Zeppa-Delta Angle posts


Never heard of the Stretch Reflex (SR)? You’re probably not alone. Even though the SR was the central focus of the research I did in 1991 with the Birdcage, I have yet to encounter anyone in skiing who knows what it is, let alone how it can function to assist skier balance by maintaining the major joint angles associated with a strong stance. The SR is what enables the world’s best skiers to ski with precision and with a fraction of the effort of lesser skiers.

After Nancy Greene Raine began supporting my work in 1978 and I started to work with world class racers and coaches I began to hear the comment that skiers like the legendary Toni Sailor or Nancy Green Raine ‘knew how to stand on their skis’. This implied that the reason other skiers could not ski like the Toni Sailors and Nancy Green Raines of the world was that they didn’t know how to stand on their skis. I found this puzzling. If it were that simple (it wasn’t and still isn’t), why hadn’t someone figured out how Sailor and Raine stood on their skis and started teaching the rest of the skiers how to stand the same way?

It was also about 1978 that the story began to take root within the ranks of the ski industry that ‘the foot functions best in skiing when it’s joints are completely immobilized in the ski boot’. The holy grail of skiing, a perfect fit of the ski boot that precisely mirrors the shape of a skier’s foot, emerged soon after. In this paradigm, if tight was good, tighter was better.

Aside from the obvious contradiction (the foot functions best when it is rendered dysfunctional?), it was a good story. On the surface, it made sense to most skiers, myself included, right up until I watched Nancy Green Raine undo all the buckles on her boots and ski better than any other skier on the hill. In observing and speaking with numerous elite skiers, a consistent pattern began to emerge; they all skied with their boots relatively loose compared to the boots of the average skier or racer; a stark contradiction to the ‘tighter is better’ story. A tight fit/loose fit paradox existed. This caused me to start to question the official position on boot fit.

By 1989, I had hypothesized that the SR was the ‘secret’ of the world’s best skiers. If I were right, these skiers weren’t flexing the shaft of their boots to put pressure on the front of the ski. They were flexing their ankles to set up the static preload that enables the SR. I had concluded that it wasn’t so much that elite skiers knew how to stand on their skis, but more a case that they were able to stand on their skis in a way that enabled them to use the SR. It seemed probable to me that these skiers had acquired a feel for the SR when they were first learning to ski. Once the feel was acquired, they were able to select boots and adjust them as required to enable the SR. The majority of skiers never acquire a feel for the SR when they first start to ski because the design and structure of their ski boots prevents this. If they don’t learn the feel of the SR early in skiing, the odds are great that they never will acquire it. If my hypothesis were correct, then the entire ski industry had gotten it wrong. The Birdcage experiments validated my hypothesis.

When Steve Podborski asked me to try and invent a new ski boot that did the same thing for all skiers as the in-boot technology I invented in 1980 did for him, I needed confirm my hypothesis that the structures of ski boots were preventing the majority of skiers from using the SR. This was especially important because preiminent safety experts had raised red flags in the Shoe in Sport (published in 1987) about the lack of sound principles in the design of the plastic ski boot. They had specifically flagged the shaft of the boot.

“The lack of proper technique seem so often is not due to a lack of ability, but to an unsatisfactory functional configuration of the shaft in so many ski boots. This is particularly true in models designed for children, adolescent and women.”

  • Sports Medical Criteria of the Alpine Ski Boot – W Hauser P. Schaff, Technical Surveillance Association, Munich, West Germany

A principle objective of my research in 1991 was to valid my hypothesis that structures of the ski boot prevent the overwhelming majority of skiers from being able to use the SR.

As far as I know, I am the first to describe how to set up the static preload that primes the SR and how to configure a ski boot so it accommodates and supports the SR. In the application of the SR to skiing, it is a powerful balance mediator and a PROTECTIVE mechanism.

The science behind the SR is complex. The best and perhaps simplest way to appreciate it is to acquire a feel for it by going through a static preload exercise barefoot on a hard, flat surface and then applying the acquired feel in progressive stages while standing in ski boots. This aspect involves correcting or removing any factors that prevent attaining the static preload. The process starts by learning how to set up a static preload on the shank-angle dorsiflexion angle.

