Understanding Crushing OTF

[Than_Bogan]

Related to some notes under MattP's OB4 review, @gator1 wrote (in part):

 

His binding system released, and he bent the bar under his boots. This means that his front foot was planted so hard that even after the rear of the HO system released, his front foot stayed on the ski with so much force that his rear foot BENT THE BAR TRYING TO LIFT the front .

 

What planted his front foot so hard? G forces of the ski decelerating crushed him down over his front foot. A release CANNOT save you in this situation. You can be standing on the ski in a tennis shoe, and once the perfect storm crushing OTF g forces are crushing you on your front foot, you are going to mash your ankle.

 

Nothing, no matter how good it is, that RELEASES, will save you in this rare perfect storm fall. You need something that EJECTS you off the ski.

 

You need a plate strong enough to pry your front foot off the ski, acting as your adjunct Achilles, and you need something to lift that plate. That's what gatormod does.

 

(end quote)

 

This is super-interesting, but I don't quite understand. If there was so much downward force on the front boot that the plate bent even after the rear pin detached, how would any system that forcibly causes the rear to eject do something different? It seems like that already happened? Or do mean he would need an even stiffer plate combined with a Gatormod?

 

I think I further understand why any local-release system cannot save you if your ankle is pinned to the ski by the forces, because effectively the "release" direction that is required is through the ski. But I'm not understanding where one force you're describing could come from. As your front knee bends past the point where the achilles can take it, what force could hold your ankle down if your boot were releasable right there at your ankle? I'm not seeing how even a huge downward force would prevent the joint at the ball of your foot from bending (as it is now the point of least resistance), thus allowing the ankle to leave the ski (assuming that it releases at said ankle).

 

I am only an amateur physicist and biomechanist, so do not hesitate to explain to me why I am completely wrong!

[gregy]

There is dynamic forces at work. If you hit a buoy and the ski goes airborne then hits the water and stops almost instantly the dynamic force caused by the deceleration would be extreme. Also does it say the injury in question is an Achilles injury. Crushing injury could just as easily cause bone breakage or lateral damage.

 

Gator design definitely addresses the Achilles over-extending but could it not be held done against the ski same as the Exo did?

 

In a crushing fall there is a torque applied by the knee to the lower leg. If you consider the balls of your foot to be the Fulcrum point so the lever arm would be a combination of your foot and lower leg. Your knee would be the point of application of the force on the lever arm. (downward force toward your foot). As you crush and your ankle bends the amount torque is going to increase due to the angle with force vector. So the torque on the ankle is increasing as you are compressing down toward the ski. The ankle or Achilles is going to be the weak point.

[gregy]

This fall on Frogman sounds similar.

 

http://www.ballofspray.com/forum#/discussion/comment/164544

 

There are some interesting comments on this thread also.

http://www.ballofspray.com/forum#/discussion/9353/what-time-of-binding-system-were-you-using-when-you-were-injured/p2

[dtm]

I agree. I don't think there is any device/system that can prevent that type of compression injury to your front ankle.

[gator1]

@Than_Bogan‌ Push your chair back from your desk (Im doing this as I type, kind of). Stand up. Put your feet heel to toe on the ground as if they were in your ski. Put most your weight on your front foot. Grab hold of the desk for balance. Now, squat, keeping your weight over the heel of your front foot. Keep squatting further until your front Achilles is stretched and stressed.

 

Now, imagine in that exact position, COM over heel of front foot, Achilles stretched like a bowstring, another Than Bogan jumped on your shoulders.

 

BAM. Your hips drop straight down. Your front knee is already past 90 degrees and therefore your front quad has very little power left to resist the weight of Than2, and your Achilles and calf does not have the power to lever your front heel off the ground. Your front knee bends more, your hips keep dropping straight down, and ping, your Achilles pops. Your hips keep dropping straight down, the ankle joint runs out of movement, and crunch, your peroneal tendon dislocates and pulls up over the ankle bone. Your hips keep dropping, and the entire ankle joint dislocates, and grinds all the carteledge off the internal joint. Finally, the bones at the front of the joint meet, compact, and crush.

 

Throughout this scenario, there is no net upward force on the front heel that could trigger a release from any binding system.

 

You are going to need your Achilles repaired, they are going to have to use a dremel tool to recut the groove for your peroneal to run in under the outside ankle bone, you'll need a plate and some screws to rebuild the bone that got crushed, and you'll need bi-annual injections of synthetic lube to replace the cartiledge that got scraped off inside the joint. When the lube quits working after 4 or 5 years, they'll fuse your ankle joint so it only moves in plane with your leg.

 

I read your post over a number of times, but I'm not sure the above scenario answers exactly what you are asking. If I missed the point, please give it another shot with different wording.

