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

I'm having a little trouble with the comment that butt to chair forces is something different than ground reaction force. Yes, it is different, but they are from the same cause. When raising the arms there must be a force from the shoulder acting on the arms to accelerate the arms upward. For every action there is an equal and opposite reaction. So there is a force from the arm downward on the shoulder. Taking the trunk as a single unit the trunk is not accelerating downward so there must be an increase in force beyond the weight of the trunk at the bottom of the trunk at the butt chair interface. Equal and opposite reaction butt pushes on chair. Chair pushes on ground.

It would be very easy to take your statement out of context. I'll try and put possible contexts around it.

When sitting in a chair and raising your leg yes there will be an increase in force under the chair. When standing or walking when lifting the leg there will be an increase in force on the opposite leg. The increase in force will be greater than the mass of the leg because you need a force greater than the mass of the leg to accelerate it upward because gravity will be pulling it downward. When running, at the instant that the leg leaves the ground and it is being lifted there will be no increase in ground reaction force, but there will be a downward acceleration of the rest of the body.

A vast majority of the time we have the additional information so that we can make an educated guess as to where the force is coming from. And we can make measurements to corroborate our guesses. In the example of sitting on the chair while raising your arms, we can look at the chair versus ground forces and it is a pretty good assumption that the increase in ground reaction force is coming from an increase butt versus chair forces and we can put the force plate there to confirm our theory.

So if we choose not to limit ourselves to just looking at the shoe versus ground forces we can make some assumptions. Why does it hurt more when you put your finger under a shoe when someone is standing and wearing the shoe as opposed to an empty shoe. It's force from the foot acting on the shoe. It can't be the leg, because the leg is not touching the shoe. (The weight of the leg is applied to the foot and then applied to the shoe.) It can't be my imagination, because it's not touching the shoe either.

The only other explanation I can come up with is explosive shoes. In this case we could just look at the force plate data and conclude that shoes are not exploding by looking at the force versus time curve. Or we could look at the shoe on top of the force plate, but that would require a little more work.

Regards,

Eric

 

Hi Bruce,

I would agree that you cannot prove that there is ankle push from force plate data alone. There will be some a/p shear without ankle push. But, you can answer the question with kinetic data.

It's not moot. Very few people have 100% ankle push, but 100% ankle push would not necessarily be a good thing. It would put more stress on the foot. In gait, energy is required to make the trailing leg become the leading leg. That energy can come from hip pull or ankle push, there are no other choices. (The spinal engine works through hip pull.) So, when people walk they choose what percentage of hip pull and what percentage of ankle push they will use. Winter has shown that the more hip pull they use the less ankle push they use and vice versa.

Now, we should examine why sagittal plane treatment could improve low back pain from the tissue stress perspective. Someone's foot hurts when they use ankle push, so they choose to use 90% hip pull. One of the biggest hip flexors is Illio psoas so it will be working much harder and when it works it will increase stress on its proximal attachments at the lower back. So, we make an ortohsis that allows the patient to increase ankle push and this would cause lower stress on the lower back. I'd be willing to bet that if you looked at gait before and after orthoses in patients that had relief from back pain with the orthoses that those with relief will show an increase in ankle joint power.

I don't think there is much difference between the two. On first step, there is forward body lean and then there is ankle joint dorsiflexion and then the trailing leg is swung forward with a combination of hip pull and ankle push. So the mechanism of stored energy in the tendons happens on both first step and subsequent steps. If you wanted to get moving faster, there would probably be more ankle push on the first step.

Regards,

Eric

 

I'm no expert on force plates either. I don't see what difference it makes regarding how the co-ordinate system is labelled as long as the plate measures forces in the 3 orthogonal axes representing normals to each of the cardinal planes.

 

Bruce:

Sorry for not getting back to you sooner. I think that part of the problem with this whole thread is that we need to agree to a definition for what a "push" is.