  • In barefeet, stand erect on a hard, flat, level surface as show in the left hand figure in the graphic below.
  • Relax the major muscles in the back of the leg (mainly the hamstrings) and allow the knees to move forward as shown in the right hand figure.
  • As the knees move forward, the hips will drop down towards the floor. The ankle joint will dorsiflex and the angle of the shank with the floor and the angle of the knee will increase until a point is reached where the shank stops moving forward on its own.
  • As the knees are moving forward, bend slighly forward at the waist. The angles of the shank (ankles) and knees will decrease as the pelvis moves back and up and the back rounds. If you bounce up and down lightly, your stance will return to the static preload position.


  • Move forward in the hips until you feel good pressure under the balls of your feet. Feel the whole system tighten up. You have set up a static preload on the shank of the leg. This is the foundation to build an SR stance on.

Try doing this in your everyday footwear. A number of factors  can prevent the setting up of the static preload that enables the SR. The ZeppaDelta Ramp Angle in ski equipment is a big factor as is drop in shoes. Over more than a few degrees of ramp angle, it is not possible for the SR to engage.

If you try the preceding exercise in your everyday shoes and the shoes have significant drop (toe lower than the heel), it is probably not possible to set up a static preload on your shank. Instead of stopping, the shank will keep going until it reaches the physiogical limits of ankle dorsiflexion.

In my next post, I will describe how to build an eccentric muscle contraction (EC) tensioned stance from the static preload shank angle.



Slowly, but definitely, the ski community are learning the positive boot board (zeppa) ramp in many boots is excessive and not beneficial to many of us. Excessive seems to be anything over approximately 3.0 degrees. The lowest ramp I have measured to date was 2.4 degrees in the latest Dalbello DRS boot in a Mondo 27. A size 23 is still over three degrees.

I believe most manufacturers have too much ramp in their boots. As explained in a recent post, the problem is worse for smaller boots, since as boots shorten, ramp increases. Still, even if boot ramp is correct, we should wonder if binding designers think boot designers need some design help, since nearly all bindings have positive ramp (delta) of at least a few degrees for Mondo 27 boots.

Like boot zeppas, as binding toe and heel are mounted closer together, delta increases and is additive with boot ramp. Further, there are additional changes to boot angle since binding delta tilts the entire boot, It also alters effective forward lean.

I believe when we demo skis, a portion and perhaps a substantial portion, of the differences we feel between skis, may be accounted for by the differences between ski binding ramp angles. At the very least, binding angles can corrupt on-snow testing of skis and/or boots. If you are working to get your boot setup perfectly adjusted to your preferences, why allow binding ramp variables to alter an optimal configuration?

In my opinion, the best solution is a 0 degree binding delta. If this is not achievable, at least set all skis bindings to the same delta. This may be easily, achievable. Most manufacturers make shim kits for at least some of their bindings. Shims appear to be available for most bindings used on race skis that allow incremental changes to 0 degrees delta. However, not all shops know of the existence of these shims since  kits are usually in the race catalogue, not the recreational product catalogue.

If alterations to either binding heel or toe height are made, an equal change must be made to screw length or there will definitely be an unsafe situation. Binding holes are between 8.5 and 9.0mm deep. Be certain screw threads do not extend more than that amount from the bottom of the binding.

One last thing to remember is that moving binding position forward or backward on the ski could subtly alter binding delta since the top surface of skis are crowned. Moving the bindings to a new position on the crown will affect the height of the heel and toe. If bindings are moved after setting binding delta, it should be checked again.

I always prefer to direct measure rather than use a calliper and calculate zeppa or delta. The photos below show a device I had fabricated that allows the angle of a zeppa to be measured between the two primary load points under the heel and the head of the first metatarsa (aka ball of the foot). The distance between the rods can be adjusted to these two points on a zeppa or to the 2 points of contact of a boot sole on the heel and toe pieces of a binding.


The SmartTool digital level shown in the photo below accurately reads to one decimal place.


