontal forces?

Regards,

Eric

 

Hi Bruce,

I'm not exactly sure what you are saying Bruce. What causes ankle plantar flexion? On a quick read it seems like you are saying dorsiflexion of the ankle causes plantar flexion later. As I see it, only two things can cause plantar flexion. Active muscle activity creates a plantar flexion moment. Gravity can plantar flex the ankle if the leg is lifted and there is no dorsiflexion muscle activity.

Correlation is not causation. Just because you see a corelation between lack of dorsiflexion in midstance it does not mean that there must be no plantar flexion just before toe off.


Cheers,

Eric

 

Hello Eric;

I think the distinction I am trying to make is in reference to "active muscle activity".

How exactly do you define "active muscle activity" Eric?

I would define it as PFion at the ankle when someone wants to stand on their toes. I would not necessarily define it as PFion of the ankle from heel off to toe off. The first I would define as "concious activity or activation of a muscle", the second as "subconcious activity".

I have had difficulty trying to find any references that would differentiate what I am saying, and you may or may not understand or agree with what I'm getting at.

Regarding the need for AJ DFion to get AJ PFion I think a comparison is in order. Kevin K. has described the energy return from a spring-like effect of running as "more like a bouncing ball". He also says " In fact, walking and running are exactly opposite each other in their use of potential energy (PE) and kinetic energy (KE). In walking, PE and KE are out of phase." Novacheck, Tom F.: The biomechanics of running. Gait and Posture, 7:77-95, 1998

I cannot find a copy of the article that he referenced to see what exactly was and was not said. Regardless, I disagree. For perpetual walking to be efficient, we must store energy , Potential energy eg AJ DFion, to be released as kinetic energy eg AJ PFion.

How often do you see non-first step "active" AJ PFion in patients Eric? By this I mean that there is AJ PFion prior to contralateral heel strike? I don't think I've ever seen this in an apropulsive gait. I think there is a reason for this and that it has to do with pre-loading the Achilles complex as well as the Fascial complex of the foot and lower leg for late midstance to propulsive phase return of energy to initiate swing.

As well, Kevin K. or you stated somewhere in these four pages of discussion something about lesser shear forces from an apropulsive gait. Of course there will be more reverse shear force in propulsive gait pattern, but you will get a longer stride length, etc. More energy in, more energy out. I think you would agree.

So, if AJ PFion is really an "active muscle activity" as you say, then why don't these patients just push off more to maintain a more propulsive gait? This is again not a challenge to switch the debate to sagittal plane mechanics... that is for another day. Instead I think that we have to define the difference between a "concious active muscle contraction" and a "subconcious active muscle contraction". Until then we will make no head way in coming to an understanding.

Be well Eric.
Bruce

 

Bruce:

It's probably not a good idea to disagree with Tom Novacheck. He has written the best and most complete article on the biomechanical differences between walking and running that I have ever read. Every clinician that treats sports injuries (and walking injuries) should have this paper memorized. I would suggest that you may want to hold off on any more comments on the subject until you have read the article a few times.

 

Kevin;
I'm sure that at this point Tom Novacheck has nothing to worry about. I did not disagree with him, but with you.

In the future I will withold my comments on papers that I have not read beyond the abstract when you begin to do the same.

Thanks for the pdf. I will look it over.
Bruce

 

Hi Eric,

Good discussion.

Eric, at heel contact we have motion at the knee and there is an eccentric contraction of the quadriceps.

You are talking about an eccentric contraction, which requires much less energy to produce the same force. The energy is of muscular contraction is used to decelerate the lowering of the center of mass. Regarding the knee, wouldn’t there be a net moment of flexion?

Eric, when you take a long enough stride, the leg goes beyond perpendicular to the ground. The ground reactive force will develop a horizontal compnonent in a negative direction. The body is going to use inefficient strategies to overcome this handicap. This effect is not a continuum.

