The innovative aspect of the FreeMotion ski boot appears to be a U-shaped spring flex-system for the shaft of the boot that is minimally affected by temperature and buckle closure tension and an exo skeleton shaft system that does not deform significantly under load. The arms of the U-spring running along both sides of the shell lower appear to act like rails in transferring force applied to the shaft to the shovel of a ski. Given the stated importance of ski boot flex and the universally accepted position that flexing the shaft of a boot applies force to the shovel of a ski to make a ski turn, the FreeMotion should have been hailed as a breakthrough technology and widely embraced. But this does not appear to be the case.

Simon’s request for assistance, in conjunction with a recently published paper on the  flexural behaviour of ski boots has provided an opportunity to explore this aspect in detail.

The design of ski and ski touring boots should consider three key elements: performance, safety and comfort. The performance of a ski boot is often equated with its (forward) flex index (my emphasis added). A parameter used by nearly every manufacturer ranging from 50 (soft) up to 150 (very stiff). Despite the widespread usage (of the flex index) there is no regulation on how to measure these stiffness indices and it is up to the manufacturer to test and rate their models. 

Whereas industry and special interest magazines  regularly perform and publish ski performance tests, very few systematically derived knowledge is available on ski boots. This is surprising as ski and boot are influencing each other’s mechanical behaviour and should therefore be treated as a system (my emphasis added).

Flexural behavior of ski boots under realistic loads – The concept of an improved test method – Michael Knye, Timo Grill, Veit Senner

  • Technical University of Munich (TUM), Sport Equipment and Materials, Boltzmannstraße 15, D-85748 Garching, Germany – 11th conference of the International Sports Engineering Association, ISEA 2016

The authors of the above cited paper note that usually boots with high flex indices are used by more experienced and skilled skiers whereas for beginners softer boots are recommended.

Based on what we have been told for decades, this makes perfect sense. More experienced and skilled skiers have stronger muscles and are more precise than beginners. Stiff boots allow more experienced and skilled skiers to make better turns because stiff boots enable them to apply more pressure to the shovel of a ski to start it turning.

Studies cited by the authors have shown high activation levels for the m. triceps surea and m. gastrocnemius were measured for various skiing situations.

The triceps surae (aka the calf muscle) is a 3-headed muscle comprised of the m. soleus and the m. gastrocnemius. These two muscles form the major part of the muscles of the (lower leg). The two muscles share the Achilles tendon that inserts into the calcaneus.

The graphics below show the m. soleus and the m. gastrocnemius.


Based on the studies cited by the authors, it seems obvious that the m. soleus and the m. gastrocnemius muscles are instrumental in flexing the shaft of a ski boot.

But then the authors cite an apparent paradox when they state:

Muscular activity of the lower leg is also affected by the boots flexural behavior showing a higher activation with softer boots.

Why would the muscles of the triceps surae show a higher activation with softer boots than stiffer boots? In the current paradigm, this doesn’t make any sense. If the muscles of the triceps surae are responsible for flexing the shaft of a ski boot, shouldn’t they show a higher activation with stiff boots than with soft boots?

One explanation for the apparent paradox is that the paradigm of boot flex is just plain wrong.

…. to be continued.


  1. Sandy is correct to a certain degree; when the skier is way out of balance (due to excessive ramp angle in the ski boot) it is necessary to have an extremely tight boot to remain standing. Many skiers ski incredibly well under the circumstances but would ski even better with better lower limb function. (Yeah I proved that, about 50 years too late but better that than never!) Just because a person wins first in a ski race doesn’t necessarily mean they are in the optimum balanced and functional position, just means they got down the fastest. (Shiffrin wins by seconds? Doesn’t mean she can’t or doesn’t want to do better. Watching her ski the slalom vs. others is night and day though!) The only way to know is to have a scientific method of checking optimum stance outside of the boots and transferring that to the skiers equipment and then fine tuning under real skiing conditions if necessary. David gives us that. I have proven it by following his instructions to the best of my ability; proving to me, not him, some things I thought were pretty hare-brained (removing large sections of the liner for example) which has my feet warmer than ever and functioning at a level I never knew was possible, not only in my ski boots but in everyday life. A good test for that would be to wear your ski boots for a day from the minute you get up to when you go to bed in full ‘lock down’ mode and see how well stiff footwear teats you:) It took a while to figure out after removing all arch impediments and increasing the toe spread and ankle glide path I was able to hike up the hill behind the house (after skiing down of course) in my ski boots better than my barn boots. So finally I realized if I applied the same to my barn boots I should be able to get rid of my chronic knee pain. I went up one size on the barn boots so I’d have toe spread and removed all arch impediments, have a dense foam piece under my fore foot to reduce toe drop (instead of the fluffy original) and now can walk pain free in barn boots for the first time since 1974:) THe only person you are limiting is yourself!! Try this tuff by following the well laid out instructions, you’ll love it when you get it right!! ( or even a bit ‘righter’)

