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

Eric,
Walking (moving forward) requires a certain amount of energy. On a 90° slope, you cannot stop. Energy is put into movement equal to gravity (32 ft/sec2). On a flat surface there is no energy put into movement from gravity. It becomes obvious that we are dealing with a sine function. So the energy put into movement is Sine of the slope of the angle X gravity. The energy of walking = Sine of a 4° slope.

We are talking about walking not standing, so if the center of mass is shifted forward, then you will have to step faster to get the leg out in front to prevent falling. The body does this by pulling the swing leg forward via the Psoas muscle, and flexing the swing knee via the hamstrings to reduce the moment of inertia

Does Winter talk about which is the preferred method for the body? Also, does he talk about the power outputs at the ankle using the gastrocnemius concentrically vs. eccentrically?

Regards,

Stanley

 

Eric,

I think we have to look at running as either accelerating or constant velocity. In acceleration the center of mass is further in front of the center of pressure than in constant velocity running. At the beginning of a sprint, the lean required to place the center of pressure anteriorly requires additional ankle dorsiflexion and the person cannot land on the heels. For efficiency, there would have to be a limitation of dorsiflexion so the lower leg does not travel backwards. At constant velocity, we have a different set of parameters. I agree with the repetitive strain of the Achilles. For the same reason there would be more firing of the gastrocnemius (which would put tension in the Achilles tendon) and resultant fatigue which I already alluded to.

Good question Eric. I have stopped measuring equinus, as I find there are three possibilities. Equinus is either present, mild or absent. In the cases of ankle sprain, it is present. How long does it last? In a clinical situation, we have a specific population where there is pathology. We do not see the self-resolvers. In my practice, I do not see the equinus resolve, but that it does not mean it does not.

Eric, please reread what I wrote, I did not say that the gastroc soleus and Anterior tibial contract at the same time (in constant velocity running).

The point I was trying to make is that as the ankle joint plantar flexes there is a greater plantarflectory moment. This is because as the foot plantarflexes the part of the calcaneus in contact with the ground moves posteriorly relative to the axis of the ankle joint. (I know this is a hard concept to follow) When this moment is resisted by the Anterior tibial muscle, the net result is a pulling of the tibia anteriorly.

Regards,

Stanley

 

No, he doesn't talk about which is preferred. My point about the foot hurting was that you have two different sources of power to move the leg forward in swing and you can choose one or the other, or a combination of both. Which you prefer will depend on the circumstances at the time. The body has a choice.

Energy is generated (chemical energy is converted to mechanical energy) at the joint when the joint moment and joint motion are in the same direction.

Mechanical Energy of body is lost (to heat) when the joint moment is in the opposite direction as the joint velocity (eccentric)

When you land from a jump and you flex your knees, the knee moment is extensor and the motion is flexor therefore there is a loss of mechanical energy. Looking at the whole situation this is obvious. Before the landing there is kinetic energy. After the landing there is no kinetic energy.

The concept of power is something that you will see in basic dynamic engineering texts. Winter's work is applying these principles to human motion and specifically walking. The work uses inverse dynamics to calculate the forces, moments and pwer at joints. These calculations are useful because you can answer questions like does the stance leg push or does the swing leg pull with existing data. It makes much more sense to use power calculations as compared EMG muscle measurements, because EMG does not take into account the direction of motion of the joint.

Joint power calculations also lets us measure some other concepts. When I was a student we were taught the concept of propulsive and apropulsive gait. An apropulsive gait had little ankle plantar flexion at toe off in gait. This gait will have low power, at the ankle, because the angular velocity is near zero. A propulsive gait will have ankle joint plantar flexion and hence add significant energy (power = change in energy) before toe off. In this case significant energy is added to the pre-swing/ swing leg. The exception to this proves the rule. A foot drop will have plantar flexion motion but low plantar flexion moment. low moment = low power.

Cheers,

Eric

 

Eric,

So the answer to may questions: Does Winter talk about which is the preferred method for the body? Also, does he talk about the power outputs at the ankle using the gastrocnemius concentrically vs. eccentrically? Is no to both.

I am curious about these calculations that will tell me all that I need to about whether the stance leg pushes or the swing leg pulls. If I have a patient in the office and I want to find out which is happening, what data would I need to collect, and what are the formulas I need to perform the calculations required to get the answer. Could you give an example?

Regards,

Stanley

 

Actually, buried deep in my overly long reply I did indirectly say that Winter addresses eccentric and concentric muscular contractions. Eccentric contractions are contractions where the moment from the muscle is in the same direction as the velocity of motion and energy is put into the system. Concentric contractions are when the moment from the muscle is in the opposite direction and mechanical energy is lost from the system. He uses energy input instead of eccentric.

An example would be the comparison of a propulsive gait versus a foot drop/ steppage gait.

But first, the measurements: You would need a force plate that could measure the vertical and horizontal components of force and calculate center of pressure. This would have to be Calibrated with a motion analysis system that could provide the position, velocity and acceleration of the segments of the body. To be really accurate you would need to calculate the moment of inertia of the segments of the body, but a rougher estimate could be done by using standardized values for moment of inertia.