 

Eric, you sure have a fancy bathroom scale.
Seriously, I am trying to figure out what happens with this horizontal vector, and this is what I get. Rotational motion goes around an axis. In the case of the foot at propulsion, the axis is the fulcrum of propulsion, the first MPJ. A moment is a rotational force, so there is a moment produced at propulsion by the tension of the Achilles tendon (and there is an equal and opposite reaction which is ground reactive force). The rotation of the foot around the 1st MPJ, is roughly that of a partial circle. If you were to trace a circle, it will change its direction, depending where in the circle you are. If you were to start at 9:00 O’clock and go clockwise, the first movement would be 100% in an upward direction and then as you progress, there would be a greater proportion of a horizontal movement until at 12 O’clock you would have purely horizontal motion. At any time, the direction would be perpendicular to a radius of the circle. Let’s now come up with an angle to tell us where we are. 9:00 O’clock would be 0° and 12 O’clock would be 90°.
There would be a 100% vertical component at 0°, and a 0% vertical component at 90°.
This is a cosine function. What this tells me is that at the initiation of push off, there is mostly a vertical component, and not a horizontal component.
What this tells me is that majority of ankle plantar flexion (at least initially) raises the center of mass.
As far as the vertical component helping the pendulum effect, it won't. It would have to be flexion of the knee. According to Neptune, the gastrocnemius flexes the knee.
One more thing. This discussion has to do with the pushing of the stance foot via the plantar flexors, and whether it initiates swing. The following article shows that the ankle plantar flexors do not initiate swing.
http://jap.physiology.org/cgi/reprint/98/6/2126



Regards,

Stanley

 

Kevin;
thanks again for the pdf. Very interesting paper indeed. I found this quote on page 11 to be quite interesting, " One should note that each of these essential power generators stretches eccentrically just prior to generating their burst of power (Fig. 9). It has been shown that tendons strectch and then efficiently return most of that energy when they recoil. In addition, it is well know that muscles that are pretensioned and then contract generate more power per unit of activation than those that are not. In essence one can consider the tendons as springs and the muscles as tensioners of the springs ( see later section )."

This reference was not made to running gait alone as he seemed to imply it worked for walking as well.

As I said before, I don't disagree with Tom Novacheck, just you!
:drinks
Bruce

 

Bruce:

I also agree with Tom Novacheck. Maybe you can show me anywhere in all of my postings on Podiatry Arena or JISC Mailbase, in any of my 2 books, 2 book chapters or 20 peer-reviewed published papers where I have written anything that contradicts anything that you have quoted above from Tom Novacheck's paper. Then, Bruce, possibly I will have a clue as to what you are talking about.:bash:

 

I agree that at push off, most of the force is vertical. It takes a lot of force to support body weight. It takes little force applied to the foot to swing the leg forward. (Think of Kevin's example of pushing a person in a swing versus lifting a person in a swing.) I also agree that the ankle of the foot relative to the ground will influence the direction of motion of the distal aspect of the foot and from that the amount of push from the ground.

That was an interesting experimental set up. In the literature review for the article they mentioned studies that supported the notion that ankle push occurs. These were in addition to Neptune's and Winter's work that we have discussed. The experimental setup was walking on a treadmill with strings attached to the foot to pull it forward in swing. This is not natural walking and I do not feel that it negates the work done without strings attached to the foot.

I'm not sure what you mean when you said "I am trying to figure out what happens with this horizontal vector" I couldn't quite follow what you said afterwards.

Regarding the vertical component of force and helping the pendulum effect you said "As far as the vertical component helping the pendulum effect, it won't. "It would have to be flexion of the knee."

Plantar flexion of the ankle can flex the knee. As the ankle raises, parts above have to move upward. If the hip does not move upward then the flexion of the knee has accomodated all the ankle motion. This upward motion can cause knee flexion if the knee is not too stiff. The knee flexors could be inactive when there is flexion of the knee, because all that would be needed is a reduction in the knee extension moment.

Regards,

Eric

 

Hi Bruce,

Active contraction of the muscle is when there is a signal from the motor neuron to the muscle and there is depolarization of the muscle and force is produced. Physiologically there is no difference between the two situations you describe.

Energy is stored in my description of gait. Energy is converted from one type to another and some energy is lost when there is eccentric contractions. Eccentric contraction = moment from muscle is in opposite direction of joint motion. Power, the change in energy is negative (energy is lost), when the joint moment is in the opposite direction of the motion. (Power = moment x angular velocity)

The classic example is landing from a jump. The instant before landing there is kinetic energy from the downward movement. After contact, the knees flex with an extension moment from the quadriceps. There is a net flexion moment at the joint, but the moment from the muscles is extensor. (If it weren't the knee flexion would be much faster.) When the downward motion has stopped, and you could choose to stop at the low point of the motion, the kinetic energy is zero. If you chose to immediately rise up again from this position there would be some potential energy stored in the elastic components of the muscle. However, the elastic energy is much less than be required to achieve the same kinetic energy in the opposite direction. Energy was lost into the muscle as heat because of the eccentric contraction.