    1. “Many skiers ski incredibly well under the circumstances but would ski even better with better lower limb function.”
      > How about natural lower limb function where the lower limbs and balance system maintain an upright posture by simultaneuslty balancing opposing forces, in the case of skiing, opposing torques in 3 planes?
      The problem is that activities like skiing and ice skating are not natural. If you try to strap your foot to a ski or ice blade by wrapping straps or some sort of unstructured medium foot (I classify what conventional ski boots or skates do ‘indiscriminate envelopment’ because there is no strategy to achieve optimum function), skiing at any reasonable level would be impossible. The fact that the human lower limbs need to be modified by attaching external appendages such as skis and ice blades in order to ski or skate opened the door to claim that rendering the foot and leg dysfunctional by immbolizing and supporting it was necessary. Necessary morphed into normal. But normal is usually far from optimal. If you put a person in leg irons or bind their feet, they will still be able to move. They just don’t move well. If you put everyone in leg irons, the way they move can be categorized as ‘normal’. But normal is not natural and normal is especially not optimal.

      My recent patent recognizes that the 3 degrees of freedom in the human lower limbs compromise function in terms of rotating a bicycle crank which requires one degree of freedom (like a piston) with the axis of the ankle joint congruent with the axis of the pedal platform. The technology I invented modifies the function of the human lower limbs to make it specific to cycling. Arch supports and indiscriminate envelopment only serve to compromise lower limb function because they fail to address the underlying issue. The same is true of skiing where specific forms of constraint need to be applied to discrete areas of the foot and leg so as to intregate the ski with the lower limbs so it becomes part of the skier system.

      1. Well constructed reply and agree that ‘natural lower limb function’ is a great way to describe it. I think there is also an optimum ‘lower limb function’ which can be applied when going barefoot; or not and then issues develop. What I mean is that as David describes how early life foot wear ruined his feet and taught all the wrong ways of walking even when barefoot. This wouldn’t be as likely in a society where barefoot or minimal foot wear is prevalent because the children don’t get taught the wrong way of walking which becomes the ‘new normal’, unfortunately not natural. That is what the evolution of ski boots has done. If you take natural limb function and then build the footwear from that ideal around it you get more functional cycle shoes, ski boots (which David had done)that perform better for the specific sport than simply walking which is fine because you don’t have to ski in your bike shoes (unless they work better than your ski boots). Ha, ha, I’m sure they do!! Then there also needs to be the changes made to everyday foot wear, then we’ll see less of the epidemic of knee and hip replacements that come from the everyday grinding of our joints misaligned by present day torture devices; 99% of todays foot wear!!

      2. Most assume that footwear is benign and that the pains they experience in their knees, pelvis and/or lower back are just a normal part of life. But once one has transitioned to barefoot, minimal shoes and foot strengthening exercises such as the short foot and the pains in their knees, pelvis and lower back suddenly vanish, it becomes obvious that the shoes were the cause and that the footwear is a defective product.

        Makers of footwear have been flying under the radar screen of product liability for decades. But this could change in a big hurry as emerging technologies that can run on smart phones allow researchers to study the effects of footwear on foot function and foot strength. One area of focus is balance in the elderly because falls incur huge costs on the system for treatment and rehabilitation. So when a study shows that dense insoles with with the right textures can dramatically improve balance and reduce falls. the study is really an indictment of the insoles that the proprioceptive potentiating insole replaced. This is the deer in the headlights that shoe makers don’t see coming. How long before lawyers clue into the fact that the effect of shoe and insole construction on foot function and balance can be proven beyond any reasonable doubt? How fast can you say, “class action lawsuits?”

  2. its an eccentric muscle activation in response to little or no support in dorsiflexion by the boot cuff or clog. this is not necessary at the same level in a stiffer boot. Stiffer boots are actually more efficient in this output regard.

    1. Here’s some bed time reading to help you rethink your position on:

      Active regulation of longitudinal arch compression and recoil during walking and running -Luke A. Kelly, Glen Lichtwark, and Andrew G. Cresswell

      Dynamic loading of the plantar aponeurosis in walking – Erdemir A1, Hamel AJ, Fauth AR, Piazza SJ, Sharkey NA.

      The Foot’s Arch and the Energetics of Human Locomotion – Sarah M. Stearne, Kirsty A. McDonald, Jacqueline A. Alderson, Ian North, Charles E. Oxnard & Jonas Rubenson's_Arch_and_the_Energetics_of_Human_Locomotion

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