The calculations: Practically, you can buy systems that have the computer that does all the calculations for you. The calculations that are done are called inverse dynamics and use Newton's second law for linear and angular motion. F = ma and moment = moment of inertia x angular acceleration. If you know the mass and you know the acceleration you can calculate the force. The same more moments.

So, in a propulsive gait there would be concentric contraction of the gastroc and soleus muscle and a high moment and a plantar flexion velocity. When this occurs there is power into system and the leg is pushed forward and or upward.

In a steppage gait, there is ankle plantar flexion velocity, but there may be little plantar flexion moment. Hence, there would be a low power output from the ankle and from the accelerations you would see that the moment to lift the leg would come from the hip.

It's hard to do the concept justice in this short post. The first site below is his textbook that is one of the clearest explanations that I have seen of it. He was the first person whose writings contained diagrams that acknowledge equal and opposite moments. e.g when the thigh applies a moment to the trunk there is an equal and opposite moment from the trunk applied to the thigh.

further reading
Winter, D. A. Biomechanics and motor Control of Human Movement 2nd ed. 1990 John Wiley & Sons, Inc. New York
Winter, DA Sagittal Plane Balance and Posture in Human Walking. IEEE Engineering in Medicine and Biology Magazine. Sept. 1987
MacKinnon CD. Winter DA. Control of whole body balance in the frontal plane during human walking. Journal of Biomechanics. 26(6):633﷓44, 1993 Jun.
Winter, DA Kinematic and Kinetic Patterns in Human Gait: Variability and Compensating Effects. Human Movement Science 3 p. 51-76 1984
Winter DA. Bishop PJ. Lower extremity injury. Biomechanical factors associated with chronic injury to the lower extremity. Sports Medicine. 14(3):149﷓56, 1992 Sep.
Winter DA. Foot trajectory in human gait: a precise and multifactorial motor control task. Physical Therapy. 72(1):45﷓53; discussion 54﷓6, 1992 Jan.


Enjoy,

Eric

 

Thanks for making me focus more on what you wrote. What circumstances will determine which of the different sources of power will come in to play normally. Not just in the rare cases of a drop foot?

I looked up the formula for power. (P=W/T, and W=F•D). Power is related to mass times acceleration times distance divided by time. Acceleration is the change in velocity (the change being a difference). You do not describe a change in velocity. What am I missing?

You have succeeded in making me more confused. :bash: I always thought that eccentric contractions were lengthening contractions, as they are resisting contractions that get over powered. So wouldn’t the moment generated from the muscle be in the opposite direction to that of the movement (velocity), or are you talking about eccentric moments around a joint being distinct from that of eccentric contraction of muscles (just as eccentric loading of bone is distinct from eccentric muscle contraction. Just like I have to be eccentric to write this stuff.:dizzy?

So we need the equipment that will make the measurements except I would have to calculate the moment of inertia of the segments. I would have to know the mass of the part to make this calculation. How do you measure the mass of each of the segments without seperating the parts :butcher:?

Eric, I thought we are measuring power. Don’t we have to know the time to move something and the distance it is moved (P=W/T, and W=F•D).

Could you expound on this?

Eric, the knee is extending, so is the gastroc lengthening or shortening? If we are talking eccentric joint moments, then it doesn’t matter. But the discussion had to do with eccentric muscle contraction, and this is important to the discussion.

In steppage gait, there is no dorsiflexion, so you brace with a dorsiflexion assist brace. If there happens to be a weakness of the plantar flexors also (as seen in MS sometimes) then a brace is made to stop all ankle motion. What this means is that there may or may not be a change in the plantar flexion moment. The position of the ankle (not moment) during swing determines the lifting of the leg by the hip.

Don’t the muscles apply the moment, not the segment?

Regards,

Stanley

 

Stanley,

A lot of questions. I hope I can get to them.

This is an individuals choice. For example, limping with a paiful foot will exhibit very little ankle push and more hip pull. My sense is that it is more energy efficient to use ankle push. Ankle push is the "spring in your step".

Time is in the acceleration (e.g. meters per second squared) part of the equation and in the power part of th equation so one of the times (seconds) in acceleration is canceled and you are left with just velocity.

My bad. I never use eccentric and concentric and got them confused. Try reading again with them reversed. I'm pretty sure I got the energy change correct.

Winter has a need diagram in his book describing the process. He also published a paper with some averages that would be scalable with differneces in size of the person. As I recall he examined the error from the estimates and found it not to be too large.

I'll try and get to the other questions later.

Eric

 

Stanley,

Some more. There's a lot to write about and I made a few mistakes by working quickly. Apologies.

the measurement. You lie on a beam that is resting on a fulcrum near the middle and a force measurement device at one end. So the beam is supported in two places. You measure the force at one end of the beam and then you move the body part and remeasure the force. For example, have both knees extended while lying face down and make the measurement. Then flex one knee and the change in distance from the fulcrum will change the force measured at the end of the platform. The mass can be calculated from this setup.

I think I addressed this earlier. The work per unit time method to calculate power gives the same result as force times velocity method of calculating power for linear motion. Same concept for the joint moment x joint angular velocity method for calculating power. There is more than one way to calculate power. Both will yield the same result.

