The most important information you will ever read about Running Form: Passive vs. Active

I've been hanging onto this post for a while now, as I wrote most of it a couple months ago for a coaching friend.  It seemed like a relevent topic and a good enough time to post it here for the rest of you.

It might seem like I’m being overly dramatic with the title, but the following two concepts are critical for understanding running form, or even human movement in general.  With the rise in popularity of running form and the increase in running form guru’s that accompanies that, I it was a good time to share what I feel is the most important lesson.  Why?  Because if you change mechanics and don’t know what you’re doing, you are begging for an injury.  As always, I’m deeply in debt to the master when it comes to this topic, Tom Tellez.  The two key lessons to learn are:
1.       What happens Active versus Passive
2.       The difference between Static movements and Dynamic movements.
What’s so important about these concepts?  Well, quite frankly it’s what separates the knowledgeable from the quacks.  It’s relatively easy to watch a lot of good people run and figure out in a general idea what good running form looks like.  What’s harder is to figure out what the runners are actually doing to get to that point.

Passive vs. Active
                The concept is relatively easy, but sometimes difficult to grasp with all the scientific type stuff, so I’ll try and keep it to a minimum.  The idea is that in dynamic movements like running, things happen both actively and passively.  It’s a little more complex than that, in that there is a large interaction between the two, but let’s stick with making it simple.  By active, we mean that muscles have to be actively contracted to accomplish some task.  For instance, in doing a bicep curl, we know that the muscles in the arm work together to lift whatever weight is in your hand.  Essentially, it’s active if you’ve got to try and do it (even if it’s so ingrained that aren’t actually “thinking” about it to accomplish the task)
                Passively is a little more complex.  You can subdivide passive into passive mechanics, and reflexive.  Passive mechanics are things like momentum or inertia, where it doesn’t matter really if it’s a human or not, objects will act in various ways.  Think back to your High School Physics class for this stuff.  Reflexive is exactly as it sounds.  It’s those things that happen without you actively thinking or doing anything.  It happens as a result of something else.  The common example is the test where they tap right below your knee in the doctor’s office and your foot kicks out a little. 
                If we look at the various passive mechanisms we have in our body, you’ll see things like the stretch reflex.  Which should essentially be thought of as a rubber band.  The muscle-tendon unit is stretched and then snaps back really quickly.  You’ve got the stretch shortening cycle, which basically means that if you stretch the muscle a bit before contacting, you’ll get a bigger power increase.  The typical example is the calf muscle during any sort of hoping.  When you hit the ground, the calf is stretched, storing energy, and then contracted as you push off. A practical example would be a counter-movement jump.  If we were to measure jump height, what would you do?  You’d start standing, then squat down and immediately explode upwards.  If you  instead squatted down, held it for a few seconds and then exploded upwards, you would not jump near as high because you didn’t take advantage of the SSC.
                There are other largely passive mechanisms that aid us in running.  We could look at the elastic energy storage and transference through the body that occurs when you strike the ground, or we could look at the interplay of the motion in the upper and lower body which can aid in movement and force production.  But…those topics are for another post, as I don’t want this to become an all encompassing 20 page post.  So we’ll focus on what I’ll call the big 3.
Passive applied to Running:
                So what does this actually mean in terms of running?  Well part of the running stride is active, and part of it relies on passive stuff.  If you are changing someone’s mechanics and you are trying to copy some other runner, and you change something that he is doing largely passively, then you just screwed yourself.  Because now you’re using muscles you don’t need to, which increases the energy cost, and increases potential injury likelihood.  Let’s take a look at the running stride and help Identify what happens passively, or at least mostly passively.
Kicking your butt:
                When athlete’s sprint or run pretty quickly, most have a high back kick where the foot folds up and almost hits their butt  as it passes underneath them.  This is a good thing because it increases the angular velocity of the lower leg.  But we don’t get there by thinking about actively kicking ourselves in the butt…Why?   
                Because the reason the foot goes close to the butt is because of the inertial force that the thigh exerts on the lower leg.  Once the thigh is extended as far back as it’s going to go, it then shoots forward (for reasons we’ll discuss shortly).  Well, since the thigh and lower leg are linked, the thigh changes directions a tad “sooner” and moves faster (because it’s closer to the rotation point…the hip) then it has a profound effect on what the lower leg does.  The thigh essentially “drags” the lower leg with it.  The faster the thigh moves, the more it “drags” it.  So what’s the effect that we see?  The faster the thigh moves forward, the more the lower leg folds up to the butt.  It’s simply an inertial force.
                If you want to get creative and see it for yourself, get some sort of simple 2 link segment to mimic the upper and lower legs.  I’ve used a longer metal hinge before.  Just play around with it making sure to rotate the “upper” part of the hinge/linked system and see what the lower part does.

