Series of blog posts from The Skiers Manifesto:
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HEEL/FIRST METATARSAL PRESSURE DIFFERENTIAL
AN INDEPENDENT STUDY IN SUPPORT OF THE UNIVERSITY OF OTTAWA FINDINGS
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Another blog post from Skiers Manifesto:
HEEL PRESSURE VS. HEEL/FIRST METATARSAL PRESSURE DIFFERENTIAL -
The guy may know his skiiing, but he doesn't know his physics. We had the following discussion
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I did have to chuckle, though, when he told you to read Inman...... classic!.;) -
More from Skiers Manifesto:
THE BOOT BOARD FACTOR -
Another post from Skiers Manifesto:
THE IDEAL SKIER’S FOOT AND LEG -
Another new blog post from them:
HOW SKI BOOTS AFFECT BALANCE -
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I'm not sure what you are asking "So where is the ground when the foot is on a platform... "
I agree that this discussion should use Newton's Laws. A good place to apply them is in a free body diagram.
Are you familiar with the concept of center of pressure? It is essentially the average point of force and Ground reaction force can be considered to be acting at the center of pressure.
Linchpin5, are you related to the author of that piece that I quoted?
Eric Fuller, DPM -
Ski Orthotics and Ski Biomechanics: The typical down hill skier's lower extremities never goes through a complete gait cycle. They ideally should have limited pedal mechanics between midstance and the beginning of propulsion, with the knee in flexion during the entire contact phase. When initiating a turn a skier will maintain their control by directing the downhill knee medially and transferring the load into the foot over the inside edge. This is done by internally rotating their tibia, causing a closed kinetic chain pronation of the foot, which transfers the pressure through the boot onto the ski edge. In ski boots subtalar joint supination and pronation are responding to knee function. With ski orthotics the goal is to control the knee function by supporting the foot anatomy. A custom heat moldable ski orthotic from Atlas Biomechanics, allow the skier to turn quicker and with more power. The skier will have a more efficient transfer of energy and will have decreased overall foot and knee fatigue. A ski orthotic from can make you a better skier. Teri Green, www.atlasbiomechanics.com -
Another blog post from Skiers manifesto:
HOW SKIING TOOK THE WRONG STANCE ON STANCE -
In the context of skiing postural responses are CNS mediated responses to perturbations in GRF that tend to disrupt skier equilibrium.
I am familiar with the concept of center of pressure as the point center of GRF. This assumes a contiguous source of GRF from ground or an extension of ground such as is present on the uppermost storey of 100 storey building where the foundations are ultimately founded on ground. However in ski maneuvers, the only time there is a contiguous source of GRF under the entire foot is at ski flat between edge change or when a skier is standing at a bar hoisting a few drinks acres ski. Although I have used the term COP in my blog posts, I am in the process of revising existing posts so indicate the load imposed by the weight of a skier as W. Here are two posts that are relevant to my posts currently listed.
THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: WHERE IS GROUND? - http://wp.me/p3vZhu-1V0
THE MECHANICS OF BALANCE ON THE OUTSIDE SKI: TIMING OF EDGE CHANGE - http://wp.me/p3vZhu-1Y9 -
Eric -
Why do think Cop does not happen when the ski is on edge?
Eric -
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Seriously? Static squatting? I have a whole series of posts on my blog on stance based on foot to core to shoulder sequential fascial tensioning. -
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>Correct. But skiing does involve load, unload and swing leg phases.
"They ideally should have limited pedal mechanics between midstance and the beginning of propulsion, with the knee in flexion during the entire contact phase."
> In the few studies that were done, the researchers found little variation in the angle of the shank in the elite skiers regardless of the stiffness of the shaft of the boot they were skiing in.
"When initiating a turn a skier will maintain their control by directing the downhill knee medially and transferring the load into the foot over the inside edge. This is done by internally rotating their tibia, causing a closed kinetic chain pronation of the foot, which transfers the pressure through the boot onto the ski edge."
> How about generating some free body diagrams showing how this works.
"Internally rotating their tibia?"
> How could this work?
"With ski orthotics the goal is to control the knee function by supporting the foot anatomy."