The orginal typo read the same more moments. should have read the same for momnets. That is, if you know the moment of inertia (or have a good guess) and you know the angular velocity, you can calculate the moment from the equation moment = moment of inertia times angular acceleration.

Multi joint muscles make things much more difficult. For the sake of the example we could just examine the soleus.

One problem with the concept of joint moment calculated with inverse dynamics s that you cannot determine which of the many muscles that cross a joint are providing what portion of what moment. For example, you could have a contribution from the Achilles tendon and a contribution from the plantaris tendon to ankle joint moment. You have to look at cross seciontional area of the muscle to esitmate the relative contribution of each. Another problem is co contraction. If there is tension in the Achilles tendon at the same time there is tension in the anterior tibial tendon you will not be able to see that with a joint moment calculation. You will know the total moment but you will not know if the plantar flexion moment from the Achilles tendon is greater than the net moment by the amount of dorsiflexion moment from the anterior tibial tendon.

The above describes the redundancy problem. If you have more than one structure that could contribute a force then the total force measured will not give you the contribution of each structure. So, looking at the gastroc, at the knee we know there is a net moment, but we can't tell which portion comes from the gastroc.

When wearing a solid ankle brace, just after heel lift, the center of pressure is anterior to the ankle joint. Therefore there will be a dorsiflexion moment at the ankle from ground reaction force. The ankle is not dorsiflexing, so there must be a plantar flexion moment from some source that is equal to the dorsiflexion moment from the ground. (Net moment = moment of inertia x angular accelration. Angular acceleration and angular velocity = 0, therefore the net moment = 0.) It could come from the brace or from the muscle or part from both.

Since the angular velocity of the ankle is zero we know that there is no ankle joint power and therefore if the leg moves higher off of the ground the power for this movement came from somewhere other than the ankle.

The muscles are part of the segment. A free body diagram helps explain the situation. Take the ankle joint and divide it into a foot segment and a leg segment. When there is tension in the achilles tendon this will cause increased compressive forces at the talo tibial joint. (They have to be equal because the acceleration of the talus relative to the tibia is 0) So, looking at the foot there is a force from the tibia pushing downward on the talus and there is a force from the Achilles tendon pulling upward at the insertion. Looking at the leg there are equal and opposite forces acting on the leg. Specifically there is an upward force from the talus acting on the inferior surface of the tibia and there is a downward pull at the end of the achilles tendon from the calcaneus. When you do a free body diagram it is important accurately label the forces. If you draw those force that I describe you will see how the muscles of the segment create forces that cause a moment from one segment to act on another.

I hope this helps.

Eric

 

Eric,

Thanks for taking the time to answer my questions. :drinksI see how F•D/T is the same as F•V. I also see how you can calculate the mass from charts, or by changing the position of a part to change the weight on two different scales.:morning:

This discussion started with whether the stance phase pushes or the swing phase pulls. My interpretation is, and please correct me if I am wrong, that the two opposing theories are (in walking, not running or accelerating {sprinting}): 1. the ankle pushes via concentric contraction or 2. the swing leg pulls, and the energy put into the system has the gastroc firing via eccentric contraction to cause push off. When you mentioned efficiency, I thought I would see if I could find an article showing a difference in efficiency in concentric vs. eccentric contractions.
I didn’t find an article on the net that exactly says that, but I found one that says some interesting things that are relevant to our discussion.

http://jn.physiology.org/cgi/reprint/86/4/1764.pdf

It compared a concentric contraction with that of an eccentric contraction. It found that: Although the elbow flexor muscle activation (EMG) was lower during eccentric than concentric actions, the amplitude of 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.
What I take this to mean is that not only that it takes less muscle energy to give a similar contraction, but the brain is alerted to prepare for movements and to modify its movements based on input from different sensors in the musculoskeletal system.

It would seem that eccentric contractions would go along with what is happening in the human body, which lends credence to the "swing leg pulls" theory.

Regards,

Stanley



.

 

The ankle does push in walking via concentric contraction. This is not theory, it is experimental fact (Neptune, R.R., S.A. Kautz, F.E. Zajac: Contributions of the individual ankle plantar flexors to support, forward progression and swing initiation during walking. Journal of Biomechanics, 34:11 : 1387-1398, November 2001).

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.

 

anyone care to share? I'd like to read the full article if possible.
thanks.
Bruce

 

Eric,

If we were to just examine the soleus by itself we would be losing the effect of knee motions on walking.

I don't see this as a problem, it is just how things work.

I agree.

Thanks Eric, that clears a lot up for me. But I am not sure how this applies to: Does the Stance Leg Push or Does the Swing Leg Pull?

Regards,

Stanley

And have a Happy Thanksgiving! :drinks

 

Kevin,

Thanks for alerting me to this excellent article. Amazing how they can make a model with that complexity.

