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tobin_kelly
Mar 15, 2007, 5:47 AM
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My comprehension of basic mechanics started to wane around high school, so apologies to anyone who finds this too dumb a topic.
Essentially, I want to understand in mechanical terms why it is possible to hang some holds, but not pull up on them?
Consider, for example, hanging a thin flat edge with both hands, hanging free without footholds. The hold is at the limit of what you can hang. In anatomical terms, you achieve this by finger and arm muscles maintaining various isometric contractions to shape the hand into a crimp (or open-handed) grip.
Now you try to pull-up. On large holds or a bar, you can pull up easily. This requires various dynamic contractions of large muscles in the shoulders and back. However when you try to pull on this small edge, you fail, before you can even initiate any significant movement.
Why does this happen? Logically the failure must be a function of limited strength in your hand grip, because the regular pull-up muscles should easily be able to perform their share of the exercise. My assumption is that the hand grip fails because of additional loading. To pull-up, there must be some acceleration to achieve an upward velocity. Acceleration requires an increase in force. The finger and arm muscles shaping the grip cannot maintain contraction with this additional loading. Anyone disagree so far?
However, if you pull-up very slowly, the acceleration should be very modest. I am not sure how to quantify this is in terms of newtons of force relative to the load the grip muscles are already resisting but intuitively the incremental force must tend to zero as acceleration is minimised. If so, it should be possible to do a slow pull-up on anything you can hang. However, it is easy to demonstrate practically that this doesn't work. Can anyone explain this? And is it possible to quantify how much additional strength is needed to pull on a hold? If, for example, if you can train to hang a small edge with a 10 kg weight-belt, should that much additional strength be sufficient to pull-up on it without the weight-belt?
[I asked the Brits this on one of their sites but the replies were pretty worthless. But you 'mericans are all properly edukated - right?]
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glytch
Mar 15, 2007, 7:10 AM
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My take:
As you do a pullup, your center of mass does not generally stay directly below the location of your hands. This is especially true on a wall: your body must come out as you pull up. As your body lifts off of the wall, your hands must apply a force inwards as well as down in order to keep you on the wall (gravity is applying a force roughly about your feet, and you have to generate a torque to counteract that). A small hold does not give you enough purchase to apply this torque, so you fall off.
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rjtrials
Mar 15, 2007, 11:01 AM
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It's been quite awhile since I had a physics class, and I dont really remember the equations for force vectors.
I do recall that the force applied (on the hold or hands) is in excess of twice the bodyweight. And that is with perfect form, not thrutching, swinging or thrashing.
RJ
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overlord
Mar 15, 2007, 11:08 AM
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rjtrials wrote: I do recall that the force applied (on the hold or hands) is in excess of twice the bodyweight. And that is with perfect form, not thrutching, swinging or thrashing. RJ
really?? that seems kindof illogical.
anyway, id say the OPs problem is the result of both the acceleration required to pull up and the shift of the center of gravity (it usually moves backwards from the vertical below the hold, making it less positive).
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rjtrials
Mar 15, 2007, 11:33 AM
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overlord wrote: rjtrials wrote: I do recall that the force applied (on the hold or hands) is in excess of twice the bodyweight. And that is with perfect form, not thrutching, swinging or thrashing. RJ really?? that seems kindof illogical. anyway, id say the OPs problem is the result of both the acceleration required to pull up and the shift of the center of gravity (it usually moves backwards from the vertical below the hold, making it less positive).
Since I dont have a physics text handy, here's a quick check of the old standbye, Wikipedia.
In reply to: The total acceleration of a body is found by vector addition of the opposite of the actual acceleration (in the sense of rate of change of velocity) and a vector of 1 g downward for the ordinary gravity (or in space, the gravity there). For example, being accelerated upward with an acceleration of 1 g doubles the experienced weight. Conversely, weightlessness means an acceleration of 1 g downward in an inertial reference frame. Therefore, the term μg-force is a comparative measure of acceleration applied to a body with respect to sea-level gravity on earth. It is a normalized force vector since dividing the resultant force vector applied to a body by the body's weight (magnitude at sea level) cancels the mass, resulting in a "fractional-g" -magnitude vector, e.g. a person sitting on a chair at sea level is experiencing "1g," due to his weight.
Once again, that is in a perfect system, with constant acceleration of 9.8 m/s2.
RJ
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overlord
Mar 15, 2007, 11:44 AM
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well, but in this case you start by hanging and not accelerating upwards at 1g.
if youre still, you only need to create enough force to counter the force of gravitiy, thus the force of fingers on the hold is -Fg, eg your mass*1g.
once you start the pullup, you are accelerating upwards (at least for a moment) and thus the force on the fingers is bigger.
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hyhuu
Mar 15, 2007, 12:46 PM
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I agree. That's why it's easier to pull up on positive crimper than a sloper.
hyhuu
glytch wrote: My take: As you do a pullup, your center of mass does not generally stay directly below the location of your hands. This is especially true on a wall: your body must come out as you pull up. As your body lifts off of the wall, your hands must apply a force inwards as well as down in order to keep you on the wall (gravity is applying a force roughly about your feet, and you have to generate a torque to counteract that). A small hold does not give you enough purchase to apply this torque, so you fall off. |
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rgold
Mar 15, 2007, 11:10 PM
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I agree that the unavoidable upward acceleration involved in a pullup increases the load on the fingers, and that performing a very slow pullup would keep the additional force low and allow it, in fact, to drop back to bodyweight, since once the body is moving, continuing the motion at constant velocity will not require additional force.