                Okay, so we got that lesson?  The thigh speed partially determines what the lower leg does.  (I say partially because body lean, the angle of the hips (the joint which everything “rotates” around) also will influence it…but that’s for another post).  So move the thigh forward faster right? Wrong!
Moving your thigh and lifting your knee:
                The problem with trying to really move your thigh forward is that once again, it largely occurs passively.  The stretch reflex at the hip plays a large role.  As the thigh/hip complex moves back rapidly as you apply force to the ground, it creates a stretch reflex effect so that once extension is complete, the whole thigh shoots forward. As I’ve mentioned previously, it’s best thought of as a pulling a slingshot back and then letting it go.  As you apply force to the ground and the hip extends/thigh moves back, you are essentially pulling the slingshot back.  Once you extend far enough back, then you “let go” and the thigh shoots forward.
                The best practical example of this is when you take patients who have spinal lesions, put them on a treadmill, and then manually force the thigh/hip to extend.  If you extend the hip enough, then let go, the leg will “magically” come forward through the swing phase in a close approximation to walking (and the lower leg will fold up slightly due partly to the inertial force exerted by the thigh).
                So, trying to move your thigh through the swing phase faster is a foolish thing to do.  It’s trying to make something that is largely “passive” into something active.  The same goes for lifting the knee actively.  If the thigh comes through with enough speed, which will be a result of the stretch reflex and the inertial force of the thigh on the lower leg (because if the inertial force is great, then the lower leg will fold up towards the butt and increase the angular velocity of the whole leg coming through…).  Put another way, the greater the force of hip extension/moving the thigh backwards by application of force to the ground, the greater the knee lift will be in the front.  Why?  Because the thigh would shoot through “quicker” because of the greater stretch reflex.  Thus…why when you sprint, you generally have a higher knee lift then when jogging…You’re not extending the hip/thigh as quickly or as much.
Arm Swing
                While there are a lot of other examples, one I’d briefly like to discuss is arm swing.  The reason I’m mentioning this is it’s relatively easy to feel the stretch reflex at play.  In this case, swinging your arms backwards will create a stretch reflex that aids in moving the arms through the forward swing phase.
                It’s pretty easy to feel.  Go stand in front of a mirror and just start stroking your arms.  Start with an easy tempo and then increase the tempo and range of motion.  If you are swinging your arms back appropriately, you can easily feel the effects of the stretch reflex.  Just mess around with it for a while and you should be able to get it.  This whole arm swing mechanism is why you primarily focus on the backswing and stroking your arms, not bringing them forward.
                One last thing I’d like to mention briefly about arm swing, is that the arms and legs work in concert.  Arm swing is a great way to control tempo and stride rate/length.  For example, why do people open up the arms and stroke them when kicking it in? To increase stride length because the arms are going through a bigger range of motion, the legs will too.
                Just a quick tip, the arms and legs counteract the angular momentum of each other.  Upper body rotation/arm swing problems can be used to identify lower leg problems and vice versa.  Most of the time what I’ve found is if you fix the arm swing, the legs will compensate and “fix” too.  It’s why you’ve got to take a whole body approach and not isolate.
So what the heck do I worry about?!
                You’ve got to make sure everything is in the right position to work correctly.  The biggest lessons are learning what NOT to do.  That being said, this isn't a post about how to change form.  The quick and dirty cheat sheet guide is to pay attention to the following:
1.       Body position –slight lean from the ground, everything in alignment, moving forward, not wasting motion.
2.       Foot strike (not only where on the foot, but more importantly where in relation to COM)
3.       Hip extension once you hit the ground (Downwards and a little back.  The key is starting hip extension once the foot hits the ground and ALLOWING it to happen and not trying to rush it or cut it off short…or as I like to invoke my inner info commercial…just set it and forget it…)
4.       Arm stroke- arms are your gas pedal.  Use them to control the tempo. 
5.       Relaxation

