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.