> How can an external intervention that limits the natural dynamics of healthy foot control knee function?
"A custom heat moldable ski orthotic from Atlas Biomechanics, allow the skier to turn quicker and with more power."
> Compared to what? This claim is meaningless.Last edited: Mar 29, 2017 -
I mostly spend time just reading and acquiring information. The reason I have become active on Podiatry Arena of late is that I see an opportunity for objective narrative, one that considers the big picture. -
Linchpin wrote:
I am familiar with the concept of center of pressure as the point center of GRF. This assumes a contiguous source of GRF from ground or an extension of ground such as is present on the uppermost storey of 100 storey building where the foundations are ultimately founded on ground. However in ski maneuvers, the only time there is a contiguous source of GRF under the entire foot is at ski flat between edge change
Eric asked:
Why do think Cop does not happen when the ski is on edge?
I did see a graphic where Center of Pressure was mislabled as center of mass. The graphic with 3 feet in different positions with a pressure plot under them. You did correctly label bodyweight W and is a downward force applied to the top of the foot as you showed. However, what you labeled as COM should be an upward force from below (footbed, ski, ground, whatever). The force from below is a reaction force to the pull of gravity on the center of mass. The center of mass is an imaginary point and imaginary points cannot apply forces. The center of pressure is also an imaginary point, but it is the sum of the applied forces that you see in the plantar pressure diagram in your picture. So the CoP can be considered to be an average of where those forces are applied. This average can be used to explain what is happening while skiing or in gait.
Trying to apply the gait cycle to skiing is getting away from physics. You can use physics to explain both skiing and gait without confusing people with the notion of having a swing phase of gait in skiing. Yes, you can unweight a leg/foot in skiing, but that is not swing phase.
So to use those diagrams in both skiing and gait. When only the heel is in contact with the ground, the center of pressure will be an upward force posterior to the ankle. Body weight is applied by the tibia to the top of the talus and will be a downward force. Those two forces create a force couple that will tend to plantar flex the ankle joint. Or, if for some reason you wanted to put your weight on your heel in a ski boot, you could use your anterior tibial muslce to create a dorsiflexion moment at the ankle that could prevent the foot from plantar flexing when ground reaction force is on the heel.
When the center of pressure is anterior to the ankle joint the combination of body weight and ground reaction force will tend to dorsiflex the ankle. Tension in the Achilles tendon can prevent that dorsiflexion. Or another way to look at that, tension in the Achilles tendon will shift the center of pressure forward under the foot. That further distal of center of pressure will tend to cause arch flattening and increase tension in the plantar fascia. -
While COM does in itself a force, it drives the moment arm resulting from the central load transfer mechanism that tensions the plantar aponeurosis through a progressive exponential moment arm. When the PA reaches peak tension the soleus arrests shank movement by going into isometric contraction. This post shows a simple model I made ub 1993 to illustrate this mechanism - http://wp.me/p3vZhu-20U. It is only recently that the mechanism of Achilles/forefoot load transfer has begun to be appreciated.
There is an abundance of information on my blog, much it as far as I can tell constituents orginal thinking. If you tell me which search words or phrases you are using I can add tags to my posts.
I will respond to the other issues you raised later. -
Influence of slope steepness, foot position and turn phase on plantar pressure distribution during giant slalom alpine ski racing.
Falda-Buscaiot T et al
PLoS One. 2017 May 4;12(5):e0176975. doi: 10.1371/journal.pone.0176975. eCollection 2017.
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So many factual errors on that page and so much contradicted by the actual scientific evidence. Lots of 'wishful thinking' - (ie make something up and wish it was true)
Love the comment that " Shoes with elevated heels, cushioning and toe spring" inhibit what they think is the 'reserve windlass' ummmm .... nope .... the opposite is the case.
and "Footbeds and Insoles" also inhibit it!!!!! Dunning-Krugar personified! -
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Comparison of EMG Muscle Activity and Turn Times
Using Orthosis and Stock Insoles During Skiing
Heidi Nunnikhoven. Montana State University, Bozeman, MT.
Presented at ACSM Mtg, 2018
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