Here is what my Encarta Dictionary says:

In definition #1, vti press against to move: to press against somebody or something in order to move that person or object, the dictionary qualifies that the pressing against something in order to try to move an object or person makes a "push". However, in physics, we do not always differentiate as to whether that force that results in the movement of an object or person actually does create motion of the object or person in the direction of that pushing force. In other words, in physics, a pushing force (i.e. compression force) on an object or person may produce (1) motion in the direction of the pushing force, (2) may help stabilize that object or person and prevent it from having any motion, or (3) may decelerate a motion acting in the opposite direction from the pushing force.

Therefore, unless we can agree on a definition of what "push" actually means, then we will continue to beat the dead horse in this thread.:deadhorse::hammer::deadhorse::hammer:

To answer your first question: "How exactly do you differentiate between an active push from the foot as opposed to a stabilization of the foot due to forward motion of the CoF with force plate data?"

Let's instead of using the term "active push" use the more precise term, "concentric ankle joint plantarflexion" which means that the ankle is undergoing plantarflexion motion and this is occurring along with a net ankle joint plantarflexion moment. I assume that what you mean by "stabilization of the foot" that this would be more precisely defined as what happens in late midstance as the ankle is dorsiflexing and the ankle joint plantarflexion moment is simultaneously occurring, which would be more precisely defined as "eccentric ankle joint dorsiflexion" . Eccentric ankle joint dorsiflexion means that the ankle joint is undergoing dorsiflexion motion and this is occurring along with a net ankle joint plantarflexion moment.

As Eric mentioned, in order to determine whether the ankle joint is undergoing concentric ankle joint plantarflexion or eccentric ankle joint dorsiflexion, then you would need to perform simultaneous kinematic analysis (e.g. 3D motion analysis) and also kinetic analysis (e.g. force plate analysis) and then use inverse dynamics to determine the net moments occurring at the ankle joint at any instant in gait. The two references I have provided earlier in this thread (Winter, D.A.: Biomechanics and Motor Control of Human Movement, 2nd ed. John Wiley and Sons, New York, 1990, p. 101; Spivack BS (ed): Evaluation and Management of Gait Disorders. Informa Healthcare, New York, 1995, pp. 31-32) both state that in normal human walking that concentric ankle joint plantarflexion occurs during propulsion.

I believe, Bruce, that if you looked at any respected textbook on gait analysis or human walking, that you will find that this is mechanical fact, that the ankle plantarflexes with a net ankle joint plantarflexion moment during propulsion, is a consistent research finding and is not data that anyone who has done research on the subject seems to have disagreements with. I do realize the that the ankle plantarflexors are rapidly decreasing in force production during propulsion, but their net effect is still an ankle joint plantarflexion moment. I also hope that any interpretation of EMG data on your part takes into account the known electromechanical delay of the initiation of muscle contractile force production and the known lag time from EMG signal loss to loss of force production by the muscle-tendon unit. (Cavanagh PR, Komi PV: Electromechanical delay in human skeletal muscle under concentric and eccentric contractions. European J. Appl. Physio., 42:159-163, 1979.)

Possibly you can provide me a reference from a peer-reviewed scientific journal or a textbook that shows me that in normal human walking that concentric ankle joint plantarflexion does not occur during the propulsive phase of gait so I can see where there is some other reasonable interpretation of this repeated kinematic and kinetic research finding from walking gait analysis studies that have been performed over the past quarter century.

 

I agree. :drinks
Usually the problem in communication is terminology. :craig:
By the way, I looked up what a force plate can measure, and it is:
Fx, Fy, Fz, Mx, My, and Mz.
Forces along each of the cardinal planes and Moments around each of the cardinal planes.
In English that I understand, I think that would be:
Fx- Anterior Posterior Shearing
Fy-Medial Lateral Shearing
Fz- Vertical Force
Mx- Rotation of the COP around the COM in the frontal plane
My- Rotation of the COP around the COM in the sagittal plane
Mz- Torsional forces in the transverse plane

I also see that the terminology is different than when I was looking at this information. When I first looked at Force plates, GRF was just Fz. Now I am not sure. It may include some other of these measures. I also see that Force plates are now used in conjunction with other instrumentation such as slow motion videography.