The SmartLevel is too long to sit properly on a zeppa. The two rods of the frame that supports the level lets me avoid toe kick or any other shapes that can distort ramp angle.  I just measure the angle directly between the two points of support.

Lou Rosenfeld has an MSc. in Mechanical Engineering with Specialization in Biomechanics earned at the University of Calgary Human Performance Laboratory. His research was titled, “Are Foot Orthotic Caused Gait Changes Permanent”.

While at HPL, he assisted with research on the effects of binding position for Atomic, and later conducted research for Nordica that compared Campbell Balancer established binding position to the Nordica factory recommended binding position.

Lou is one of the invited boot-fitters on the EpicSki forum “Ask the Boot Guys” and has authored articles on boot fit, balance, alignment and binding position for Ski Canada, Ski PressSuper G, Calgary Herald, and Ski Racing, USA. He is a CSIA Level 2 instructor and CSCF Level 1 coach. He currently resides in Calgary where he owns and operates Lou’s Performance Centre. A selection of his articles may be found at www.Lous.ca.



The universal boot truisms that David puts forth in his blog is a ‘blueprint’, ‘computer program’, what ever you want to call it which is applied on an INDIVIDUAL basis. Each individual is measured with the system and then the results are applied to the boot. Unlike ‘one boot fits all skiers’ which is the current system the industry provides. I can’t believe that anyone would buy a boot based on a ski magazine test but that probably is hardly worse than the info one gets in the ski shop which gave me nerve damage in my feet (gratefully healed now since I gave up on ‘race fit’). David talks about using flat shims of varying thicknesses to fine tune ramp angle just like canting shims or duct tape are used for lateral experimentation; 2.5-6 is his starting point while I would start with zero but with current ski bindings only millimeters of fine tuning can be done when some individuals need centimeters from the current setups. Obviously if one changes the ramp angle the shank angle may have to be adjusted also which is why skiing is believing, David gives a perfect example in his reply on what lower ramp angle did for his skiing. Also the extremely high starting point on ramp angle makes it impossible for many skiers to loosen their cuff for normal forward flex because they need to be clamped tight to prevent falling one their noses; that’s where I got fooled for a couple of decades.

Having built an adjustable plate for ramp/delta in the early 2000s I can tell you one thing for sure; the skier knows instantly if things are better or worse. That by no means indicates an optimum net ramp because there are so many other aspects of the boot that are factors such as toe crunch (race fit) and ankle flex restricted to virtually zero. I started from the wrong end with ramp/delta whereas David starts in the boot first which is what I would do but took me about a decade to loosen my boot cuff significantly enough to make a difference; that due to a hip joint that was killing me from skiing. 2 months after loosening the boot cuff and removing the power strap which is only good for carrying the boots (my opinion), I was introduced to The Skier’s Manifesto and learned from that how to create an ankle glide path, free the toes, free the arches, etc. What amazes me is 2 things; first that when I first decided to build the BalancEnhancer as I call it (due to a friend’s prodding), that it actually worked , and second, how hard it is to even get skiers to try something different and the number that do try it and then don’t even try to modify their own equipment to their needs based on what had made there skiing better!!

  • Michael Pupko


A follower of skimoves posed the following;

“I’m trying to determine my optimal boot shaft angle and ramp angle given my physiology – i.e. what works best for me. I’ve done some of this work on my own by adjusting binding ramp angle (last season). What is interesting is the shaft angle of my newer Head vs. Lange boots”.

As discussed in recent posts, the importance of the cumulative effect of boot board ramp (zeppa) and binding ramp (delta) angles on stance is becoming increasingly recognized. Although binding ramp angle (delta) typically varies widely from one binding to another in recreational bindings, boot board ramp angle seems to be coming into line with functional reality in race boots. Reliable sources in Europe tell me that the boot boot board ramp angle in World Cup boots is in the order of 2.6 degrees. After I eliminated the arch profile in boot boards for a 23.5 Head race boot, I calculated the ramp angle at 2.35 degrees, a far cry from the 5 degrees claimed for the boot boards. I calculated the boot board ramp angle of an Atomic race boot of a local ski pro at a little over 2 degrees. I have also been told that shim kits are available for all race bindings that allow the delta angle to be zeroed.