While it may be possible to generate 1 G upward accerleration, there is no reason to suppose that the load on the fingers must under all circumstances be double body weight.
While it is true that body position changes during the pullup, the idea that the climber's center of gravity moves out of line with the climber's hands has it backwards. It is precisely because the center of gravity must stay in line with the hands that the body attitude has to change. (One could accelerate the center of mass of of line with the hands briefly; this would induce swinging.) In fact, the natural urge to draw up the feet is, I think, at least partially related to an inclination to keep the torso vertical and avoid the front-levering stresses on the arms that would be a consequence of the more prone position required to align the center of gravity with legs straight.
So in a slow pullup with no swinging, there is no additional force on the hands resulting from change in body attitude. (The situation is completely different if the feet are on holds and taking some body weight.)
I suspect that physiological factors are more critical than the purely mechanical ones. Ignoring grip issues, slow pullups are considerably harder than fast ones, for reasons that are probably both mechanical and physiological. Control is more critical, since any body motions that induce swinging will make it harder to hang on. Hand strength may not be constant as the angle at the elbow changes and may be further diminished by having the arm and torso in motion.
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cartelryder04
Mar 16, 2007, 2:20 AM
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in reply directly to tobin_kelly: i only read the rest of the replies partially...
you are correct when you say:
"Logically the failure must be a function of limited strength in your hand grip, because the regular pull-up muscles should easily be able to perform their share of the exercise."
the force needed to start the inital pull up is actually an impulse problem which relates to a forcing function of time.. which i will omit for this discussion and assume there is a constant force being exerted. anyways, the force needed to do a pull up is the same regardless of the hold. imagine a vector diagram..in the simplest case: all your mass is in one point. the force due to gravity is directly downward. your pull up is actually you pushing down on the rock- the rock pushes back on you. this "responding" force is called the normal force. this is the force that is actually "fighting" the force due to gravity. the difference of the two forces (in the vertical component only) is the actual resultant vector. and if you're push was strong enough, the resultant is "up" and, thus, you are doing a pull up.
here's where you were correct in your logic. the force of you doing a pull up is coming, obviously, from the muscles in your body. even though the force that it takes for you to do a pull up is the same no matter your hold on the rock, it is coming from different parts of your body (shoulders, arm, finger). this is why various pull ups are harder- your muscle groups are formed or shaped differently. this difference you feel directly in your body, which registers as an easier/more difficult pull up.
the reason why you can sometimes hang on a hold and not do a pull up is because when you are hanging, your body is in equilibrium. the force due to gravity is equal (in magnitude) to the force you are exerting on the rock. by you hanging, you have compensated for the force due to gravity pulling you down. to pull up you must exert an additional force that will "overcome" the force due to gravity; giving you an acceleration upward.
now i know in many cases that your body's CM is not directly below your hands. this gets into torque (torque == r x F.. thats the cross product by the way.. not multiplication) which makes your pull up harder because your muscles now have to accomidate for the extra force on your body.
sorry for the essay. i'm a physics major and i get carried away. i held back on most of the details of the actual mechanics. hopefully this helps some..
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zeubanks
Mar 16, 2007, 2:53 AM
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cartelryder04 wrote: now i know in many cases that your body's CM is not directly below your hands. this gets into torque (torque == r x F.. thats the cross product by the way.. not multiplication) which makes your pull up harder because your muscles now have to accomidate for the extra force on your body.
It does involve torque, but the main thing that makes you use different muscles is the fact that when your C.O.M. is not directly under your hands, you must exert a force that pulls you inward, toward the wall (or you fall off backward). In the case of a crimp or any ledge-like hold, that force comes from the friction force between your fingers and the hold itself. The force of friction is equal to the product of the coefficient of static friction (a constant that depends on the surfaces in contact) and the normal force (the downward force that you apply on the hold). This means that you have to pull down harder than usual so that you can pull yourself inward. Because of the change in body position, the weight belt is useless for helping to pull up if you're just hanging there. Try bigger holds and work your way down instead.
(This post was edited by zeubanks on Mar 16, 2007, 2:55 AM)
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glytch
Mar 16, 2007, 5:09 AM
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cartelryder has it right.
If you can hang on a hold just fine, and you're capable of doing pushups, 90% of the time your hands aren't going to give out because they aren't able to provide the small additional force to move your body upwards (and an arbitrarily small additional force is all that's needed to move upwards!) - your hands are going to five out because your CM has moved out of line w.r.t. the hold and you have to apply additional force to counteract the resulting torque. The important feature of this force, as cartelryder describes, is that it is a force into the wall that you have to apply.
The above description is certainly true - the one other possiblity (and this I can't comment on with any authority) is that certain arm configurations allow you more or less grip than other configurations. i.e. if your wrists are bent at a sharp angle, it's possible that you aren't able to grip as well with your fingers. As you pull up, you change the configuration of your arms; if certain positions are weaker than the dead-hang, this could cause you to fall off in a purely strength-based way.
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patto
Mar 16, 2007, 6:11 AM
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Yep it is due to the positioning of your centre of mass.
This is brought home by a crimper above a door frame i have at home. I can do pullups on the crimper if the door is open but not close.
With an open door it allows my legs to swing in through the doorway and thus keep my CM below the crimper.
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tobin_kelly
Mar 16, 2007, 9:01 AM
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Thanks for all these responses. Combined with comments I have got elsewhere, it seems like there's general agreement that this is a complex issue in which body positioning and neural/ muscle coordination are as important (or more important) factors than quantifiable grip or pull-up strength.
If so, is there actually much value in classic dead-hang type training? Should all finger-strength training include a kinetic element?
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