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Where is CoM?

post #1 of 9
Thread Starter 
Seems fairly obvious, huh?

I propose that most of us, predominantly:
a) use visual cues
b) use sensation from the part of our body that has the most weight on it.

I was corresponding with one of the bears (cantunamunch) about a biking topic, noseless saddles, specifically in regards to this paragraph from www.roadbikerider.com:
We're occasionally asked about noseless bike seats, which are presumed to be "safer" because body weight is supported only on the sit bones. There's no narrow nose than can press into the crotch. Some riders are interested for health reasons, others are seeking more comfort. Noseless seats have been around for more than a decade without much acceptance. We've tried noseless seats in our role as product testers. When asked our opinion, we say it feels like pedaling a bike while sitting on a soccer ball. Noseless seats make a bike feel tipsy, particularly when you're reaching down for any reason, like to grab a water bottle or adjust a shoe strap. There's nothing between your thighs, nothing to slide forward on. When you turn your hips, the bike doesn't turn with you. You feel like you're on a perch, separate from the bike instead of one with the bike.

RBR's Ed Pavelka knew all that and still went riding last week on a noseless seat. It was a favor for a friend in the industry. The guy is thinking of importing the seat from Europe to market to American men worried about conventional saddles causing perineal problems. Ed was dubious but the first ride went okay. Then the next day, disaster. He lost control while riding at about 16 mph and signaling a left turn. The bike went out from under him as suddenly as if he'd hit a patch of ice. Fortunately, the truck right behind stopped in time. Other people ran up. "Are you okay?" Ed wasn't so sure. His helmet was cracked, he was bleeding from the usual places -- knee, hip and elbow -- and it felt as if a linebacker had speared him in the ribs. And he was really mystified. "What happened?" Ed asked the driver that saw the crash. "I don't know. You must have hit something." But the street was as clean as Martha Stewart's kitchen. Ed hadn't ridden over anything. He had simply angled into the left lane with one hand on the bar and the other signaling -- a move he'd made a thousand times without winding up in a heap. This time one thing was different. This time he was on a noseless seat.
Now. Does anything strike you as really really strange there? Why should a rider who can stand on the pedals and make that same arm move hundreds of times fall over when he's seated (on a noseless saddle)?
- His feet are the same width apart
- They are still available for balance
- Same core strength
- Same disturbance to CoM

Applying Principle (b) above we would immediately notice that, whilst seated, the majority of his weight is on the saddle, not on the pedals. Not having a strong reference point, he couldn't find his CoM in order to correct mispositions.

I wonder if we can apply principle (b) to ski poles? As we never really have the majority of weight on them it would seem to state that the function of ski poles is adjustment after sensation, not sensation itself.

So, as a first experiment I suggest eyes-shut one-leg standing with both ski poles touching the ground. Corrections or sensations?

As a second experiment I suggest sitting on a Swiss ball, eyes shut. First with feet on ground then with ski poles. Corrections or sensations?


If a skier drops their gaze to the snow nearby during weight transfer drills are they having trouble finding CoM from the stance foot?
post #2 of 9
Comprex - I can only tell you what we know from my attempts at learning balance

first thing to remember though is I cannot detect COM in any normal way quite simply I don't have any idea where my body parts are in relation to each other..... so for instance when asked to "lift a foot" in rollerblade lessons I do JUST as instructed LIFT... not as intended ADJUST WEIGHT TO OTHER FOOT & LIFT (everyone else moves weight before lifting it is sensible yes? - not if you are unaware of weight.... I'm a great guinea pig I TRY to do exactly as ordered... results often bizarre because I make none of the usual compensatory moves)

Anyway - back to your touches....
This goes back to the gym people trying to teach me to stand on 1 leg.....
We KNOW I cannot stand on 1 leg naturally - so they train me to do so....

1) Stand on 1 leg & hold onto something
2) Stand on 1 leg & TOUCH something (not hold)
3) Stand on 1 leg and touch something with ONLY 1 FINGER
4) Stand on 1 leg and touch with 1 finger LIFTING FINGER for a fraction of a second
5) Stand on 1 leg and touch with 1 finger for a half of the time (eg one second on one second off)...
6) Tap with one finger while standing on 1 leg
7) Stand on 1 leg NEAR something & tap ONLY if off balance....

I can tell you that the tap gives my brain data it needs to help it balance.... - they DO NOT rebalance me (point of my index finger is not that strong).... if I am well off balanec I need my hand to help stop me falling.... but the info does help me work out where balanced is....

