The subject matter of this post may be too technical for some readers. The impetus for the subject matter was the recent publication of a paper by a group of Australian researchers on the findings of research they did using an arch-loading device. This is the first study of its kind that I am aware of. The reason the study is significant is that in 1980, only months after reconstructive ACL surgery, Steve Podborski used an arch-loading device I invented that not only allowed him to compete after being told he was out for the season, but to win on some of the most difficult downhill pistes on the World Cup circuit. At the time that I conceived this device my thinking was not so much in the context of a arch-loading device but in the context of a device that prevents the arch from the unloading that occurs during typical ski maneuvers in response to perturbations in ground reaction force. In studying the potential mechanisms of knee injury, it seemed probable to me that load-unload cycling was putting stress on the knee and possibly decreasing the effectiveness of the muscles that act to protect the knee. Steve was easily able to ski with minimal knee stress with a Lange ski boot fit with this device whereas he was unable to ski at all using a conventional ski boot.
US Patent No. 4,534, 122 – August 13, 1985 – Filed December 1, 1983 – Inventor: MacPhail
“According to one aspect of the invention, there is provided a fit and support system for sports footwear, comprising a lower support shaped and dimensioned to bear evenly against the dorsal surface of the foot………”
“The system of the invention applies significant pressure to the dorsal (upper) surface of the foot over the instep………. ”
With the exception of overt pathological conditions, the story that the arch of the human foot collapses under the weight of the body, implying that it is somehow intrinsically weak and in need of support, is a myth of gigantic proportions. From an overly simplistic perspective, the arch of the human foot roughly equates with a 3-dimensional architectural truss system. As with all truss systems, it functions in a compression/tension relationship. The arches of the human foot are compressed by the weight of the superincumbent body (ergo, the body stacked over the foot or feet). The soft tissues resist the compressive load by becoming tense. When the weight or load is removed from the foot, the ligaments and muscles, which have been stretched in tension, recoil and, in so doing, release their energy. Here is an abstract of the Australian paper. A fee is required to obtain the full paper which is 10 pages long. I have bolded sections that are pertinent.
Intrinsic foot muscles have the capacity to control deformation of the longitudinal arch; Luke A. Kelly, Andrew G. Cresswell, Sebastien Racinais, Rodney Whitely, Glen Lichwark
Published 29 January 2014 doi: 10.1098/rsif.2013.1188 J. R. Soc. Interface 6 April 2014 vol. 11 no. 93 20131188
The human foot is characterized by a pronounced longitudinal arch (LA) that compresses and recoils in response to external load during locomotion, allowing for storage and return of elastic energy within the passive structures of the arch and contributing to metabolic energy savings. Here, we examine the potential for active muscular contribution to the biomechanics of arch deformation and recoil. We test the hypotheses that activation of the three largest plantar intrinsic foot muscles, abductor hallucis, flexor digitorum and quadratus plantae is associated with muscle stretch in response to external load on the foot and that activation of these muscles (via electrical stimulation) will generate sufficient force to counter the deformation of LA caused by the external load. We found that recruitment of the intrinsic foot muscles increased with increasing load, beyond specific load thresholds. Interestingly, LA deformation and muscle stretch plateaued towards the maximum load of 150% body weight, when muscle activity was greatest. Electrical stimulation of the plantar intrinsic muscles countered the deformation that occurred owing to the application of external load by reducing the length and increasing the height of the LA.
These findings demonstrate that these muscles have the capacity to control foot posture and LA stiffness and may provide a buttressing effect during foot loading. This active arch stiffening mechanism may have important implications for how forces are transmitted during locomotion and postural activities as well as consequences for metabolic energy saving.
COMMENT: The last sentence applies to skiing which is predominantly a postural activity as does the reference to the LA arch becoming maximally deformed and muscle stretch (eccentric contraction) plateauing at 150% of body weight. In other words, the arch of the foot reaches its minimal height at 150% of body weight. There are significant implications in skiing to preventing the arch from becoming maximally deformed through devices placed under the arch that intentionally prevent this.