The default barefoot ramp angle for humans is zero. It has been unequivocally established that anything more than a small amount of ‘drop’ (heel higher than forefoot) in footwear will have a detrimental effect on stance, balance and movement patterns. This especially true for balance on one foot, something that is fundamental to sound ski technique.

Elevating the heel relative to the forefoot will cause the muscles in the back of the lower leg to contract. Over time, these muscles will become chronically shortened. The key muscles affected are the calf muscles; the gastrocnemius and soleus. But the small muscles that stabilize the knee and pelvis are also adversely affected, not a good thing.

If I want to find the optimal boot shaft angle and compare the shaft angle of two or more boots, I start by making the boot boards perfectly flat with the transverse aspect horizontal with the base of the ski. I set the boot board ramp angles for both boots at 2.5 or 2.6 degrees. Since it can take a long time for the body to adapt to even small changes in ramp angle underfoot, the angle is not hypercritical.   I have settled on 2.5 to 2.6 degrees of total ramp (zeppa + delta) as an arbitrary starting point. Although there appears to be a positive effect of a small delta binding angle in SL and GS, I prefer to work with a zero delta angle initially since a positive or negative delta affects the shaft angle of a ski boot.

When moving from one boot model to a different model or to another boot brand, the first thing I do is remove the boot boards and calculate the ramp angles with the top surface monplanar. If the boot boards are not flat, I plane or grind them flat. If a new boot is to be be compared to a current boot with a boot board angle of 2.5 to 2.6 degrees, I modify the boot board of the new boot so it has the same angle as the current boot.

Next, I compare the shells and the angles of the spine at the back of the shaft of each boot. Even if the angles of the spines of the boot shells appear similar, there is no guarantee that what I call the static preload shank angle (more on this in a future post) will be the same.

A quick check of how the structure of the shell of the new boot is affecting the functional configuration of the foot and leg compared to the current boot, is to put the current boot on one foot then put the new boot shell with the liner from the current boot on the other foot. If a significant difference is perceived, the source is the new shell.

At this point, it may be apparent that there is a difference in the shank angles of the left and right legs when comparing the current boot to the new boot. But whether one boot is better than the other or even if one boot eanables the optimal static preload shank angle would not be known. I will explain how I identify this angle in my next post. For now, study this recent video of Lindsey Vonn starting off by skiing in what appears to be a strange ski stance. In fact, the exercise Vonn is doing is a familiar routine to me, one that I do before I start skiing – https://www.facebook.com/LindseyVonnUSA/videos/10154672700589728/

Why is Vonn skiing this way? What is she trying to do?

Also, check out this screen shot of Anna Fenninger. Note her compact, forward in the hips stance.


Finally, watch this video in which Brandon Dyksterhouse compares Shiffrin and Fenninger – Shiffrin GS Analysis – https://youtu.be/phchHWwDhdY

What do Vonn and Fenninger have in common? Why?



The reason I started this blog was to stimulate an intelligent dialogue on the issues pertaining to the design and modification of ski equipment and the formulation of ski technique based on sound principles of science. The comment below on BOOT BOARD (ZEPPA) RAMP ANGLE VS. BOOT SIZE was received today from Michael Pupko. I have underlined key statements.

Excellent presentation on boot length and the binding’s effect on net ramp. Anyone that is going to experiment with lowering their ramp angle needs to be aware that they will have to do something about the restriction on ‘ankle glide path,’ and the ability to flex their ankles within the ski boot shaft or they may end up with a false conclusion.