We have repeated this in many situations... (Australian Institute of Sport guys use same idea in FITBALL classes & my gym programs)...

I used it in learning to ski also... & in rollerblade lessons....

Note that most of teh above have NO POLES..... only my finger/hand etc....

Re the visual clues...
Some smart instructor decided to "test" drive me to check what they taught him at university on human movement stuff....
So they decided I use visual cues to help substitute for my loss...
One morning I found my balance suddenly "off" in the gym.... I cursed thinking I was either very tired or getting some virus(I compensate badly when tired or unwell).... everytime I tried to do my quad stretch I started to fall & needed to grab the chin bar I was standing next to....
THEN I saw the instructor on the desk giggling..... turned to find the OTHER instructor on other side of chin bar....

he tells me he was pushing & giving the bar a lean - I leant too (& fell)
So then he thought a "sway" would be interesting - again I had the SAME sway as the bar.... and again I started to fall quite unaware what was going on.....

After this same gym instructors were MUCH more helpful re my dodgy balance issues.... seems their lecturer at uni had found the "test" on me quite interesting & they were given a big description of the implications.... (I love gym instructors who are studying physio/human movement etc.... they are MUCH more useful most of the time)
post #3 of 9
while pedaling, the inner thighs continually are reminded of the saddle's nose and its presence.

some of us actually pressure the saddle with our inner thigh for handling improvement.

remove the saddle nose and you remove a touch point for activity reference.

I can imagine finding it difficult to pedal a nose-less saddle.

riders who pedal with a huge amount of bowlegged femoral posture might find it less difficult.
post #4 of 9
Thread Starter 

using your training progression, as above, would you be able to use ski pole contact with snow to ski in a whiteout?
post #5 of 9
note: the noseless saddle is a solution in search of a problem.
post #6 of 9
Originally Posted by comprex

using your training progression, as above, would you be able to use ski pole contact with snow to ski in a whiteout?
All I know is that as an intermediate skier I ALWAYS skied faster in a white out than with good visibility.... (No trees to see - don't know my speed)....

Nowdays I seem to be less affected than others around me - but as I rely on sensation from feet & those pole touches just to stand on skis and to cue all my movements on skis it sort of follows that I am in general more trained to use feet than to use visuals .... remember that if I used visuals I would need to react to the visuals - which requires better movement skills than I may possess.... also the biggest use for visuals in everday life is looking for vertical lines to help me stay upright.... on a ski hill the vertical lines are trees... they do NOT represent balance & i have probably learnt to ignore the visual input a fair bit
post #7 of 9
Thread Starter 
Originally Posted by uncle crud
note: the noseless saddle is a solution in search of a problem.
'K. The standing balance vs. seated on a Swiss ball is more direct to visualize anyway.

Just attempting to see whether -untrained- prime activity reference isn't strongly correlated to the prime weight bearing surface.
post #8 of 9
My guess - NOT

You rely greatly on proprioception.... It is inherent & not apparent - until you don't have any.... ever spain an ankle/injure a knee & NOT feel it was "safe" even though it was strong again? NO proprioception..... Whole body worth gone = very stange sensation... check out "man who mistook his wife for a hat" chapter called "disembodied lady"....

Alternatively OD on Vitb6 & check it out!

I have none & rely greatly on visuals

Since learning to ski I have been TAUGHT to rely on feet more (weight bearing surface).... but that was learnt... BTW - it helped my everyday balance immensely to learn this.....

If bodies used weight more dominantly I think I would have compensated better
post #9 of 9
Originally Posted by comprex
'K. The standing balance vs. seated on a Swiss ball is more direct to visualize anyway.

Just attempting to see whether -untrained- prime activity reference isn't strongly correlated to the prime weight bearing surface.

this is why the nose-less saddle is a solution in search of problems.

a properly adjusted saddle (height, fore/aft position, tilt) provides a saddle nose that is nothing more than a touch point... not close to a weight bearing point.

improperly adjusted saddles -- too high, tilt angle set with too much nose height, saddle too far aft making rider perch on the nose -- cause too much pressure on the perineal area. they turn the touch point into a weight bearing point.

the solution lies in getting a saddle shape that matches your sit-bone -- perches are spaced similar to your pelvic girdle's breadth beteween the sit bones... and then adjusting it properly. once this is done, the saddle remains an important touch/reference point, and not a primary weight bearing point.

a saddle that effectively is noseless (adjusted the opposite of nose-high) results in too much weight put on the hands and feet, typically causing bad shoulder/back/arm cramping.
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