Regards,

Stanley

 

Hi Eric:

Eric, so we agree that in this case the increase of ground reactive force is from raising the arms. I agree that the butt to chair force is essentially the same as GRF, but just looking at GRF you wouldn’t necessarily know that without some assumptions.

I agree. So when you look at an increase in GRF, it can come from a result of several different things (lifting a leg in this case or raising an arm in the previous case)

Exactly what I have been saying. You can make some guesses but you would need additional measurements beyond just GRF to confirm this.

Again more assumptions and more testing (shoe by itself vs. shoe with foot in it on a finger [acting as a sensor])
Eric, I agree with your thinking :drinks, but in context with:
Kevin Post 174: Just goes to show you: the stance limb does push against the ground throughout the stance phase of gait! (i.e. if it didn't push, then there would be no ground reaction force!!), there are shortcomings in this statement regarding the causation and its analysis.

Regards,

Stanley

 

Very interesting article and applicable to our discussion

Some points

Turkey gastroc absorbs energy going downhill and produces energy going up hill.

Compliant tendons and aponeurosis enable fascicle length
changes to be uncoupled from that of the whole muscle–tendon
unit (MTU) length

Numerous investigations on different terrestrial species have
demonstrated that during locomotion, the ankle extensor
muscles act nearly isometrically (constant length) or concentrically (shorten) whilst the whole MTU lengthens.
(Griffiths, 1991; Ishikawa et al., 2005; Fukunaga et al., 2001;

Positive work was done by the GM fascicles in all conditions.

Some comments:

For me a fascinating concept is that electrical signal ends long before the force output ends. This means that when we use a muscle, we have to make a guess at how much contraction/force is needed before the activity takes place. This would have to be a learned amount of contraction. It's kind of like throwing a ball at a target. Once you let go, you have no more control over the ball. Once the EMG stops no more force can be added. It has been shown that people who are running and have an unexpected change in surface are able to adapt to the characteristics of the new surface in one to two steps. The muscle output would have to change that fast. It would be nearly impossible to hard wire walking on different terrains.

It looks like they found isometric and concentric contraction of the muscle fasicles even the whole musculotendinous unit lengthens. My take on this that it is more important to look at the force requirements for the given situation and less important to look at what is the most efficient in terms of muscle contraction. If you need more energy to perform a certain movement, the body will use that energy.

Very interesting article on the inner workings of the Gastroc. It does corroborate the notion of ankle push.

Regards,

Eric Fuller

 

Eric,

It's so comforting to know that when you look at an article you see things that I don't. :dizzy:

Eric,
I agree that the Turkey gastroc absorbs force going down hill and produces energy going uphill. But if you look at page 2954 of the Daley article you will see “Neither muscle contributed as much energy as expected for its mass to increase the PE of the body COM during incline locomotion. This suggests that proximal muscles of guinea fowl may modulate work more than distal muscles.” In other words there is extra work needed to go up an incline, and the distal muscles are not doing its fair share, as it is not designed to do this. You can see this also on the bar graphs on this page. Also on page 2956, it says “However, sonomicrometry measurements show that proximal muscles of various species strain substantially during and immediately following muscle activation, suggesting that they generate work or absorb significant energy (Carrier et al., 1998; Gregersen et al., 1998; Gillis and Biewener, 2001). In addition, the biceps femoris and vastus lateralis of rats increase their active shortening when the animals run up an incline compared to on the level (Gillis and Biewener, 2002). Measurements of joint work also suggest that muscles at proximal joints may commonly contribute more work than those at distal joints (Pandy et al., 1988; Gregersen et al., 1998; Belli et al., 2002). Taken together, these results are consistent with the view that proximal limb muscles play a central role in modulation of limb mechanical work.”
I am glad to see that you now agree about the importance of the stretch of the Achilles tendon. I am confused that I found an article that speaks to the human Achillles tendon, and you pick out the parts that have to do with terrestrial animals. There are differences in function between the guinea fowl and human ankle extensors, that being that is humans the calcaneus contacts the ground for part of the gait cycle, while the equivalent of the fowl does not.