 

The conditions have to be "perfect" for the true crushing OTF to happen: COM over front heel, COM over ski, g-force directly perpendicular to ski, skier already squatting. But they ARE perfect too many times, and the older you get the less perfection is required to overcome aging muscles and inelastic tendons.

 

@gregy‌ The mod forces the plate to release from the pin at the rear of the binding, but it also lifts the plate if your front knee keeps bending and your hips keep dropping. Its designed to never let your ankle bend past the Achilles breaking point.

[gator1]

@Than_Bogan‌ I read your post again, Yes, you would need a stiffer plate so that the mod can lever the front foot off the ski via the plate. Fogman/stealth is a lot stiffer than HO. The mod @ob is testing is also very stiff after it lets the Velcro plate flex a little with the ski.

[Than_Bogan]

Perfect. That does indeed answer my question. I just couldn't figure out where the tendon got "trapped" so your (almost) static example is very helpful.

[gregy]

I'm envisioning that there could be something added to the ob4 system where a strap ran up the back the boot to a wedge under the heel on the bottom of the plate.

 

gatormod.

 

http://www.ballofspray.com/forum#/discussion/10154/gatormodded-powershell-v2-0

http://www.ballofspray.com/forum#/discussion/8089/gatormod-patent-is-filed

[gator1]

@gregy‌ Yes, the patent anticipates such an embodiment.

[Than_Bogan]

Ok, now I must engage in the extremely dubious exercise of trying to design something for a field I am not expert in ... over the internet!

 

What happens if we place a little hydraulic piston between the shin (but attached to the top of the boot) and the toe. Sufficiently compressed, this becomes essentially ridgid (but I expect should have a continuous ramp up of resistance force as opposed to hitting something solid which is effectively a threshold of force).

 

I think it's "obvious" (see lack of expertise in the field) that this can prevent an Achilles tear, because it simply stops the joint from bending any further.

 

BUT the question is what have we given up? Might such a device just cause your shin to break? I seem to recall snow skiers tried higher boots a while back and found exactly that -- no more broken ankles; broken shins instead.

 

I guess I'm now thinking of the main issue here as one of rotation. If the force on us is trying to effect a slightly backward ("fall back") rotation, then it drives into the heels. If we rotate that significantly more in that same direction (to "out the back"), then the toe pops out and no big deal. If we rotate that significantly in the opposite direction ("out the front") then the heel can release from an independent release system, just like I was envisioning before I understood the slightly backward rotation pinning the heel.

 

Would I be wrong to say that the Gatormod does effectively the second one: it takes a force that was driving your knee into the ski and changes it into a torque that creates enough forward rotation to save your ass (or your heel)?? (Man, I hope I'm right because then I understand a critical aspect of the design and why it can save something that a release-only cannot.)

 

I can't yet see how to do it, but is "all" we need a similar mechanism to introduce a forward rotation on an independent release? Maybe some analysis by you will show that my piston idea does just that, but I don't think so. Maybe we just need to point the piston(s) in a different direction(s)?

 

Man, at times like this I wish I knew everything about every science...

[gregy]

@Than_Bogan‌ suppose you could use a hydraulic cylinder mounted to the boot to actuate another cylinder that would cause the boot to release.

[gator1]

@Than_Bogan‌ The stiff pre-curved tongue on the Stealth works exactly as you want your piston to work. Lets say you wanted to buy an OB4 and crush-proof it.

 

Add the Stealth tongue to your front boot. It attaches to the toe of your boot just as it does on the stealth. It is curved away from your shin such that it only contacts your shin when your ankle is getting close to overflexing, and it is a VERY stiff tongue. That tongue replaces your piston in providing the torque as the knee keeps bending.

 

The strap that goes around the center of the tongue, which holds it tight to the "crotch" of your ankle, is attached to a lever at the back of the OB4 plate. When your ankle overflexs, the tongue hits your shin. Strap goes tight. Pulls on lever. Lever defeats OB4 release. Knee keeps crushing down. Strap pulls lever farther, now only has to overcome g force to lift your heel. So you don't break your shin. Bingo, it works just like a gatormod, only on single plate, not double. In fact, that is what is included in the patent.

 

Then, since you are a smart guy and don't want one-foot-out-one-foot-in torsional fractures in twisting falls, you pin one side of OB4 center crossbar, and radius slot the other side. If one foot releases, the crossbar swings away from the remaining foot, and its release force goes to 0.

 

 

[Than_Bogan]

The good news is: I think I understand!

 

The bad news is: I want one! :) I really can't imagine myself building that, though.

[MattP]

@gator1‌ do you have any video of your mod in a dock test?

[gator1]

@Than_Bogan‌ Well, hell, just call up OB4 and tell them you want the OB4-M model.