Getting back to the question that has been puzzling us "Does the Stance Leg Push or Does the Swing Leg Pull?", upon close reading of the article,

http://www.musculographics.com/pdf/2001_ankle.pdf

I draw your attention to page 1392 on the end of the page:

In late single-leg stance through pre-swing (40–60%
gait cycle), GAS and SOL both provide forward progression (Fig. 4) and deliver power to the trunk (Fig. 6), but SOL much more so. Although excitation in both muscles has ceased by mid pre-swing, both muscles produce force throughout pre-swing because muscle deactivation (e.g., Ca2+ uptake by the sarcoplasmic
reticulum) is not instantaneous. Since both muscles are shortening then, they produce positive power (Fig. 6, SOL, GAS: solid lines are positive). While the energy produced by SOL is delivered mostly to the trunk (Fig. 6, SOL: solid line E dotted line), almost all the energy produced by GAS is delivered to the leg for
swing initiation (Fig. 6, GAS: dashed lineEsolid line) rather than to the trunk(Fig. 6, GAS: area under dotted line is small).k

It appears that in this study the ankle both pushes (from the soleus) and contributes to the swing of the leg (gastroc).

This is the first article to evaluate the plantar flexors individually. As a group, the two competeing theories are Winters (confirmed by Kepple) and

By the way, I didn't read how he models the red vs. white muscle fibers. Since the white is not an endurance muscle, I wonder how long the soleus would be able to maintain this energy to the trunk.



Regards,

Stanley

 

Bruce and Stanley:

Here is a list of articles that show the fact that ankle pushes via a contrentic contraction during propulsion:

1. Neptune, R.R., S.A. Kautz, F.E. Zajac: Contributions of the individual ankle plantar flexors to support, forward progression and swing initiation during walking. Journal of Biomechanics, 34:11 : 1387-1398, November 2001.

2. 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.

3. Winter, DA: Biomechanics and Motor Control of Human Movement, 2nd Ed. Wiley, 1990, pp. 1809-183.

4. Sasaki K, Neptune RR: Differences in muscle function during walking and running at the same speed. Journal of Biomechanics 39(11): 2005-2013, 2006.

5. Zajac FE, Neptune RR, Kautz SA: Biomechanics and muscle coordination of human walking Part II: Lessons from dynamical simulations, clinical implications, and concluding remarks. Gait & Posture 17(1): 1-17, 2003.

Happy Reading!

 

Kevin,

Thanks for alerting me to this excellent article.

Getting back to the question that has been puzzling us "Does the Stance Leg Push or Does the Swing Leg Pull?", upon close reading of the article,

http://www.musculographics.com/pdf/2001_ankle.pdf

I draw your attention to page 1392 on the end of the page:

In late single-leg stance through pre-swing (40–60% gait cycle), GAS and SOL both provide forward progression (Fig. 4) and deliver power to the trunk (Fig. 6), but SOL much more so. Although excitation in both muscles has ceased by mid pre-swing, both muscles produce force throughout pre-swing because muscle deactivation (e.g., Ca2+ uptake by the sarcoplasmic reticulum) is not instantaneous. Since both muscles are shortening then, they produce positive power (Fig. 6, SOL, GAS: solid lines are positive). While the energy produced by SOL is delivered mostly to the trunk (Fig. 6, SOL: solid line E dotted line), almost all the energy produced by GAS is delivered to the leg for swing initiation (Fig. 6, GAS: dashed lineEsolid line) rather than to the trunk(Fig. 6, GAS: area under dotted line is small).k

It appears that in this study the ankle both pushes (from the soleus) and contributes to the swing of the leg (gastroc).

This is the first article to evaluate the plantar flexors individually. The prior work evaluated plantar flexors as a group. The two competing theories are Winters (confirmed by Kepple) and Hof (confirmed by Meinders).


By the way, I didn't read how he models red vs. white muscle fibers. Since the white is not an endurance muscle, I wonder how long the soleus would be able to maintain this energy to the trunk.



Regards,

Stanley

 

Stanley:

I agree, Rick Neptune's articles are all excellent and he seems to be one of the world's leading researcher in this topic currently.

It is quite obvious from the articles above that the gastroc/soleus complex both push the trunk forward and help accelerate the swing leg by their concentric activity during propulsion.

In regards to fast-twitch (white) vs slow twitch (red), if I remember correctly, the gastrocnemius is a predominantly fast twitch muscle and the soleus is predominantly a slow twitch muscle. Therefore, the soleus, being more of an endurance muscle would be better adapted to repetetive slower contractions with less chance of fatique whereas the gastrocnemius would be better suited to faster contractions but would fatigue more rapidly. This is fascinating stuff and when I was taking my exercise physiology classes at UC Davis during the late 1970s, this was already an established science, muscle fiber typing. Maybe we can start a separate thread on this......on the physiology of muscle.

In regard to the idea of many sagittal plane theorists that the gastrocnemius-soleus do not have the capacity to push the body forward during walking throughout the day, here are some questions and answers:

1. When comparing the muscle mass of the gastrocnemius-soleus complex to the combined muscle mass of the rest of the muscles of the leg, which leg muscle group mass is the greatest? (answer: gastrocnemius-soleus complex has more muscle mass than the combined masses of all the other muscles of the leg)

2. What is the largest tendon in the human body? (answer: the Achilles tendon is the largest tendon in the body in cross-sectional area)

The point of these questions is that the gastrocnemius-soleus-Achilles tendon complex is by far the most powerful muscle-tendon unit in the leg and is designed for power and prolonged activity against weightbearing forces. If the patient does not have strong gastroc-soleus muscles, as they should, they will not be able to walk normally. That is all there is to it. The gastrocnemius-soleus muscles, and their normal function, are absolutely essential for normal gait function.