Static vs. Dynamic
                Passive and Active are very much related to static and dynamic.  If you’ve suffered through this post long enough, you’ll realize that the body has to work differently in a dynamic state then a static one.  The stretch reflex, SSC, and inertial forces for example all rely on relatively quick movements.  They don’t happen if we move really slow through the same range of motion.  That means that the way we work dynamically is completely different then how we might work statically.
                This has a couple major implications.  First, you can’t really use static tests or static range of motion to tell you much about a dynamic movement.  My favorite example is that Carl Lewis couldn’t touch his toes, yet watch him long jump and his dynamic range of motion was incredible.  If you don’t believe that story, I’ll demonstrate on myself…

Ask anyone who has run with me and they’ll tell you my static flexibility is horrible.  I’m completely useless.  Even when I wear high socks to run in, I can’t come close to being able to bend down and touch them.  The pictures below illustrate this.  In the first one, I’m actively raising my heel as close as I can get it to my butt.  In the second one, I’m pulling my heel to the butt as far as I can without feeling like my quads about to be ripped.  It's pretty apparant that if you looked at these pictures, you'd realize my static range of motion sucks...it's horrible... :)
 

 
But, what happens when I sprint?  Well, my heel somehow is able to make it right under my butt with no problem.  It doesn't feel like my quads going to rip or like there's anything abnormal about the range of motion it's going through.  It is a perfect example of Static VS. Dynamic!



The point in mentioning this is to not fall into the trap of doing stuff like static flexibility or range of motion tests and then using those to explain someone’s movement in a dynamic situation.  For example, if a runner doesn’t have a high back kick and a trainer/coach notices they have horrible static hamstring flexibility, the conclusion is often that the kid needs to stretch more so he can get that range of motion…Well that would be wrong.  Runner’s muscles are often “tight” statically because a tight musculo-tendon unit is “stiffer” which means it can work as a spring better, storing more elastic energy than a completely loose muscle.  Our body isn’t dumb; it tries to become efficient at what we do.  That’s the reason why research studies show that runner’s with worse sit and reach scores have better Running economy.
Does that mean no static or dynamic stretching? No.  Because remember that most of our mileage is run at relatively slow paces, so we primarily adapt to that.  That’s why you need some dynamic flexibility work, or more importantly some sprinting or faster running where the range of motion is much higher throughout the year.

To finish the static vs. dynamic section, I'll quote a dissertation by James Smoliga (2007) where he said, and backed up with research:
"Functional MRI imaging has revealed coordination between the upper and lower body to be a complex task controlled by multiple areas of a motor network, distributed across cortical and subcortical regions of the brain61. Coordination of the arms and legs is task-specific, with a reflex pathway active during locomotion73, 112, but not during tasks performed while standing or seated73 "

So What?
  The whole point is to be informed.  It's easy to look at a picture or a video and realize what you see.  But what matters is the why and how what you see is happening.

Not being able to distinguish the above things is what seperates the so called "guru's" and the people who actually know what they are doing.  Sorry if it sounds harsh, but with the rise of the barefoot running stuff and the interest in running form, there are a lot of people trying to make $ of runners who don't know any better...

