It is important to note that, as far as I have been able to ascertain, the two University of Ottawa pressure studies were not funded by the ski industry. Nor was the study that used the six internationally certified Canadian ski instructors presented at a ski seminar or symposium on ski science. Instead the study was presented at an International Symposium on Biomechanics in Sports.
The following are excerpted from the study presented at the symposium. I have underlined key statements and added comments in brackets ( ).
- The averaged maximal pressures were relatively similar for all types of turns ranging between 28 and 38 N/cm 2 (Table 2). The maximum pressures obtained during all types of turn reached up to 45 N/cm2 for one of the subjects while performing Short Radius turns. (7 N/cm 2 higher than the averaged maximal)
- The trajectory of cop was consistent between both feet for all turn types. Results showed the cop following a near linear trajectory for the Dynamic Parallel, Short Radius and Basic Parallel turns. This trajectory had the cop move from the head of the first metatarsal at the beginning of the turns, and progressively migrate towards the medial aspect of the longitudinal arch near the end of the turns. (COP stayed close to the head of the first metatarsal)
- Interestingly, the average peak pressures are quite similar for all types of turns. The similarity is more evident when the higher end turns (Short Radius, Dynamic Parallel and Giant Slalom) are being considered.
- Another equipment variation which may have affected in-boot measurements, is that some subjects (n=5) wore custom designed footbeds, while the other did not. (This comment suggests that researchers noted significant differences in the pressures between those who wore custom designed footbeds and those who did not. But the study did not present the pressure data. Nor is it possible to draw conclusions without conducting comparative tests with flat and custom designed footbeds.)
The portion of the arch, that forms the beam I referred to in my last post, is the medial or longitudinal arch (LA). The paragraph that follows is excerpted from the paper, Intrinsic foot muscles have the capacity to control deformation of the longitudinal arch. – Kelly LA, Cresswell AG, Racinais S, Whiteley R, Lichtwark G. 2014: J. R. Soc. Interface 11: 20131188. http://dx.doi.org/10.1098/rsif.2013.1188.
The paper was published in 2o14. I have inserted my comments in the text.
“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.
The graphic below shows 3-dimensional views of the three largest plantar intrinsic foot muscles referred to in the above study in the same L-R order.
(click on graphics to enlarge them)
COMMENT: The Ottawa pressure studies found high pressure under the heel and head of the first metatarsal of every elite instructor in both studies. Some instructors applied higher pressures than others. The study cited above found that the recruitment of muscles peaked at 150% of body weight (1.5 Gs). The University of Ottawa studies found some instructors created pressures as high as 3 Gs in typical recreational turns. One and a half Gs is the threshold at which intrinsic foot muscle activity peaks.
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: Studies have unequivocally established that the muscles that support the foot respond to loads imposed on the foot by becoming stronger. The cited study further found that the intrinsic muscles have the (potential) capacity to control the foot posture and LA (Beam) stiffness with active arch stiffening and that this effect is sub-maximal below loads of about 1.5 Gs. Active arch stiffening facilitates the management of the pressure differential between the heel and head of the first metatarsal and the transmission of forces during postural activities (stance) including skiing.
In my next post, I will provide my explanation of the high pressure heel-first metatarsal COPs seen in the University of Ottawa pressure studies and the effect of changes in the pressure differential between the 2 load points.