 

Kevin, I hope you had a nice Thanksgiving.:drinks

Kevin, As we discuss "Does the Stance Leg Push or Does the Swing Leg Pull?", the theory that the Stance Leg Pushes would be supported by a concentric contraction. Any other contraction would support the Swing leg pulling. According to Neptune, the only time that the Gastroc or Soleus muscle is concentrically contracting is late single-leg stance through pre-swing. The rest of the time the Gastroc or Soleus muscle is not contracting concentrically. During the late single-leg stance through pre-swing Neptune says"The production of substantial energy by GAS in pre-swing and its delivery to the leg is consistent with the net role of the combined plantar flexors being to initiate swing (Hof et al., 1993; Meinders et al., 1998) because GAS delivered more energy to the leg than SOL delivered to the trunk"
If we were to think of the gait determinants, how much of the energy delivered to the trunk is to raise the center of mass at the end of the stance phase rather than for propulsion?
Overall, Neptune's paper supports the notion that both the Swing leg pulls and the Stance leg pushes. However, the majority is the Swing leg pulls (in walking).

Kevin, it looks like you were less incorrect on the muscle fiber types of the gastroc and soleus. It must be too many years for the both of us. :morning:
Soleus muscles have a higher proportion of slow twitch fibres (70%) than their synergist, gastrocnemius. The gastrocnemius contains about 50% slow twitch fibres.
http://www.springerlink.com/content/t7446n300207km7m/
The first time I heard about muscle fiber types was a lecture by Dave Costill from Ball state in 1974 or 1975. He talked about the quadriceps muscles.

Regards,

Stanley

 

My reference to efficiency was not related to eccentric vs concentric contraction, but to causing the swing leg to swing. To swing the leg forward you need a moment (force x distance) at the hip. The distance of the attachment of the Psoas to the hip is very small compared to the distance of ground reaction shear. Therefore a small ground reaction shear force (posterior to anterior force from the floor applied to the foot) will produce the same moment at the hip as a much larger force from the iliopsoas muscle. That was what I was getting at with efficiency.

There are two different efficiencies here. There is energy required to move the leg and the amount of force produced with a given contraction. You get more force with eccentric contraction. But, less force is needed when you have a longer lever arm.

By definition, power cannot be added with eccentric contraction. In eccentic contraction the moment produced by the muscle is in the opposite direction of the motion. Therefore energy is being absorbed into the muscle with eccentric contraction. Power can only be added with concentric contraction.

I don't see how this helps the swing leg pulls theory.


Regards,
Eric

 

Stanley,

Thanks for your interest.

I posted the above in response to your question about how one segment can apply a moment to another segment. This is relevant in validating the joint power concept. It also is a deeper explanation of Newton's third law of angular motion. For every moment there is an equal and opposite moment. So, when the leg applies a moment to the foot, the foot applies a moment to the leg.

Regards,

Eric

 

Kevin, Stanley,

The problem that I have with the article is that it makes the assumption that activity of the ankle plantar flexors is a constant. Winter showed that there is variation in the relative activity of the hip flexors and the ankle plantar flexors. When you use ankle plantar flexors you need less effort from the hip muscles. He also showed variations from one individual to another and variation within a single individual from one day to another.

The point of this is that every individual who has both hip and ankle muscles will choose what portion of each to use. You can verify this by walking and seeing if you can make your ankle push more.

I've said this many times, but I will try one more. Yes, the swing leg pulls the body forward during the period of gait when the center of mass of the swing leg is anterior to the center of pressure of the stance leg. That fact is not in question. Now if we look at the whole picture we can answer the question of the thread. When the swing leg is posterior to the center of pressure of the stance the swing leg will pull the trunk posteriorly. So, if there is no ankle push then there will be an equal amount pull backward and pull forward. That's ok. The momentum of the body will not be changed. With ankle push, you can take a longer stride so that the swing leg wont significantly slow the body during the first half of swing. It is possible to walk with no ankle push. It's just hard to take long strides without ankle push. Try it.

Regards,

Eric

 

Good point Eric. At this point of the gait cycle, the gastroc is bending the knee and lifting the leg. The force of gravity starts to pull down the leg (a pendulum effect) and assist with swing in this position.

Agreed

In two jointed muscles, as one joint is moving in an opposite direction greater than that of the other, even though the muscle is lengthening, there is power added to the joint moving less. To prove this, go to a train station and wait for a train to arrive. Take a bungee cord and attach it to the last car of a train. Take the other end and wrap it around your waist. Wait for the train to leave the station, and tell me that no power has been added to your body.

