Suppose a person who weighs 110 lbs gets a weight reading of 77 lbs with the hands on the scale in the standard pushup position. Does this mean: 1. That a pushup for thaqt person is comparable to lifting 77 lbs in a bench press? 2. That their center of mass is at 0.7 of their height?
Pretty much, but I think you would find that the the weight reading would change depending on if your arms were extended or not. Sounds like a quick experiment is in order.Please Register or Log in to view the hidden image!
I weigh about 165 and tried that experiment and got 110 with my arms extended, which is probably how Jennifer was doing it. It would be hard to read the number with your face down on the scale! 110/165 is about the same proportion as 77/110. Maybe girls would have the center of mass slightly higher because - well....you know.
Thanks, guys. I did try getting a reading both with arms extended (easy) and in the down position (harder). As far as I could tell, the reading were very close -- maybe 1-2 lbs different. My scale (digital) doesn't like the weight changing. Once it settles on a weight, adding more weight will not change the display. So the readings not be accurate. The other problem is that I am not strong enough to hold myself in the down position long enough and stable enough for the scale to settle down. This is one of the reasons for the original question. I can only do a few "real" ("guy") pushups. If I push it, my shoulder sometimes feels a little twinge. So my plan is to build up my strength by (a) bench presses and (b) doing inclined pushups with my hands on a desk or chair until I build up more strength. This got my curiosity up as to (a) how much weight in a bench press is equivalent to a push up and (b) how that varies as I raise the height of the inclined pushup. So, can someone answer the original questions? Please Register or Log in to view the hidden image!
1) Yes. Your arms are "holding up" 77 pounds whether its a pushup or a bench press. Clever way to test, btw. 2) Presuming you are on your toes (and not your knees) then, yes. Don't forget that the inclined pushup works different muscles than a flat one. Just saying it won't quite be apples-to-apples when you switch over to flat bench but, generally, one can flat bench quite a bit more than incline bench anyway so it probably won't matter.
I made an error in my center of mass calculation. I didn't take into account the moment arm of the lifting force (my shoulders). Let F1 = the load (weight) L1 = the moment arms for F1 (center of mass) F2 = the lifting force (effort) L2 = the moment arms for F2 (location of my shoulders) Then the equation for balanced forces is F1 x L1 = F2 x L2 Solving for L1, we get L1 = (F2 x L2) / F1 For me, F1 = 110 lbs, F2 = 77 lbs, and L2 = 54". So, L1 = (77 x 54) / 110 = 37.8 So my center of mass is 37.8" from my toes or at 0.59 of my height (64"). That's a good bit less than the 0.7 I had. Another way to do it is to specify the moment arms as a fraction of my height (H). Then L2 = 0.84 (54/64) and L1 = (77 x 0.84H) / 110 = 0.59H This form makes it easy to see where I went wrong. Solving the balanced forces equation for L2, yields: L2 = (F1 x L1) / F2 = (110 x 0.7H) / 77 = 1H My result would be correct if my shoulders were mounted on the very top of my head. Please Register or Log in to view the hidden image!
Right, I was speaking in very idealistic terms (treating your body as a plank of wood suspended from the ends) but, I agree, 0.7 still didn't quite pass the "smell test", regardless of what your body looks like. I figured there may have been some weight disclosure fudge factor involved and wasn't going to pursue it. Anyway, what you've done here looks more reasonable.
origin: Using my new formula, I think my head is factored in. It uses my shoulder height and the effective lift weight at the shoulders. The weight and moment arm of my head should be included in the latter. I've attached a graphic illustrating the geometry. Did I do it right? RJBeery: Weight disclosure fudge factor? Me? I would never do that. I would adjust my height. (I'm actually 4' 6" Please Register or Log in to view the hidden image!)
Have you tried them with your knees bent at 90+ degrees and your feet in the air? So that the knees take the weight instead of your toes? That makes it easier, and the direction you're pushing is more like a normal pushup than an inclined one, so the strength gain may be more transferable from the easy form to the difficult. A pillow is advisable on hard floors. "Twinge" sounds bad. Shoulders are complex and delicate. I hope you're warming up first, makes problems less likely.