[gator1]

@MattP remember what happened last time I tried to upload video? :.)

 

Awhile back I started to make a video of the mod, then realized I was tired of trying to evangelize it to people and quit. But guys keep getting hurt so I start posting again. I'm kind of waiting on @OB to finish testing before I get reinvigorated. But if you want a video I'll make one tonight and send it to you.

[gregy]

I'm bring this over from the Matt's OB4 review. This seems like a more appropriate place. I think the OB4 system is a great design.

 

@gator1 has obviously put a lot of thought into this. I understand his point of the rear heel lifting causing the damage to the Exo system on that thread.

 

I think maybe a combination of heel lift and torque. My thought is that if it was pure rear heel lift the bend in the beam would have been under the rear heel. The bend is clearly under the ball of the rear foot which is the pivot of the torque when force is applied at the knee. If there was a torque on the rear foot it would result in a down force at the pivot point and a torque applied from the bend forward. These together would actually push the plate down harder against the ski (from the pivot point forward).

 

I want to add this: Gator, your mod to the these single plate release, I think makes the system many times safer, much more than I even thought at first. Looking at the way that failed anybody using a single plate release is putting themselves at a huge risk without gator1's mod. Not sure how its going to play out with the velcro release but in my option anybody using a Exo, stealth, frogman etc needs to be using Gator's modification. And I don't know Gator1 and have no interest in promoting his product. I just see a serious weakness in these systems that it address. The straps or ropes from the knee to the lift mechanism will need to be fairly strong. What it does is transfer the force of the front knee away from the front ankle to the back of the plate making the front of the plate the new pivot point (instead of the front ankle).

[gator1]

@gregy‌ : Rem acu tetigisti.

[mmosley899]

Interesting thought lines here. However with the combination of torque and heel lift, the OB4 system will release. You should not have the release tension set at a pressure that exceeds the tear strength of your Achilles' tendon. Individual release plates proved to be most reliable with less stress in testing. No OB4-M needed.

[gregy]

@mmosley899‌ are you'll going to have booth at Nationals? If not I'd like to get with you and look at your setup first hand. It's amazing that its taken so long for the water ski industry to evolve compared to snow skiing.

[gator1]

@mmosley899‌ : ".......with the combination of torque and heel lift...."

 

But that is the point of all this internet rambling. In a crushing OTF, there is no heel lift.

 

While there is torque in the right direction to cause your binding to release at the front heel, the g-force crushing the heel to the ski is in the opposite direction. And, the g force crushing the heel down on the ski is greater than the heel lift caused by the torque created by the strength of the Achilles. The result in no heel lift force.

 

Your binding (one of the best) only senses net heel lifting FORCE. It does not sense bending moment on the ankle, or load on the Achilles, or the angle the ankle joint is being forced into. Since it does not sense these inputs it cannot respond when one of these variables exceeds the strength of the joint or tendon or ligament.

 

You could set your release tension to 0 and it would not release in a perfect crushing OTF.

 

How often your customers will face a perfect crushing OTF is a different question. The one time I did in 40 years of skiing was more than enough.

 

 

[gregy]

@gator1 yes on the front foot and if my thinking is correct the rear heel lift and torque would drive the front part of the plate even harder into the ski making a single plate system even more dangerous in a Crushing OTF. The gatormod gives your ankle a chance, without it its toast. In the case of the Exo failure the front attachment point is a good 3 plus inches in front of the ball of the foot so it effectively lengthens the lever arm and increases the torque that achilles and ankle joint will have to overcome. If I was on a single plate release I'd be talking to Gator.

 

Hey gator what you get your degree in. I never got to brake legs in school, sound interesting. We just played with plastics, metals, and such. I was a ME concentrated in Mechanism design.

[mmosley899]

@gregy‌ yes, we will have a booth at Nationals, and i look forward to showing the OB4 System to you and everyone else.

 

@gator1‌ extremely unlikely that our release would not release in any type OTF fall, the problem is having both feet on one plate... See you at Nationals.

[gator1]

@gregy‌ BSME at Uof I Champaign/Urbana. We got to do all kinds of weird stuff.

@mmosley899‌: How would your design release in a perfect crushing OTF as I laid it out for Than? If it would, I'd buy one today.

[BraceMaker]

@‌gator1

 

@mmosley899‌

 

I think one part I'd want to study is how much displacement might be generated "fore/aft" with a system with a front spring vs. a fixed front mount.

 

Say OB4 Vs Reflex. In both situations you have a front boot that releases (lets focus on a RTP and ignore dual boot scenarios)

 

In the perfect crushing OTF, does the boot get pushed forwards on the ski - and if so would it compress the pin on the OB4 loosening the boot heel ledge?