When I walk, I can choose to plantar flex my ankle before contralateral heel strike. It puts quite a bit of strain on my foot, so I usually choose not to plantar flex my ankle before contralateral heel strike. Bruce, my definition of a propulsive gait is one with ankle plantar flexion. An apropulsive gait may or may not have heel lift. The heel lift will occur with forward rotation of the tibia yet with no ankle plantar flexion. What is your definition of a propulsive gait?

It is possible to plantar flex the ankle without preloading. Therefore, I think there is some other reason for your observation. People subconsciously choose a different gait pattern, probably to avoid pain in the foot.

I just want to make sure we are talking about the same thing when you say reverse shear. I think what you are talking about is a posterior to anterior horizontal force from the ground applied to the foot. (There is an equal and opposite force from the foot applied to the ground in the opposite direction. When we are looking at swing leg mechanics we want to examine the former and not the latter.)

Bruce, until you explain the physiologic difference between "conscious and unconscious" contractions let's just assume there is no difference and continue the debate. These four pages of discussion are all about one of the explanations of sagittal plane theory and we are talking about it today.

Swing leg powering gait is not an important part of sagittal plane theory. You can still use the F-Scan to alter how you make orthotics without ever talking about the swing leg powering gait. In fact, stating things that are contrary to the best available detracts from the rest of what you say.

I believe the reason that patients walk with an apropulsive gait is that they have learned that walking with a propulsive gait hurts. They subconsciously make the calculation that walking inefficiently is better than walking with pain. It's kind of like limping on a smaller scale.

Respectfully,
Eric

 

Hi Eric,

Eric, the paper was done on real people using EMG’s to assess what the muscles were doing. I agree it is not natural walking, but how else would you test to see which muscles were contributing to swing in humans. The advantage of this paper is that we do not have to worry about whether the model is correct.
In the paper Winters was referenced by saying “Before the experiment, subjects performed a series of contractions to ensure cross talk was negligible” and Neptune was referenced “Remarkably, we do not have a definitive understanding of which muscles are responsible for initiating and propagating leg swing in running. In walking, ankle plantar flexor muscle(s) contributes to swing phase initiation”. This is not a strong affirmation of ankle push. In fact the conclusion of the paper states “These results suggest that the plantar flexor muscle group is not responsible for leg swing initiation in running. This finding is in contrast to the idea raised in several walking studies that the ankle plantar flexor(s) initiate leg swing. We attribute the activation of the ankle plantar flexor muscles in late stance to the functions of forward propulsion and supporting body weight.”

Eric, basically the vector's horizontal scalar is a function of the direction of the force. The more horizontal, the higher the scalar is. Since the direction of the rearfoot is nearly vertical at early propulsion, there is very little horizontal scalar.

If there is a push off, it means that the foot is progressing quicker than the leg. This would cause the knee to straighten.


Regards,

Stanley

 

Eric,

You wrote "Swing leg powering gait is not an important part of sagittal plane theory."

Swing leg function is ESSENTIAL to understanding sagittal plane theory. Swing phase motion is hard wired in the nervous system. What is so critical about its function is that the power for propulsive gait is ALWAYS present....its the inability to utilize this power and then the requirement to either store or dissipate it that is the essence of sagittal plane theory.

Howard

 

Hi Stanley,

A better way to measure whether or not the ankle plantar flexors contribute to the energy of the swing leg is measure joint power. What model are you referring to? Joint power is a measurement made from inverse dynamic calculations. One of my main contentions in this debate is that it is possible for the ankle plantar flexors to contribute to swing leg energy. Sometimes they do and sometimes they don't. There has to be plasticity in the system.

What is interesting about the last quote is that the plantar flexors function go provide forward propulsion. The sagittal plane theorists have said for a long time that plantar flexors cannot provide forward propulsion because the knee is flexing at this point in walking. Was the last quote referring to walking or to running. Any way forward propulsion of the body at toe off acts to move the swing leg forward.

Some terminology; scalar is a measure of something that does not have a direction associated with it. For example, mass is a scaler quantity. Vectors are used to represent the magnitude of someting that does have a direction associated with it. For example velocity is expressed in vectors. If we abritrarily define positive direction as left to right. When an object travels from right to left it will have a negative velocity. Mass cannot be negative.

I agree that the horzontal component of force at push off is small compared to the vertical component. However we need very little force to accelerate the swing leg forward. We need a lot of vertical force to counteract the pull of gravity.

We also are not talking so much about the force in early propulsion. It is middle and late propulsion when the angle of the foot to the ground, and the position of the foot relative to the rest of the body, is more optimal for providing push.