The danger with using EMG:
Lastly, a section I’ve added on because Pete Larson requested it…EMG is a way we measure electrical activity in the muscle. EMG simply tells you when there’s an electrical signal going to the muscle.  So it can basically be used to tell if the muscle is “on” or “off”.  It’s a tremendous tool.  What it doesn’t tell you is important to understand.  It doesn’t tell you where the nervous impulse originated, whether it was through higher motor centers, from a Central Pattern Generator in the spine, or as part of a reflexive adjustment based on feedback.  More importantly, it doesn’t tell you what the muscle is doing.  Just because it’s “active” doesn’t mean it is being used in a primary contraction/force production way.
                For instance, if we look at the concept of muscle tuning in the lower leg, an EMG might show that there is some pre-activity going on in the muscles of the lower leg for example before we even strike the ground.  That doesn’t mean the lower leg muscles are necessarily being actively contracted by the individual to dorsiflex the foot or try and get the “force generation” going early.  Instead, what is going on is that the whole lower leg complex is “muscle tuning” meaning it’s adjusting the characteristics of the musculature such as stiffness to prepare for the force absorption that will occur when the foot collides with the ground.
                Another great example is in the hamstring muscle group during the swing phase as the leg swings through, unfolds and is getting ready to touch the ground.  As the leg swings through, you often get an EMG activity signal.  This is sometimes interpreted as the hamstring being active so that the lower leg unfurls and then there is a large active pawback where the runner tries to contract the hamstrings a ton to get the foot moving in the same direction as the ground.  What’s really happening is that as the leg comes through and the lower leg unfurls, the hamstring is simply active to slow down the lower leg unfurling.  Because the thigh and lower leg swing through with a lot of momentum, the hamstrings work to counteract the lower legs unfurling momentum so that it doesn’t just keep going until the leg is straight.  If the hamstring didn’t contract to provide just enough “braking” while the lower leg unfurls, we’d have a lot of runners landing with their whole leg in a straight line far out in front of them.

These are only two examples, but many more can be given.  The “core” is an excellent example that I’ll try and delve into some other time.  Additionally, it is hard to seperate out the stretch reflex mediated signal from a higher level motor signal.  The stretch reflex will elicit an EMG response, so unless the study is designed to, it's hard to figure out how much the stretch reflex (or even if one contributes at all) contributes to the EMG signal. The point is that EMG only tells us when the muscle is on/off, not what it’s role is or where the signal is coming from.

29 comments:

  1. Anonymous9:10 PM

    great food for thought and again, thanks for sharing. Great post.

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  2. Great post Steve.

    As I am sure you already know, ChiRunning has been teaching both the active and passive points for some time. The hip extension recoil from pelvic rotation and a heel lift "whip" are primary examples of passive points. The author suggests that "allow is a huge study word in ChiRunning"; and describes what can happen passively if they let it. It does of course take practice to develop both the alignment and relaxation needed for the passive movement to occur.

    If you study any martial art including T'ai Chi, you also know that active/passive are key concepts there as well. A key principle is simply more active from one's center, more passive (or cooperation) everywhere else.

    Thanks for a great post.

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  3. Hi Steve,

    Great post. Explaining motion with via active or passive functions is a great way to look at human movement. I worked with EMG and exercise for ten years and I could not agree more with the limitations you highlighted.

    I have a related post on hamstring and knee flexion during running that explains some of the research behind some of the concepts you discussed if you or your readers are interested.

    http://thebodymechanic.ca/2011/02/02/running-biomechanics-the-knee-is-not-flexed-by-the-hamstrings/

    All the best,

    Greg

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  4. Great post Steve,
    I've been using Tom Tellez's idea's on running form since I came across your blog over a year ago.
    After spending over 2 years trying different methods of running form-Pose,Chi Evo, BK etc I came to the conclusion that they were all highly flawed!
    I feel upset that runners are getting taken for a ride by these Guru's.
    I'm really well educated now on running biomechanics thanks to you and always try and steer everyone I know away from said money grabbing Guru's.
    Keep up the good work.

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  5. I have a question regarding the typical Mach drills that runners do: As and Bs and such. Does what you are saying here render them pointless? Because it seems to me that the movements that we teach there (high knees, lift heels to butt, the swing through) are all what you call reflexive or passive. So is there any benefit to trying to practice them actively? I guess you could focus on bringing the leg down and driving the back leg instead... Maybe there is still some neurological benefit, in some way? Am I way off here? What kind of drills do you do with your group?