[gator1]

@BraceMaker‌ Very perceptive. In a perfect crushing OTF, there is no forward compression. However, in a slightly less than perfect crushing OTF, there is. And the less perfect, the more compression.

 

This means that OB4 has an advantage over Reflex in less than perfect OTF. The perfection required- dead nuts centered COM, dead nuts downward load over ankle-to hurt you is more perfect in an OB4 than that required to hurt you in a Reflex.

 

So your odds are better in a OB4.

 

Additionally, on the two feet in world, the levers on the gatormod pry the plate forward and up at the same time, so they also work with the natural compression of the front pin to increase your odds.

[gregy]

@bracemaker, I was hoping you'd get in on this. This is exactly what I was thinking. If there is any forward momentum, the ob4 system should compress the front release freeing the heel. At this point how strong is the ankle joint/achilles, force is applied by the knee on the lower leg. Either the torque lifts the heel and frees the leg from ski or you continue to crush and the ankle fails. Not sure what the answer would be there and it will most likely depend somewhat on the individual crash and angles of forces. I think the Ob4 system looks very promising.

 

The Reflex system is going to be forced into the loop with forward momentum, I'm thinking this could trap the boot down against the ski. I don't have any evidence of this, just speculation.

 

The cool thing about gator's design is it does 3 things. 1) limits the range of movement the ankle can experience. 2) causes a release of system. 3) It takes this huge force that is generated from the impact and resulting deceleration and uses it in a positive way - spreads the force of the knee on front lower leg and uses it to lifts the plate from the ski.

[BraceMaker]

What I am angling at is your example with a gym shoe.

 

Consider a crop of extremely low tension waterski boots.

 

Prior to said crushing OTF there is that moment where it appears the person will OTF, the tibia is advancing, the knee is flexing and you are heading towards a crushing OTF.

 

No matter how perfect this extremely low tension release should still toss the boot free prior to the crash.

 

What I am convinced happens is that the release tensions are set high enough to prevent pre-releases almost all hold you through this zone into the danger zone.

 

Your gator mod has I believe the ability to force an otherwise stable release to let go early. Another concept would be to locate the boot on the plate so it was confined from rotation/sliding etc. separately from retaining it on the plate.

[gator1]

Yes, the mod forces the release to let go earlier, perhaps keeping you from getting into the perfect OTF position. And then, if that didn't work, and you keep crushing, the mod pries your foot up off the ski.

 

The cool part is that we are not injured by heel lift force. We are injured by dint of the position our ankle joint is forced into. The mod releases based on position, not on force. And, in fact, YOU CAN RUN YOUR RELEASE SETTING HIGHER and be safe. You don't have to walk the fine line between inadvertent release caused by a loose setting and injury caused by an overly tight setting.

 

This is especially intriguing on a Velcro setup.

 

This situation is interesting to contrast to a snow skier. You cannot set the release at the toe and heel of a snow ski boot loose enough to protect an ACL while retaining the ski as you hit ruts at high speed. In order to protect an ACL you need to measure forces at the knee, and release based upon THOSE forces, not the forces measured at the toe/heel of the boot. Can't do that yet. (I'm working on it though).

 

With a mod on a water ski, we have the advantage of being able to sense the strain on the weak point in our system (ankle not knee) and release based upon that strain level, rather than the load on the heel of the boot, which is a piss poor proxy for the ankle load.

[dchristman]

I just woke up this morning thinking about crushing OTFs (what's wrong with me !?). I don't recall this being discussed before - @gator1‌ I'm wondering how much you think the release at the human/handle link contributes to the perfect storm scenario. Does the super-human grip provided by the extra leverage of clincher style gloves and accompanying potential delayed release greatly increase the chances of this type of fall? What about a power-vest?

 

It seems to me if you don't try to hang on to something your hands can't handle, your risk of this type of fall is greatly reduced. Unfortunately, perhaps also your performance. How much of the pure momentum of the body rather than compression from pulling on the rope can be attributed?

 

The Goode April Fool's video struck a chord ... has anyone ever suffered a COTF (new abbreviation) with a flat wooden ski and old CG style rubber bindings? probably not. In the quest for performance, new mechanical advantages move the weak link to a new point in the body. Will we not be happy until we have an Ironman Slalom suit?

[gator1]

@dchristman‌ Yes, you are a sick man.

 

We just had another blown tendon out here in the PNW, W4 nationals skier classic COTF. So maybe you're not that sick to be thinking about it. Seems like we're on the one a week pace like last year.

 

I think, in order of magnitude:

 

1) better skis. Old woodies wouldn't hold an edge enough to create COTF

2)Age. 50 is the new thirty, but your tendons don't agree

3)Bindings, more restrictive means less than perfect COTF will still get you

 

I don't think its better handle/hand interface, because its almost all momentum But I have no data to back that up.