I didn’t have the smarts to start with the fact that the ski boot locks up the ankle and also ‘race fit’ etc. restricts all normal foot function. So my approach to not being able to flex my ankles enough to get out of the back seat was to raise the ramp angle because it does 2 things; first it ‘artificially’ gives the skier more forward lean without using up the limited ankle flex available thus getting them out of the back seat and second, from that position I found I could ‘drop’ into the tongue of the boot using body weight rather than the tiny muscles in the front of the shin that aren’t designed for bending rigid plastic ski boots. As I removed the obstacles to foot function, first by trial and a lot of error on my own and then with the help of this blog in regard to ankle glide path, allowing toe spread, and removing arch impediments I have greatly reduced my ramp down to a third of what it was at its’ literal zenith. The trouble with starting within a boot with too much ramp in the zeppa, one has to really tighten the boot cuff to prevent falling forward out of the boot. This is what David addresses before the skier steps into the bindings and those experimenting with ramp angle must do so also. I originally improved my fore/aft ‘balance’ by increasing the ramp angle but it kept my feet locked up or even increased that issue. Everything has to be fixed where the problem is to get the optimum results; adding oil to the car’s engine won’t help if the gas tank is empty.

I’ve been playing with body position while running this summer. Due to my knee issue, the only way I can run without stressing the knee is to land on the ball of my foot; forefoot strike. Going down hills this has been fine but I started playing with the position of my COM due to a discussion with a friend. I discovered that I was running downhill very much like walking down steps until I lowered my COM and that really smoothed out the foot strikes. What I realized is that in a sense I’m stepping down as if my foot is ramped but adjust it according to the slope and needs. Thus I can see why there needs to be some ramp in the ski equipment for going downhill but the industry has gone way overboard with net ramp and forward lean because someone decided to not bother with allowing natural ankle movement in ski boots. I tried to find my last pair of leather boots to see what was underfoot in them, but unfortunately had no luck. I have noticed for years from seeing old pictures of skiers in leather boots, how they flex normally like other sports. That still didn’t make me figure out that we should do likewise in ‘modern’ plastic ski boots because I believed they were made and fitted by experts that knew what they were doing. Thanks David for helping me get over that misconception!!

  • Michael Pupko

In my next post will explain why it is important for ankle flexion to occur within the confines of the boot shaft and how to determine optimal boot shaft angle.


It is becoming clear, the angle the boot board (zeppa) establishes for the skier’s foot relative to the ground, is vitally important to the ability to balance and function on skis. Therefore, knowing boot board angle (ramp angle) and skier preferences should become part of every boot setup and purchase. Yet there appears to be a fundamental error in the understanding of ramp angle in boots. This is evident when someone states, for example: “The head Raptor has a ramp angle of 4.5 degrees”. The statement may only true if the angle is linked to the boot size.

There are production controls applied to boots just as controls and standards are applied to all other things mass produced. In boots, it means the first prototypes are designed to a specific size (generally Mondo 26). All other sizes are scaled up or down from it. Each Mondo size is a change of one centimeter. Zeppas are fixed in both rear foot and forefoot height in the prototype standard. Only the zeppa length changes as boot size changes.

It means; if the prototype size is twenty six, the zeppa of a twenty three is three centimeters shorter with the same toe and heel heights. Therefore, the ramp angle of the zeppa of a twenty three is steeper than the ramp angle of the zeppa of a twenty six. Since many women’s boots are scaled from the twenty-six Mondo standard, boot set-up problems can be more difficult to solve for women than for men. This is the reason women are more adversely affected by boot configuration than men. The graphic below compares the boot board (zeppa) ramp angles of larger and smaller boots to the standard Mondo 26 boot.

Zeppas Mondo 26


Bindings obviously confer the same effect, since with most models heel height is greater than toe height. As the heel and toe change distances from each other according to boot size, binding angle (delta) changes and its angle is additive with the boot ramp angle to determine gross equipment angle as shown in the graphic below. Binding delta has a double effect, since as delta increases boot cuff angle relative the ground also increases.

Zeppas Mondo 26 bindings

When talking about boot boad ramp, we should include the boot size or always use the ramp of the Mondo 26 as a known reference.

Lou Rosenfeld has an MSc. in Mechanical Engineering with Specialization in Biomechanics earned at the University of Calgary Human Performance Laboratory. His research was titled, “Are Foot Orthotic Caused Gait Changes Permanent”.

While at HPL, he assisted with research on the effects of binding position for Atomic, and later conducted research for Nordica that compared Campbell Balancer established binding position to the Nordica factory recommended binding position.