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  6. I was thinking the same thing.
    Are traditional drills not useful as a dynamic warm-up and drills like high knees must strengthen the hip flexors, resulting in a stronger reflex action.
    So Steve are drills useful?

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  7. Thanks for the comments all-
    Greg- Excellent post. thanks for sharing.

    Rick and Montreal- I like saying drills are useless to overemphasize a point. The reality is drills are great for a dynamic warm-up, increasing dynamic range of motion, or even specific strength like Rick mentioned. What they aren't good for is changing form. A lot of coaches just throw a crapload of "form" drills and think that they are working on form. The reality is that does almost nothing for form changes. What it does do is help with warming up, strengthening, increasing range of motion, and working reactiviley if the right drills are done.

    The point is to know what whatever you are doing is actually doing...Drills aren't for form work, they are for other stuff..

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  8. Really good and insightful article once again on the running mechanics. For someone who has been try to change the running form while also going towards minimalistic / barefoot running, your current and previous articles on running mechanics have been extremely valuable. I completely concur with you when you say there are a lot of quacks out there, who make a lot of claims about the proper running form.

    I have a somewhat related question to the posters above. Would there be any benefit in doing some sort of strength training directly for the hip flexor with weights? Or should short sprints / strides / drills take care of that aspect?

    Once again thanks a lot for sharing this wealth of information

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  9. Anonymous10:15 PM

    You help alot of people who run. Thanks.

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  10. Steve, your comment that runners with poor sit and reach scores having better running economy makes me wonder about yoga as a supplement to running - do you think practicing yoga does more harm to a competitive runner than good?

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  11. Anonymous5:01 PM

    @Lisa - Most yoga styles involve actively and dynamically stretching the muscles, interwoven with brief periods of completely passive "in"activity, such as in savasana and various meditation and restorative postures. Combining this information with the information from the above article, it would seem that adding in yoga which involves dynamic stretching would fit in nicely with a running program.

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  12. Steve, thanks for a very interesting and helpful post. I would imagine that one element that helps in having the "right" passive elements in our running is to have the appropriate balance of strength and flexibility in key areas such as the hip flexors and glutes.
    In suffering through a recent hamstring injury, the therapist I was working with that most professional athletes in any sport involving full-body motion (baseball, golf, basketball, running, etc.) have the ability to initiate motion from their glutes instead of their hamstrings or quads, thus developing greater power and reducing the risk of injury in the generally-weaker leg muscles. This is not something one can consciously do, it has to come from having strong-enough glutes that the natural tendency is to initiate motion in this manner. This would seem consistent with the points you make in your post.

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  13. Nice post, Steve. One comment - you discuss the stretch reflex that occurs during the swing phase....are you aware of any studies that investigate this reflex in runners? My qualm with your example is that it deals with spinal cord patients. These patients are ambulating at a much slower speed than that of runners. There is no doubt in my mind that this phase of gait is mainly passive for them. My thoughts are that hip flexion might be more active at either higher speeds or uphill. That the stretch reflex is not enough to overcome gravity when running up a steep slope. Would be interested to hear your thoughts. Thanks.

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  14. Rich D1:10 PM

    This is a lot of great food for thought, Steve. I am really intrigued by the discussion of arm swing. I had never really thought about it. Your points about focusing on the backswing because of the stretch reflex (an SSC in the arms, like a boxer cocking back for a punch?) and about the arms coordinating with the legs are fascinating. It occurs to me that the (left) arm backswing coincides with the (right) footstrike. I wonder if there is a connection between the power of the backswing and the power of the footstrike, that is, if a power footstrike and power backswing go together. This seems plausible to me. I don't know what to make of the observation, but I love thinking of the whole arm-leg system powering the stride. It may even be that a way to focus on stride/form is to focus on the power of the backswing, as the leg power will naturally follow. If you had any thoughts about that, I would love to hear them.