Sure, I've done "girl" pushups. Please Register or Log in to view the hidden image! And you're right that the motion is more similar to a real pushup, especially if the angle of inclination is large (>45°). But even if I start with a "wall" pushup (almost vertical), as I gradually lower the incline height, I will be using more and more of the same muscles as for the regular pushup. And it's easy to vary the resistance gradually and continuously. (Also true for the bench press.) And, using different muscles has its own benefits, too.
Well, it appears that I made another error. This is turning out to be a lot more complicated than I thought. (Or I am even more math impaired than I feared.) When I was making a drawing of the angles, it occurred to me that I didn't make a note of where my hands were relative to my shoulders. I knew hand placement can make the exercise much harder, but I thought that was because my muscles were having to work at a greater mechanical disadvantage. I didn't think it would make any difference to the scale. I was, apparently, very wrong. I did weighings in the "up" position (arms extended) with my hands at 2" intervals from as far back as far forward as I could keep my body stable ebough to get a decent reading. The results are in the table below. The CoM column is the Center of Mass as I calculated it above L1 = (F2 x L2) / F1 Since these results vary, I have to conclude that my CoM formula is wrong as the real CoM is not moving around. :bawl: Code: Hands F1 F2 F CoM 40 -- -- -- -- Could not straighten body 42 -- -- -- -- Could not straighten body 44 87.5 87.8 87.6 35.1 Body may not have been straight 46 85.5 84.0 84.8 35.4 48 82.3 82.3 82.3 35.9 50 80.0 80.8 80.4 36.5 52 77.8 78.3 78.0 36.9 54 76.0 77.0 76.5 37.6 Directly under shoulders 56 74.5 74.3 74.4 37.9 58 71.5 71.5 71.5 37.7 60 68.8 69.0 68.9 37.6 62 -- -- -- -- Could not hold weight stable 64 -- -- -- -- Directly under head Can someone help me make sense of this data?
Actually, without analyzing or understanding your analysis above, your center of mass would move around if you moved your arms around.
That's a good point. It sure would. I wonder if it could be as simple as that? I always prefer to overlook the simple answers if a much more complicated one is available. :bugeye: My arms are pretty skinny, which is why I can't do any pushups. The weight reported by the scale varies by almost 20 pounds as a result of moving the hands 16". That seems like more than could be caused just by moving me hands. The arms would move proportionately less with the thickest part moving the least. I can't spend any more time on this right now. If I get time later, I'll try to construct a drawing and work out the angles and force components -- unless some kind soul posts a solution first. Please Register or Log in to view the hidden image!
I think it's the other way round. We have a much larger percentage of our muscle mass in our chests and arms than women do. That's much denser than breast tissue.
http://speedendurance.com/2014/03/02/how-much-weight-do-you-push-with-push-ups/ A comparison of load weights using different push up techniques.
I don't trust those numbers - they don't square with experience. Something's been overlooked. How widely your hands are spaced makes a difference in the leverage - when I want to cheat, I spread them a little wider. This will not register on a scale before or after the pushup - the weight moved doesn't change, only the distance over which is moved and the flex at greatest load. A lot of scales have a memory function - put a pad on the scale, lower your forehead to it in guy pushup mode, pick your hands up and support your weight on toes and forehead, use that number to calculate your own center of gravity. imho: The effective difference between feet elevated (at ordinary heights, so the trig numbers of the pivot angle are not much affected) and feet on ground is mostly arm angle - the hand placement relative to shoulder height. The feel of greater weight is in my observation created by the fact that placing your hands in the same relationship to your shoulder as customary with regular pushups - the angle of best biomechanical leverage - moves them closer to the body center of weight, so you're either lifting more of your weight or lifting it at lesser biomechanical advantage. Women's center of gravity is much closer to their hips than men's, on average. This makes it easier, in the abstract Newtonian physics sense, for a woman of a given weight to do a pushup - she's moving less of her weight, or moving it a shorter distance, depending on how you want to think of it. The catch is that the weight distribution advantage is a consequence of smaller skeletal structure in the upper thorax, including shorter collarbones etc, which decreases the leverage available far more than it reduces the weight to be moved.
Yes, you are technically pushing 77lbs of weight but I would disagree that it is comparable to the equivalent bench press. The upward barbell bench press generally involves many more stabilizing muscles than a push-up and I can guarantee you it would be much more difficult to bench the same 77lbs (if you can even do it at all).
