Discussion: 1 extra # is 25 extra # on your knees

Here is something what I read in Nov-2006 issue of the "Runner's world"

Each extra pound of body weight is equal to four pounds of presue on the knee when walking and 25 pounds when running. (Source: Arthritis and Rheumatism)

Another reason to stay fit.

What is the definition of "extra pound[s] of body weight"?

What is the definition of "extra pound[s] of body weight"?

Article does not elaborate on the definition. But I figure if I used to be 160-170# and now I am 195# then - based on formula - my knees are getting 625# to 875# extra to work with when I attempt running. Sounds kind of scary.

Althouigh there's no denying that weighing more puts more stress on your joints, this particular quote sounds to me like old bits of wisdom along the lines of "if you swallow chewing gum, it stays in your stomach for seven years". I'm still trying to figure out exactly what it's claiming in order to decide if it makes any physical sense. Part of the problem is that "pounds" is not a unit of pressure. I'm guessing that they're referring to peak forces, but these factors of 4 and 25 seem pretty high to me. One way or another though, if you weight 10% more, the loading on your knee is going to be 10% higher. So if you go from 175# to 200#, that could mean that the loading increases by 625#, but bear in mind that this would be a change from to 4375# to 5000#. (Well, a bit less on both coulnts, because you can deduct the weight of your lower leg.)

So should I stop carrying water on my runs? Is it bad for my knees? Oh God, I can't believe I've been trying to stay hydrated for all these years while all I was doing was destroying my poor knees.

This is the sort of nonsense that arises when people try to quantify things that any reasonable person intuitively knows. Heavier is bad. We all know this.

One of the reasons I've been driving my weight down this year is because my forefoot (which endured two broken bones a couple years back) has taken up the nasty habit of aching really bad on long runs and rides. I figured that less weight -> less pressure -> further into the run/ride before it becomes a problem. Seems to have worked, although it still aches sometimes.

http://health.dailynewscentral.com/content/view/00...

http://www3.interscience.wiley.com/cgi-bin/abstrac...

Nonsense or not but some people get paid to come up with such numbers

Well, the abstract is not very well written, although I don't know what the full article looks like. The mass-media reporting on this is awful. This business about 48000 lbs per mile? If I put a one ounce weight on my desk, and leave it there for a day, it will be there for 86400 seconds, so there will be a cumulative weight of over 2.5 tons? I don't think so.

This does not sound correct if u look at it scientifically.
If you were exerting a force on your knees 25 times grater than your weight then the resultant force on your body would be 24 times your weight upwards.
Given energy is absorbed by your muscels and tendons and even bones you would still have a force pushing up on your body many times greater than that of your weight.

This sounds impossible to me because for one reason why arnt our steps when running lanching us high into the air.

I will atmit the forces are greater but the only way to measure this accuratly would be on a treadmill with a senson to measure the force of each step downwards.

By newtons third law for every reaction their is an equal and opposite reaction. In this case upwards on your body through your knee.

Now lets c ppl tell me i did not study hard enough for my science final.

Actually, I have no problem believing that the strike force is close to 25 times your body weight, probably more when running downhill. Your foot is moving downward fairly quickly and in the space of about 1/4" comes to a complete stop and loads with all your weight. That force has to be in the tons (albiet for a very short time).

Again, so what? Our bodies have evolved to handle it and we can see by example that many overweight people are capable of running quite a distance without injuring themselves.

Naturally, that risk is less when you are lighter, although there is surely some point well above zero where bone and muscle mass have deteriorated to the point that any further weight loss would render them too fragile.

Having written a few NIH grants myself, I can assure you that getting paid is no guarantee of non-bogus research.

I am almost through my physics degree.
And using various formulae, this result actually makes very perfect sense. I won't post the math of it unless someone actually wants to see it. It would probably be slightly lower than 25, somewhere around 20 for distance(orienteering), th 25 is more for a shorter distance.

Roughly speaking, you'd have to be looking at loads of 25X if your foot were on the ground for only 4% of the time. You certainly do get this kind of effect for racehorses, which is why they so easily break bones when things get out of proper alignment. Seems like 4X for walking and 25X for running are on the high side, but I had been thinking about the kind of running that I do, but you're right that they're probably referring to sprints.

This doesn't improve my opinion of the article, though.