Regarding the concentric contraction of the gastrocnemius while the tendon lengthens, this is not occurring at propulsion. The rest of the quote as “Compliant tendons and aponeurosis enable fascicle length changes to be uncoupled from that of the whole muscle–tendon unit (MTU) length (Griffiths, 1991; Fukunaga et al., 2000).Numerous investigations on different terrestrial species have demonstrated that during locomotion, the ankle extensor muscles act nearly isometrically (constant length) or concentrically (shorten) whilst the whole MTU lengthens (Griffiths, 1991; Ishikawa et al., 2005; Fukunaga et al., 2001; Hof et al., 2002; Lichtwark and Wilson, 2005d). This acts to stretch the relatively compliant tendon of this muscle group during the stance phase. Because tendon is an elastic material, energy is stored in the tendon whilst it is stretched AND THIS ENERGY IS RETURNED LATE IN THE STANCE PHASE WHEN THE TENDON (AND SUBSEQUENTLY THE ENTIRE MTU) SHORTENS RAPIDLY.” (Emphasis added) Eric, with the muscle contracting and the MTU lengthening, what would inverse dynamics say is happening?

What page did you see this quote on?

Yes this is a fascinating concept. So in the gastroc, we have a contraction that is isometric, and the COM is progressing forward causing energy to be stored in the Achilles tendon. As the COM goes further forward, there is less force keeping the calcaneus on the ground. As the calcaneus starts to rise, this is the time the muscle turns off and the delay is in effect. We see that the ankle is plantarflexing which is due to two things: 1. the spring from the Achilles tendon, and 2. what appears to be a slight concentric contraction of the gastroc. As you have pointed out, “Once the EMG stops no more force can be added.” So what is going on? As the weight on the leg decreases because the opposite foot is in contact with the ground, there is less resistance to the isometric contraction (If you were to resist a force and all of a sudden the force was to be eliminated, you would change from an isometric contraction to a concentric contraction).

Eric, there is an article that Dave Smith sent me regarding the primitive nervous system for robots to walk. Gait is basically hardwired, but there is an override from the visual system and the vestibular system. In other words, the robot is walking and it sees a log. It will pick up the leg so it doesn’t trip, or if it is walking and the road turns to an incline, the muscles will change to meet the needs.

Eric, you are referencing terrestrial animals prior to propulsion in this statement. If you remember the Neptune article stated that the Gastroc and soleus were nearly isometric in midstance. But with the knowledge we now have, we can both see that there are deficiencies in the modeling that he did. There is no modeling of the stretch of the Achilles tendon. So I am not sure how relevant his statements really are. So we really do not have a good reference for whether there is an isometric contraction and the momentum of the body stretches the Achilles or whether the concentric contraction assists in the preloading of the Achilles tendon. In either case, storage of energy in the Achilles is done in the most efficient manner. If you want to look at the force requirements, then remember this equates to what the muscles do, and given a choice, the body will use the most efficient way possible.

According to the definitions that Kevin gave, I would have to agree. But in relation as to what powers gait-the proximal (swing) muscles that are designed to modulate work or the muscles that power push off. It still seems to me that the muscles that power swing provide for the energy that is used by the gastroc/soleus in an isometric contraction to stretch the Achilles tendon to store energy that will be used to raise the heel.

Regards,

Stanley

 

I have no problem wiht the notion that proximal limb muscles play a central rolw in modulation of limb mechanical work. Ankle plantar flexors are part of the equation of toal limb work. Sometimes more, sometimes less.

This article does say that ankle plantar flexors add power to the human limb. My point about quoting the fowl articles was to show that it does happen across species.

Energy change = Joint power = joint moment x joint angular velocity. When moment and velocity are in the same direction there is energy gain and when moment and velocity are in opposite directions there is energy loss. Your final question is an important one. Joint power does not really care exactly what is happening at the tendon or the muscle fasicles. It cares about joint velocity which is more correlated with the length of the whole muscle tendon unit.