Could you explain the above some more? As I see it, ankle plantar flexion, just before toe off, will either flex the knee or push the entire body.

Regards,

Eric Fuller

 

Howard,

You are right swing leg power is essential to the theory. What I should have said is that swing let power is not essential to protocol used for making orthoses. Sagittal plane theory is one theory to explain the success of the treatment protocol. I believe there is a better explanation and that is why we are here discussing it on podiatry arena.

Regards,

Eric

 

Eric,

Wrong again....net swing power is also essential for orthotic Rx protocol. I am trying to build a device which permits motion...and rather than one that will only attempt to control it. No wonder we have these endless circular discussions.

I have published outcomes on lower back pain subjects using this protocol...so its not about belief of why it works, but rather knowing its effective and attempting to improve upon it. I use far less orthotic posting and control, and achieve a very high cure rate. Using the same philosophy and protocols, my cure rate for plantar fasciiitis is also very high, with surgery required on less then .01% of patients.

That said, I do appreciate a large part of these discussions, and I do learn from them. I hope that this is a two way street.

Howard

 

Howard,

One of the things that I have learned from our discussions is to be able to handle disagreements and to parse out what the exact disagreement is. From our past discussions I know exactly where you are coming from and I am now able to handle being told that I'm wrong without going nuts.

Keep on making the good observations and I would bet that we come up with similar orthoses with different protocols.

However, I still disagree with you on the explanation of why your protocols work. It has been an awesome learning experience.

Eric

 

Eric;

I plan on responding to the rest of your post in a day or so. I would like to make a comment to you on what you said above.

First, this is a debate forum list. We are encouraged to challenge what some believe to be right and what has been written and tested and published.

I will continue to challenge what you call "the best available" so long as I disagree. I think that you are very foolish to think that this has ever detracted from my reputation except with those who only see their way as correct and choose to discredit or belittle those who disagree with them.

You have now made this comment to me twice at least in recent weeks. I suggest you worry about your own reputation and less about mine.

My suggestion to you is that you stick to the debate and avoid personal comments or reccomendations in public. Your reputation might be elevated by utilizing a little more diplomacy that way.

Respectfully;
Bruce

 

Hi Eric,

Eric, I am still waiting for the Winter article. The only articles that we can discuss, because we have access to them, are the Neptune article, the Modica article, and the Anderson and Pandy article
In the Anderson and Pandy article, the Winter article is referenced. It says: It has been decades since the work of Saunders et al.[1] was published, yet we still do not have a comprehensive and quantitative picture of how muscles contribute to the vertical ground-reaction force and therefore to support, even during normal gait. Considerable insight in this area has been offered by a number of researchers, but findings have been limited because the analyses have largely not been based on estimates of muscle forces. Winter [2] based his observations on similarities between the shape of the ground reaction force and the sum of the net extensor moments applied at the hip, knee, and ankle.I would gather from this statement that the Winter article is a deficient model in looking at the plantarflexors. Is this the Winter article you are talking about? To say that the Winter article is a better way to look at the muscles than the Modica article which is using EMG’s to measure whether these muscles contribute to the swing phase is puzzling to me. This brings me to the point of what are we discussing. Is it the measurement of power at a joint or is it the muscular contribution to the power of a joint? For instance, if a tendon has a certain amount of stored energy, and contributes to the plantarflexion of the joint, would you consider this the contribution of the plantarflexors?

They do have a good point. If the knee is flexing, it is not in contact with the ground. I don’t see how you can push if the foot is not in contact with the ground. Are you agreeing with Kevin Miller?

Or is the corollary what matters?- The swing leg provides for forward propulsion at toe off.

Thanks for the refresher course. Eric if a vector is at an angle to the horizontal plane, then there is a vertical and horizontal component. For example let’s say you have a vector on the horizontal with a scalar of 5kg. What would the horizontal and vertical components of that vector be if it were in a direction of 36° elevated to the horizontal plane?

That is why we do not have to pick up our leg very high in walking.

What might theoretically happen is not what really happens. In the Modica article it clearly shows that “Neither MG (medial gastroc), Sol (Soleus), nor RF (Rectus femoris) muscles contribute to leg swing initiation; however, RF does play a significant role in leg swing propagation”.

Eric, your contention is that the plantar flexors exert a push at the forefoot which propels the foot into swing. For this to occur, the foot would have to propel the leg forwards. If this were to happen, the distal end of the leg would travel faster than the proximal end causing knee extension.

Regards,

Stanley