Also keep in mind that most studies on running are done with people that are actually not technical runners but just weekend runners. If studies were done with runners that have actually been coached and use proper technique, those forces would be a lot less.
Most runners have never been coached and tend to land on the heel with the foot in front of their center of gravity, forcing the leg to break the stride and decelerate the body to speed zero. They also spend a long time on the ground, taking their body from behind that foot, moving it over and beyond the foot, once again increasing the amount of forces and stress absorbed my the body. Redo the study with a very technical runner and those forces would be reduced by quite a bit.

Very interesting analysis, Marie-Cat....uncoached runners decelerate to speed zero with each step? I always pictured myself gliding along with great efficiency....not starting and stopping in a herky-jerky manner.

However, for us "Super G" types, may I mention one good reason to carry some extra weight? Particularly as an older orienteer: Bone health.

All older women and most older men find they must begin to worry about osteoporosis and osteopenia - a decrease in bone mass, enlargement of bone spaces producing porosity and fragility, that results from disturbance of metabolism as you age. I was recently shocked to be told, after a lifetime of running and cross-country skiing, that I had early osteopenia in my spine at age 65. A recent QCT bone-mineral density test <http://www.nof.org/prevention/index.htm> was very good at determining this.

Weight-bearing exercise can be very good at delaying this process. And if you're fat, you naturally get more weight-bearing exercise, hence good bones. True?

When I took anatomy and physiology class 11 or 12 years ago, my professor mentioned how strength training during one's younger adulthood helps develop high bone density and thereby helps delay/prevent osteoporosis. However, she said that it is necessary to use great resistance, approximately 90% of one's maximum lift (in weight training terms).

Regarding the force of impact when running, if it's really in the thousands of pounds, doesn't that mean that we should be able to support those thousands of pounds of weight, for example on a leg press or squat machine in a weight room? The most I've ever leg pressed was 700-something pouinds (nominal weight on a machine), so thousands of pounds seems like a lot, but I guess we're only talking about a fraction of a second, taking place while the leg is mostly extended and therefore in a position of muscular mechanical advantage.

By the way, I read the title of this thread as, "one extra number is 25 extra number on your knees". That confused me for a minute!

i think the bones/cartilage/stringy bits absorb quite a bit of the force - one can jump down 6 feet but not up.

I think there's some confusion between peak force and average force. Obviously, the average vertical force is equal to one's weight or, as previously noted, you'd go flying up in the air. However, for most of the running stride, the force is zero because the foot is not on the ground.

J-J's assertion of 4% contact assumes that the force is constant throughout the period the foot is on the ground. This is certainly not true - we all know the "jolt" that travels through your body with each foot stike. The foot is on the ground for quite a bit of time after that. The peak force during the impact only lasts for a tiny fraction of a second. The muscles do play a part in resisting this force, but it's essentially a "negative", in that the muscles are catching your weight and resisting while extending rather than contracting. Muscles can hold off much greater forces than they can push, especially for very short intervals. As Mr. Dobbs pointed out, the skeletal support structures bear a great deal of the force as well.

All these little spikes do add up though, and it's one of the reason that pro cyclists can race day after day whereas a marathon runner needs weeks to recover.

"J-J's assertion of 4% contact assumes that the force is constant throughout the period the foot is on the ground. This is certainly not true - we all know the "jolt" that travels through your body with each foot stike. The foot is on the ground for quite a bit of time after that."

I beg to differ, the foot is actually on the ground a very short period of time. And again, using physics, your foot is causing/reacting to forces to accelerate your whole body back up and forward to the same verticle speed that you had coming down.
If you look at a superball that compresses and rebounds back up, it is more or less the same thing with the leg and foot. Though there is a bit of a curve to a force - time graph, peaking in the middle do to the compression. It looks like an inverted parabola. There is always a significant force acting. And the slope of the parabola would be relatively small so the force is close to constant.
IMO force is the wrong way to analyse this though. Work, which takes distance (that the foot and shoe compress) into account too, that would give a more realistic answer because we know that shoes, compress to help with the shock, and or feet compress slightly as well.

Just to clarify, I meant that if your foot were on the ground for only a short time, then the forces would indeed have to be large, not that large forces would imply that your foot is off the ground most of the time.