So to answer your question directly, when the whole MTU is lengthening (ankle dorsiflexion) when there is a plantar flexion moment, kinetic and potential energy of the limb is lost. However, this article shows that some of the kinetic and potential energy that is lost is stored as elastic energy in the MTU.

This is both in the summary on the first page and in the first paragraph of the discussion section. I think, the authors think that this is an important point.

I have a minor issue with your analysis above. It is not the progression of the center of mass that causes heel lift. If there was infinate range of motion of the ankle joint then the heel would never lift. The heel lifts because of a plantar flexion moment at the ankle joint. The cause of the plantar flexion ankle moment can come from the plantar flexion muscles or the structures that cause the end of range of motion in the direction of dorsiflexion (e.g. talus hitting tibia).

I agree with your anlaysis that when you have a release of opposition force in the presence of an isometric contraction that you will get a concentric contraction. Joint power = joint moment x joint angular velocity. There is still force in the tendon, which causes a plantar flexion moment and there is plantar flexion velocity so we have concentric muscle contraction causing an increase in energy of the soon to be swing leg. If there were no force in the MTU as it lengthened there would be no ankle push.

I agree that walking cannot be 100% hard wired.

The momentum of the body cannot stretch the tendon unless there is activation of the muscles. Again inverse dynamics does not really care what happens within the musculo tendonous unit. It just cares about the output of the whole unit. I agree that the body will usually try to do what it needs to do in the most efficient way possible. However, to make an omelette you have to break some eggs. To move, you have to use concentric contractions and that is not a bad thing.

Again, the muscle has to be active, and this article showed that there was shortening of the muscle fasicles, to create a force to store the elastic energy. With no muscle force the muscles would passively lenghten (up to a point).

Regards,

Eric

 

Hi Eric,

According to this it is always less.

There is another way to look at this. If we look at the Daley article, they look at two different ankle extensors, the lateral gastroc and the DF-IV. Since there are obvious differences in anatomy, the question is which one is more similar to the human gastroc. Guinea fowl run on their toes, so the Lateral gastroc functions as a more proximal muscle than in humans. The DF-IV can be thought of functioning more similarly to the human gastroc. Also “The LG shortened throughout most of force production, becoming relatively isometric during the latter half of support and then stretched slightly at the end of support. In contrast, the DF-IV typically lengthened until peak force, and shortened rapidly during force decline.” (P2947-Daley). What happens to the human gastroc is more similar to the DF-IV. Furthermore “the DF-IV averaged little net work per stride on the level”.
So which on is the equivalent functionally to the human gastroc? This is a debatable point with no perfect answer, but we see the DF-IV tendon with more elasticity and the tendon doing the work, while the muscle has little net work per stride. If this is more similar to how the gastroc functions in humans, then the muscle is not powering the ankle, but rather the tendon.

I think this summarizes what we are both saying in this discussion. You are looking at your data from inverse dynamics and are saying that the ankle joint can power gait. Looking at tendon length and EMG studies it looks like the isometric contraction of the gastroc causes a stretch of the Achilles which returns energy in push off with a little concentric contraction as the resistance decreases faster than the force decline. Both are correct. The key to this is exactly what information you need to know. I think the original question is moot, as we learned a lot from this discussion and now we know what is going on. We also now know the deficiencies in any one measuring technique.

I agree. So the storage of elastic energy comes from the deceleration of the leg by the gastroc and/or soleus.

Thanks I found it. It says “The results of this study have shown that the human GM muscle fascicles produce force in a consistent fashion, despite changes in gait and incline. In each condition the Achilles tendon length (and hence force) increased with no change in muscle fascicle length. The change in locomotion condition did however alter the GM muscle fascicles’ length and velocity at which force was produced during the stance phase. Positive work was done by the GM fascicles in all conditions. This was achieved because the muscle fascicles all shortened during force decline.”Wouldn’t this mean that first the muscle is functioning isometrically putting storing energy, and then the after the tendon has put the energy into the system and is shortened, the muscle fascicles shorten with declining force? It doesn’t seem that this would be the way the gastrocs should power gait.