I wrote ”What I take this to mean is that not only that it takes less muscle energy to give a similar contraction, but the brain is alerted to prepare for movements and to modify its movements based on input from different sensors in the musculoskeletal system”. Walking requires balance. To do this you need to have sensory input and the ability to respond to it. If not, you would walk around like a drunk all the time.:drinks What this means is that eccentric contraction is required to walk with balance. According to Neptune, this happens for most of the stance phase with the exception of late single-leg stance through pre-swing.

Regards,

Stanley

 

Eric,

Didn't Howard answer that with his explanation of the crutch walking? If not can you expound on your question.

Regards,

Stanley

 

Eric,
I didn’t want to complicate the discussion with what I saw as a problem with the paper, but the modeling is in 2-D. There is no subtalar joint in his model. This means that the rotational component of resupination is not accounted for. (This is why the engineers that do research on the lower extremity should have a podiatric consult). This accounts for an increased use of the soleus in midstance in this model. Therefore, he is modeling a post triple arthrodesis patient. I don’t remember the last time I saw a patient that was happy with his triple arthrodesis, and I am sure if you asked the model, it wouldn’t be happy with the way it walked, except it has no head to respond with.

Could you send me a link to the Winter article, or somehow post it for all to read? I would like to see what you are talking about. It would seem that in walking the body wants to conserve energy for efficiency. The basic mechanisms are eccentric contraction, and something I read years ago in a Kinesiology book called the Lambrinudi paradox. Simply stated it says that all the muscles work together to shorten the leg (as in swing), and they all work together to lengthen a leg. Get up out of a chair and feel your hamstrings and quadriceps. They will both be contracting. In the article it shows that at this point of the gait cycle the other muscles of the leg are not contracting, which would lead one to eccentric contraction, except the paper shows it to be concentric contraction. The question this begs to ask is is the model wrong, or is the body doing some ineffecient activity for a reason?

I think that Howard addressed the no ankle push vs. hip swing. I also think that we all realize that having ankle plantarflexion is required to prevent the center of mass from dipping too low at push off. The question is whether it is due to the energy of the swing leg, or from the soleus concentrically contracting. The bigger question is whether the swing leg pulls the body, or the stance leg pushes.

Regards,

Stanley

 

Stanley:

Had a great Thanksgiving, thank you. Hope you also had a good one.

I did remember correctly that the soleus had a higher percentage of slow twitch fibers than the gastrocnemius. I do know that in some sprinters that the gastrocnemius has a higher percentage of fast twitch than slow twitch fibers. Thanks for the reference..that was a good one.:drinks

 

This has been one of my points. The pendulum effect, at the initiation of swing, will cause a rearward force on the body from the leg. At the initiation of swing, the swing leg pulls the body backward and at the end of swing it pulls the body forward. So, if the pendulum is the only effect there is no net gain of energy from the swing leg. Therefore you cannot make the statement that swing leg powers gait.

Power is only added if the angular velocity is in the same direction as the moment. So, I agree, it is possible that the joint moving less is adding power.

Regards,

Eric

 

It may not be a problem because the amount of STJ rotation relative to amount of ankle motion is very small. Additionally, the amount of tendon movement is related to the lever arm at the joint. The Achilles lever arm at the ankle is much greater than the lever arm at the STJ.

I'm having a good Thanksgiviing at my in-laws except for the weak internet connection. I'll try and search the links for the sites that I posted earlier. There are several papers and they are all related. I believe Winter combined them together into a book.

Well, we can sit and debate whether Howard was right, or we can look at what research has been done to answer the questions. Winter's and Neptune's work definitely show that there is no net energy gain from the swing leg. (At initiation of swing, body energy is lost to the swing leg and then regained later in swing). Winter also showed that ankle joint push happens some of the time.

I'll see what I can do about getting the articles.

Regards,

Eric

 

I agree with you here, Eric. When the swing leg accelerates forward during early swing, this motion of the swing leg will tend to pull the head-arms-trunk (HAT) segment posteriorly. However, when swing leg moves anteriorly forward of the center of pressure (CoP), the ground reaction force vector plantar to the foot will be directed superior-anteriorly (i.e. an inferior-posteriorly directed pushing force is exerted by the stance phase foot on the ground) due to the shift in the center of mass (CoM) of the body anterior to the CoP.

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. This is especially true when one carefully and objectively considers the considerable research evidence that shows that most people exert a concentric ankle joint plantarflexion contraction from the gastrocnemius-soleus complex during propulsion that occurs concurently with a posteriorly directed force from the stance phase foot on the ground......i.e. a push!.

 

I see your point now.:morning: Because the leg is behind the center of pressure, if you lift it, the body will tend to go backwards. That’s a good point, however the body is moving at a constant speed at the time this is taking place, and there is momentum to stop the body from falling backwards. From a standing start we have a different story. The body leans forward to move the center of mass anteriorly, and then raises the leg to start the process. If the energy was put into the system by the push off of the triceps surae, then you would start with a push off from the triceps surae. This is not how we start to walk.

Good we are getting closer.

Regards,

Stanley

 

The point is that we need to raise the center of mass at propulsion. One way is ankle plantar flexion. STJ supination is another. I agree it is a smaller amount, but it diminishes the amount that is required. Also the windlass effect will plantar flex the forefoot (which is something else not modeled in the Neptune paper). Again, this is a minor contribution, but it still diminishes the amount of plantarflexion required.