Clark, I like your therory. The heck with trying to build muscle mass, let's get fat, works just the same - at least for bone health ;-)

I'm used to seeing short-duration forces analyzed using impulse/momentum rather than work/energy. I've been trying to stay out of this because the 25 pound claim doesn't have proper units and therefore it's not credible. If it is a pressure, then I suspect the bearing area is 0.04 in^2, and a 1 G runner would have a pressure of 3325 psi, and a 1G+1lb runner would have a pressure of 3350 psi, just standing around on one leg. 0.04 in^2 is a square 0.2 inches (5mm) on a side.

Can we just go back to talking about nail polish and lipstick now? I just found this new orange that matches the Silva flags perfectly, but sort of clashes wtih the red of the old ones... anybody would know what to do?

I don't know, Barbie. Too much nail polish and lipstick will add extra weight your knees will have to deal with. I guess that explains why men or faster then women in general...

Point taken! I am taking all this nail polish off as we speak! Watch me fly now!

Damn, I don't have any nail polish or lipstick to remove. Guess that means I'll never have a chance to fly past the plastic wonder.

Would a dramatic hair cut accomplish the same thing?

Would a dramatic hair cut accomplish the same thing?

I think so. If you ever watch track and field competitions - most of the elite sprinters shave their heads bold...

Unfortunately, my wise employers consider shaved heads for females to be "faddish", which means... I'm stuck with hair.

I am Canadian, and it was once explained to me that pounds were not mass units but force units, equivalent to newtons, so I thought it was in correct units for force.

The original quote said 25 pounds of pressure. I find engineering in metric units to be easier than "traditional" units. There is a whole category of trick questions professors can ask regarding how you account for the mass or force usages of pounds. The short answer is pounds is a force or weight, and not a mass. The longer answer is we sometimes use it as a mass. When that happens, I convert to metric to ensure I don't screw it up.

Yeah, I am all about the metric, everything makes sense and conversions are easier, prefixes and factors of 10.

Yes, pounds is force. Stones is the English unit for mass.

Nope. Slugs is the English unit for mass. Stones is also force, I believe.

A stone is 14 pounds.

The discussion so far has ignored the most significant effect of all. Between the push off and landing, the center of mass of a runner is marginally further from the center of mass of the earth, hence in a lower gravitational field, hence, time runs faster. Therefore, the change in momentum of the runner during the hang-time is greater than we have previously assumed (dp = F*t). On the other hand, the runner must cancel this momentum during the time that their foot contacts the ground, when the center of mass, and particularly the location of the knee, is in a larger gravitational field. Thus, the force provided by the runner is integrated over a shorter period of time than we have previously assumed, implying that the force is higher.

Obviously, you should run on stilts.

But wait, there's more to it than just general relativity! Your knee is in a quantum superposition of the states |broken> and |unbroken>. Every time you take a step, you perform a measurement of the B operator (B stands for broken, duh). By this flawless reasoning, if your steps were infinitesimally separated in time, the likelihood of measuring your knee in the state |broken> would go to zero since the knee would not have time to evolve from the state |unbroken> after each measurement (wow, quantum mechanics proves that a faster cadence is better!). However, the time between steps is finite, so, taking the above argument into account, we need a quantum theory of gravity to describe the evolution of the state of your knee.

[insert string theory or loop quantum gravity here, while keeping your wits about you]

=> 42.

I asked a Harvard physicist, and he said this model isn't disallowed by any known measurement of the Higgs scalar field, so it must be right.

The answer is: left-over Halloween candy!

Brendan, you _are_ a Harvard physicist. Talking to yourself again?

it was a collaboration...

If you're going to open the door (so to speak) of quantum superposition, then why not also ask, "Does the knee really exist, and are its actions selfishly motivated?"

WOW!!! they actually know stuff at Harvard physics!! I am surprised to find this out. At McGill, we really just rag on about the slackers at Harvard and MIT, we have been told what little work they do. Oh and if you see Horowitz or Hill, or that other H guy that all worked on the Art of Electronics text, tell them there was an error referencing one of the figures from the excercise book to the text, and for the third edition they should bold the in text excercises, they are hard to find when italicized.

So since I have spent most of my life right side up, my legs are older than my upper body!!! Wow!!! I've never thought of that before!

No, it's the other way around. Time runs slower in a stronger gravitational field. The extreme example is the surface of a black hole, where time stops, so things get trapped at the surface, instead of falling to the center. Or at least that's the conceptual picture. Who knows what actually happens. Anyway, so your legs are younger because time has been running slower for them. Good luck trying to measure it, though.