I can see how you can misinterpret what I said. What I should have said is that the tension in the Achilles tendon to raise the calcaneus is reduced as the center of mass progresses anteriorly.

I agree with what you are saying. Can you clarify what the “it” is that you were referring to that lengthened in the last sentence?

I can agree with that.

I agree. Of course concentric contractions are required to move how else could you move the swing leg.

It seems that the “muscle fascicles all shortened during force decline” This is not during the time the energy is being stored in the Achilles tendon.

Regards,

Stanley

 

Hi Stanley,

This was my point earlier. In looking at inverse dynamics you don't care whether it is the muscle or tendon. Together the muscle and tendon produce moment. There is no reason to split the muscle from the tendon for determining whether or not there is ankle joint push. It's all the same if the muscle powers the tendon and then the tendon powers the gait.

I agree. Between heel lift and toe off the muscles are shortening, there is a plantar flexion moment and there is a plantar flexion motion, therefore the gasroc muscle is powering gait. There is also some elastic energy stored in the tendon so that the motion of the joint can be faster than the fasicles can shorten.

Regards,

Eric

 

In reading the article there is some fasicle shortening as energy is stored in the tendon. From my read of the article the release of tendon elastic energy and the muscle shortening force are occur at the same time. I don't quite see why the gastroc should not work this way.

MTU = musculo tendinous unit (i.e. the combined tendon and muscle).

Regards,

Eric

 

Hi Eric,

Eric, it is all the same if you say the ankle helps to power gait, but if you want to know if the swing phase muscles or the stance phase muscles are what power gait then you have to look deeper.

Eric, it would be nice if it were that cut and dry. The gastroc only becomes concentric at the time of force decline. You have looked at these curves and you see how rapidly the force declines. If the force is decreasing at the time the muscle is beginning to become concentric, this means that the resistance has decreased markedly. So at the time the concentric contraction is occurring, both feet are in contact with the ground.

Where did you see that?

Thanks Eric, I am not sure I agree with the last sentence. The muscle is isometric at the time prior to heel off, which means that the tendon is lengthening at this time. The push off that occurs due to the tendon releasing its elastic energy will result in a decrease in length and tension, both of which we see.

Regards,

Stanley

 

Hi Stanley,

The inverse dynamic calculations say that the ankle plantar flexors power gait. Why must one look deeper?

I don't see how your comment is inconsistent with what I said. If you remember the article with the external swing assisst 3% of body weight was around the amount of force needed to make the leg swing. Even if that was not correct, and swing required 10% of body weight, this is still considerably less than the force needed to support the body against gravity. There is nothing wrong with the trailing leg being pushed when the force is declining. Energy is being added to the trailing leg and that energy is given to the whole body as leg slows before heel contact.

I don't understand why you are disagreeing with my statement. The point in time that I am talking about is after heel off.

Regards,

Eric Fuller

 

Eric,

It looks like we are really splitting hairs at this point. I think we are in agreement on so many aspects, that the differences are relatively minor at this point.

Because you miss important concepts. Inverse dynamics says that the moment around the ankle powers gait, but it doesn’t say what does the work. If you don’t look deeper, you would never know that the tendon has a spring effect on propulsion.

My understanding is that there is a period of time after heel off and just before double stance. It appears that this is the time of the contribution of the tendon. When the opposite touches the ground then the force declines rapidly, which means the potential to do work declines rapidly (W=FXD). So most of the work is done by the tendon, and the work done by the gastroc is a result of the other leg touching the ground and decreasing the resistance so the previously isometric contraction becomes concentric. The work done by the gastroc is almost a fortuitous finding.

I reread it, I agree.:drinks Sorry!!

Regards,

Stanley

 

Push or Pull? A Look at Running Propulsion

 

How exactly does a swing leg pull? In order to accelerate an object with a "pulling force" then that object must have a tension force acting on it in the direction of the acceleration.

How does a foot pull on the ground when it is not contacting the ground?