Eric, what happens happens. If poorly designed research shows something, then it doesn’t mean it is applicable to another situationigs:. A correct theory will be able to answer all the questions. It turns out that the idea of pushing with the triceps surae does not answer all my questions. So I looked a little deeper until I found something that makes sense to me.

As I said in my previous post that there are two muscle strategies for better utilization of energy: Eccentric contraction and the Lambrinudi paradox-neither of which are working at propulsion. That means that there is another mechanism working. We know in running that there is a spring effect of muscles that accounts for 30% of the efficiency of running.
There is also a spring effect in tendons
http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=4415
See post #3

Also Dave Smith (who I consider the world’s authority on the plantar fascia) was surprised that the plantar fascia has a greater stretch than he anticipated.

In the Neptune article:
http://www.musculographics.com/pdf/2001_ankle.pdf
P1392 states:

In late single-leg stance through pre-swing (40–60% gait cycle), GAS and SOL both provide forward progression (Fig. 4) and deliver power to the trunk (Fig. 6), but SOL much more so. Although excitation in both muscles has ceased by mid pre-swing, both muscles produce force throughout pre-swing because muscle deactivation (e.g., Ca2+ uptake by the sarcoplasmic reticulum) is not instantaneous.
This strikes me as a little weird. I thought they were using actuators.

and finally I found this article when looking for the Winter articles (he finds fault with Winter and Neptune so he uses more actuators and 3-D:dizzy :
http://www.bme.utexas.edu/faculty/pandy/Gait&Posture2003.pdf
page 163:
The ligaments crossing the metatarsal joint generated all support from metatarsal-off to toeoff

So looking at all this, the energy put into the system by lifting the leg is utilized during eccentric contraction and stored in the muscles, tendons, ligaments and fascia for utilization at propulsion to raise the center of mass.

Looking forward to it. Thanks. :drinks

Regards,

Stanley

 

Kevin,

I am glad you had a great Thanksgiving. :drinks

Regarding the strength of a concentric ankle plantar flexion contraction, I want you to remember that in walking we have a GRF of more than 100% body weight at propulsion.
http://www.cl.cam.ac.uk/research/dtg/publications/public/rph25/PUI2001_Headon.pdf
There are 1000 foot strikes per mile in running, and even more in walking due to a shorter stride length. I know you can easily walk a mile. So let us say that there is 1BW per leg, so to duplicate this in doing a toe raise using two legs, you would need to put your body weight on a bar and to duplicate a mile you would need to do 1000 toe raises starting with the foot 10° dorsiflexed .
When you do this, I would be willing to say that the swing leg somehow powering the gait forward is a podiatric myth. igs:

Regards,

Stanley

 

The foot during propulsion does not need to lift the full body weight vertically upward, it only needs to push the center of mass forward and accelerate the leg into swing. The next time you push someone on a swing, Stanley, compare how much force is required to lift their full weight so that the swing rope slackens, versus the amount of force required to keep the swing moving back and forth at constant magnitude. This swing analogy only serves to reinforce the points that Eric and I are making. Saying that the swing leg powers the forward progression of walking gait and that the stance leg does not push the body forward is a podiatric myth.

 

Kevin,

When you start to walk, the body leans forward to move the center of mass anteriorly, and then raises the leg to start the process. If the energy was put into the system by the push off of the triceps surae, then you would start with a push off from the triceps surae. This is not how we start to walk. If you were to say there is energy put into the system by the lifting of the leg by the gastrocnemius and all the leg and hip muscles, then that would be something that would make sense.
As far as lifting the body, the center of mass is raised via plantarflexion. It is one of the gait determinants.
This concept of the swing is a good one. The hip flexors do not have to work that hard to pull the leg forward. Swings store most of its energy as potential energy, and then the potential energy becomes kinetic energy due to gravity. Lifting the leg increases potential energy which is then converted to kinetic energy due to gravity. Pushing a leg whose knee is bending doesn’t seem to make a very effective swing. You wouldn’t be pushing in the correct direction. When you push someone on a swing, you push perpendicular to the ropes that are hanging down. Therefore you would be pushing downwards. The ankle does just the opposite pushing upwards, raising the center of mass.

Regards,

Stanley

 

Stanley:

When the ankle is concentrically plantarflexing during the propulsive phase of walking exerting a pushing force on the ground, the center of mass of the body is falling not, as you stated, rising. So the swing analogy still holds....the ankle plantarflexors are pushing in the direction that gravity is already accelerating the body, not lifting the body upwards.

 

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, as Eric has stated repeatedly, comes either from a hip pull or an ankle push. If you want to say that energy is put into the swing leg by the gastrocnemius, soleus and hip flexors, then I would agree with you. However, if you were to say that the swing phase limb somehow "pulls the body forward" without the posteriorly directed pushing force from the stance phase limb, then I would disagree with you.