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  15. I've been doing a lot of DWR (deep water running, not touching the bottom of the pool) to supplement my training. Usually high-intensity intervals of 1 to 5 minutes, rest 30 seconds to 1 minute.

    Your post makes me wonder just how running-specific these DWR sessions are. No touching the ground certainly means not amortization of any stretch reflex. To enter "flight phase" the leg is "actively" lifted by the hip flexors. Hip extension is a "kick-back" motion. Hmm.

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  16. Fantastic post, thanks.

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  17. Anonymous11:20 PM

    Excelente artigo!!!!!

    Parabéns.

    Marcelos

    marceloscorrendo.wordpress.com

    ReplyDelete
  18. Great article. I hope you don't mind, we've linked to it over on our blog, where we often discuss running form.

    readingrunningredsox.blogspot.com

    Have you considered a follow-up post about the five things you should worry about?

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  19. Great points on upper body arm swing, I often tell my runners to focus on the back swing and have the front swing take care of itself. This confirms I'm teaching the right thing. Thanks Steve.

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  20. great info. I love searching this site. I was wondering what exercises or drills you would recommend to improve and strengthen the stride? Thanks.

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  21. At the risk of sounding stupid instead of just uninformed, when you say

    "Foot strike (not only where on the foot, but more importantly where in relation to COM)"

    what exactly is COM?

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  22. Anonymous4:51 PM

    I often print out lots of fitness info, sometimes hundreds of pages, from various websites that I find informative. I usually expect to find 1 or 2 pieces of useful information for every 100 or so pages I read but I found lots of great stuff in this one article and what's more I actually understood it. Great work.

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  23. yeah agreed with everyone amazing article just book marked this blog really appreciated thanks for sharing

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  24. Brilliant article. My running coach (I'm not at a particularly high level) often uses my running as an example to the other group members because I have a good heel lift. I then see the other consciously flicking their heels up to the point where they're creating motion in the wrong direction. I try to explain that I do it subconsciously and if anything I'm thinking more about lifting my knee and moving forwards with as little lateral movement as possible. I'm going to forward them all a link to your post and take a hinge / leg simulator to play with too.

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  25. Anonymous1:28 PM

    So, is a high heel lift just window dressing for slow jogging, making it look pretty rather than actually improving function? Maybe running well involves moving at a certain pace--faster than 8 min per mile, for example...

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  26. Really useful and interesting article!

    I tried the pose method and whilst it helped fix some issues in my form like the usual suspects - heel strike, center of mass, posture, lean etc what I really struggled with was feeling sufficient propulsion and finding a way to stop my excessive push off/bouncing running style.

    Your article was just what I needed. My hips really needed working on and telling what to do a lot more whilst my legs/feet needed to stop working so hard. In effect changing the focus from passive hip extension/active recovery to active hip extension/passive recovery is really helping me to get the desired propulsion and calm down the bouncing/excessive push off.

    Thanks!

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  27. Anonymous5:57 AM

    "then the lower leg will fold up towards the butt and *increase the angular velocity of the whole leg* coming through"

    No, it will not increase the angular velocity of the whole leg, only of the thigh. The angular velocity of the lower leg decreases so that the total angular momentum is exactly the same regardless of the knee angle (supposing an idealized physical model with the same input energy). The reason one running form is better than another is strictly because of muscle composition, i.e. some muscles are stronger and have better elastic properties than others. In this case flexing the hips is more physiologically efficient than extending the knees.

    What's the next thing that running form physiologists will tell us? That gravity pulls us forward? Oh wait...

    -physicist

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  28. Very true on the arm swings. Can you watch this video by balanced runner? I'm not exactly sure if they are correct in what they are saying about arm swings.
    http://www.youtube.com/watch?v=9zkapadQwHk&lch=email_reply&feature=em-comment_reply_received&lc=ULtPzivjVqULZryJnP1c4dMTmeKjoMg1pL6LpkLf7DU
    They say you are strongly mistaken. I don't believe that.

    ReplyDelete

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