Lou is one of the invited boot-fitters on the EpicSki forum “Ask the Boot Guys” and has authored articles on boot fit, balance, alignment and binding position for Ski Canada, Ski Press, Super G, Calgary Herald, and Ski Racing, USA. He is a CSIA Level 2 instructor and CSCF Level 1 coach. He currently resides in Calgary where he owns and operates Lou’s Performance Centre. A selection of his articles may be found at www.Lous.ca.


At the time that I designed the Birdcage research vehicle in 1991 with a biomedical engineer, I was aware that a Net Ramp Angle (boot board + binding ramp) of more 3 degrees had a significant, perceivable, negative effect on skier stance and balance. Although I didn’t know what the range of the optimal Net Ramp Angle was (and still don’t), I knew that Net Ramp Angle is affected by the length of the Achilles tendon and that this aspect affects the synchronizing of peak arch tension with peak Achilles tendon tension that occurs just before the heel separates from the ground to initiate propulsion. I refer to this as the Reference Shank Angle. It is the foundation on which to build a strong stance from the bottom up.

Since my experience prior to 1991 had demonstrated that more than 3 degrees of Net Ramp Angle was too much, a decision was made to fix the Net Ramp Angle of the base of the small Birdcage, shown below, at 2.5 degrees and the base of the large Birdcage at 2.35 degrees.

Screen Shot 2016-08-09 at 3.30.12 PM

The small Birdcage fit US men’s size 4 to 8 feet. The large Birdcage fit US men’s size 8 to 12 feet. Thus, all skiers with feet in a Birdcage size range had the same Net Ramp Angle.

Since the base of the Birdcage acted as the boot board, there was no removable boot board as in most ski boots. The base shown above was made from high grade aluminum and was in the order of many times stiffer and more torsionally rigid than was necessary to withstand the expected loads of skiing without deforming. This is an important factor that will be discussed in a future post.

Since 1978, I had suspected that the plastic shells of most, if not all, conventional ski boots were undergoing significant deformation under loads typically of racing. So I began stiffening the bottoms of ski boot shells with a torsion box structure similar to those used to stiffen skis. Recent studies have not only confirmed my suspicions, but shown that the deformation that occurs can be far worse than I suspected. Consistent with good practices of science-based research the entire Birdcage was engineered with excess structural capacity so as to ensure that it could easily withstand the maximum loads imposed on it without significant deformation as this could disrupt the processes of skier balance and control.

An important aspect of the Birdcage was continuity of the surface structure that the foot rests on. A one piece top sheet comprised of 5 mm thick high grade aluminum formed into a tub was secured to the base with screw fixations. The 3 black strain gauges shown mounted on the base (2) and side plate (1) in the photo below of a left foot Birdcage show the continuity of the surface under the balls of the feet. Continuity of the surface under the ball of the large toe is especially critical.


In order to ensure each test skier was as close as possible to the Reference Shank Angle, the start and end points of shaft rotation and the forward end point at which resistance was introduced were adjusted to peak Achilles tension – Shank Reference Angle so as to make the effect of ramp angle as consistent and neutral as possible without fine tuning it to each test skier.

The photo below shows the rotation resistance control mechanism on the back of the Birdcage.


This important aspect is discussed in detail in US Patent No. 5,265,350. FIG 56 below from the patent, shows the means to adjust the rearward travel limiter of the shaft to set the shaft forward lean angle, forward travel limiter of the shaft to set the limits of forward shank movement and journal resistance means for 10 to 12 degrees of low resistance shaft rotation.

Fig 56

A reasonable starting point for a boot board standard would include the following:

  • A ramp angle of 2.5 degrees with a  shim kit with 0.1 degree shims for the heel and forefoot to facilitate fine tuning based on ski testing.
  • A top plate surface that the foot rests on that is monoplanar (flat in the long and transverse planes) with the transverse plane parallel to the transverse aspect of the base of the ski.
  • Torsional qualities that when integrated with base of the boot shell maintain deformation of the boot board and boot shell base within agreed upon acceptable limits.
  • A top plate surface that the foot rests on that is contiguous under the heel and the balls of the feet, especially under the ball of the big toe under the the 5th metatarsal.