As long as the ankle is plantarflexing at a rate that increases the metatarsophalangeal joint (MPJ) to hip distance at a rate greater than the decrease in MPJ to hip distance caused by knee flexion, the overall effect during propulsion is an extension of the limb during propulsion. However, even with no ankle joint plantarflexoin during propulsion (otherwise known as an apropulsive gait), the foot still pushes posteriorly on the ground as the center of mass of the body advances forward of the center of pressure acting on the foot. However, this posteriorly directed pushing force does not occur with the same magnitude as that seen during the normal ankle joint plantarflexion of the propulsive foot during gait.

 

I believe Winter did a paper on the initiation of gait as well. It was excellent example of using center of pressure versus center of mass to explain human movement. What he saw was from static stance at the initiation of gait there was a posterior shift in the center of pressure, then this caused forward rotation of the body at the same time one leg was lifted and swing was initiated. (the posterior shift in center of pressure could come from the anterior tibial muscle.)

As the swing leg begins its motion it could certainly have some ankle push. Sometimes people use ankle push and other times they don't. If you have a fused ankle you don't use ankle push. There is more than one way to make the leg swing.

So when tension in the Achilles, coupled with ankle plantar flexion, does aid the initiation of swing we can divide the force created at the forefoot into component vectors. There will be a horizontal component and a vertical component. The horizontal component will directly swing the leg forward at the hip. The vertical component will increase the potential energy and help the pendulum effect.

Of course, we could add the the increase in potential and kinetic energy together with the independent vectors, or we could just look at the joint moment x joint angular velocity to get the same answer.

"Pushing a leg whose knee is bending doesn’t seem to make a very effective swing." Well this is where it is important to look at the forces and moments and not just position and velocity. How stiff is the knee when it is flexing? How are we going to figure out if knee is stiff enough to allow the ankle to push the leg and thigh forward. Well, we can look at the power equations and see exactly how effective pushing a swing leg with a flexing knee is. If the positive ankle power is greater than negative knee power then there is a net push on the thigh from the ankle. (The power is positive when the joint moment and joint velocity are in the same direction and the power is negative when the joint moment is in the opposite direction as the joint velocity)

Regards,

Eric

 

Didn’t Newton say for every action there is an equal but opposite reaction? If there is a force pushing down on the ground, isn’t there an equal and opposite force which would be an upwards force?
If you remember Saunders classic work on gait, he talks about gait determinants. He discusses how ankle flexion lengthens the leg at propulsion so the transition of the center of mass is smoother. Without the lengthening of the leg, the center of mass would drop further. (Just watch someone with an ankle fusion or wearing a rigid AFO for drop foot, and you will see the hip drop at propulsion). I guess to be more accurate ankle flexion at propulsion (via the ankle plantarflexors) diminishes the lowering of the center of mass.

Kevin, now you really have me confused. The direction of gravity is down to the center of the earth. Are you saying that the plantar flexors are accelerating the body downwards? By the way regarding the swing concept, when you have a little one in a swing, you sometimes pull him back and up to get him started (like the gastroc starting the swing phase), as the impulse required to get him going that quickly by pushing would be a little traumatic.

Regards,

Stanley

 

What I meant was that the effect of gravitational acceleration at the initiation of swing is to swing the leg forward, which is the same forward direction that the swing leg has been pushing the preswing leg at the end of propulsion.

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.

 

Kevin, we are pretty close in what we are saying. However, there are minor differences and interpretations that lead us to this discussion.
We agree that both the stance and swing legs are important.
We agree that the swing leg puts energy into the system.
We agree that there is a rearward directed force from the stance phase.
I am not sure where you stand on: the effect of ankle plantarflexion on the center of mass
I think we disagree on the following:
The energy put into the system by the lifting and pulling of the swing phase leg powers gait.
As Howard said:

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.
I feel that due to neurologic and muscle physiologic reasons that the concept of the contraction of the soleus (I think we agree that the gastrocnemius is involved in lifting the leg and puts potential energy into the leg which is converted to kinetic energy [gait]) cannot power the gait cycle through concentric contraction. I feel that the main reason for ankle flexion is to lengthen the leg so as to relatively raise the center of mass (decrease the dropping). I have given the possibilities as to the mechanisms that are involved that would be consistent with the known physiologic and neurologic mechanisms.
You are basing your understanding on engineering principles, such as inverse dynamics which Eric mentioned, moments, and modeling.

I think this states our positions. Do you want to make corrections to this?

Regards,

Stanley

 

I agree with this. The anterior tibial is also important in lifting the leg via the Lambrinudi paradox. By the way, I found an excellent teaching tool on kinetics.
http://www.sportsci.com/adi2001/adi/services/support/tutorials/gait/chapter1/1.1.2.asp
One thing that is mentioned here in regards to inverse dynamics is: Such a model is based on several assumptions, e.g.: that the joints are frictionless pin-joints that the segments are rigid with mass concentrated at their centers of mass that there is no co-contraction of agonist and antagonist muscles that air friction is minimal.
This seems to invalidate the accuracy of the papers you mentioned that use inverse dynamics for calculations on human gait, as there is always co-contraction.

What determines the proportion of the horizontal and vertical components of the vector?

When you do these calculations, tell me how you are going to account for the co-contractions.

Regards,

Stanley