Why is a bench-press harder than a push-up for a given load?

Factual Questions
1

I was curious about how much weight I was lifting when doing a push-up, so I got into a push-up position on the bathroom scale, measuring the load on my feet and arms.

It turns out that for me, a 250lb guy, I have 190 lbs on my arms, and 60 lbs on my feet in proper push-up form (same for arms up or down, as far as the scale can tell).

So with a trainer making sure I use proper form, and with my hands placed on the floor right below my armpits (so as to not recruit too much shoulder muscles) , I can do 8-9 push-ups. (yeah, only. I know, it’s embarrassing, but I’m working on it :o )

But there’s no way in hell I can do a 190llb bench press (80 lbs, for 10 reps. if you must know) yet it seems the movements are nearly identical.

Why?

Starting with arms extended shouldn’t make that big a difference, I would think. What other muscles get recruited in the lifting effort (not stabilizing, like abs and core) in the push-up? Is it a question of angle of the force vector? of the movement? of the length of the lifting stroke?

Can you explain this in a way that this former mechanical engineer can both believe and understand? So far the answers I have gotten are suspiciously vague and unconvincing.

2

The mechanics of a pushup are much different than a bench press. A pushup is an example of 2nd class lever, although the mechanical advantage is small, there certainly is one. By contrast, a bench press is done without mechanical advantage, and further, you have to use additional muscle groups to stabilize a freeweight.

3

Mechanical advantage? The OP made the excellent move of measuring the precise weight on his palms, he’s not using his body weight.

My guess is that it would be those shoulder muscles you’re not excercising. There’s many other muscles involved as well. Some push up, many others keep balance. So the places which are strained are different, and there are many more places for your arms to fail (or just get tired). This is probably why people do bench presses over pushups in the first place.

My suggestion? Learn hand-stand pushups. The benefits of brench-presses… in your very own home :wink:

4

Just because he knows how much weight is on his feet and on his palms, there no mechanical advantage to lifting the remaining 190 pounds? Look, it’s a 2nd class lever, plain and simple. The effective weight is located in his center of gravity somewhere near his solar plexus, while the force is applied under his shoulders. His shoulders move a greater distance off the floor than his center of gravity. Mechanical advantage.

Your guess is just that. A guess.

5

Hmm. at first, my reaction was, like Alex_Dubinsky’s, to say that discounting the weight from 250 lbs to 190 lbs accounted for the leverage / mechanical advantage effect, but your second post above illustrates the difference better:

Bench press: center of force is directly aligned with lifting arms. Shoulder travel = CoF travel .

Push-Up: center of mass (CoG) is closer to pivot point (feet) than the lifting arms. Shoulder travel = CoF travel *R/r where R= radius of rotation of shoulder about the feet and r = radius of rotation of CoG about the feet.

The 60/190 distribution is just the distribution of weight between the hands/shoulder & the feet. it does not account for the fact that less work is done because the CoM travels a shorter distance in the push-up.

This also explains why push-ups done with knees on floor instead of feet are so much easier: the R/r ratio, and thus the mechanical advantage, is much greater.

This would suggest that if I do 190 lbs * r/R I would come up with the equivalent BP load to a push up. Hmm, anyone have any ideas on finding the height of my center of gravity?

6

The CoM traveling a lesser distance is already taken into account by you not using the CoM in your calculations.

The only thing that’ll happen during a pushup which will offset the 190lb figure is the fact that the angle of the body changes a bit and hence the distribution of total weight between feet and hands. However, this won’t make a very large difference, and could be measuring by weighing with hands extended and hands flexed. An intermediate value should be used (or hell, let’s do integrals).

Your arms are counteracting a force. A force measured to be 190 lbs (maybe a bit less if you discount things like weight of forearms or varying angles). That force is pushed through some distance, equal to the length of the upper arm (given a proper pushup/benchpress position). That force has been observed, that distance measured. No arguments regarding the motion of the CoM or the lever action of your body will change that.

7

You obviously confused the CoF with 190lb. It is equal to 250. If you use the value of 3/4 for r/R and 250 for CoM, you get 190lb, your so-called equivalent BP load.

On another note, if you measured your weight while on your knees, the scale will read much lower (however, you’re now dealing with bigger variations in angle so mind that).

8

Buddy, levers change the amount of Work that needs to be done, they aren’t weight altering magical devices. They change the ratio of force and distance for an equivilent amount of Work.

9

I thought levers kept the amount of work to be done constant.

Work has to do with displacement and force. You’re decreasing force at the cost of increasing displacement.

But, I haven’t been in a physics class in a long time.

(pulling up a chair for this thread, especially after “buddy” has been thrown out)

10

You also have to take into account that for most people, the bench press is not a natural movement. It’s definitely more of a learned skill which becomes easier as you learn to align your body properly (this is not to say that it’s not a strength-dependent movement, I’m just saying that it gets easier as you learn to stabilize and move the weight more efficiently).

A pushup, however, is a pretty natural movement for most people. Drop a person onto the floor, tell them to do pushups, and they’ll pretty much fall into a good position to do them naturally. Well, at least a better a position than most people’s first attempts at a bench press. :slight_smile:

And all of that stuff about leverage, of course.

11

Are you doing pushups with your elbows in but benching with your elbows out? If so, that’s part of the difference there.

You meant to say military press, right?

12

You have it exactly backwards. You even said as much in your next sentence. The amount of work stays the same. The weight/force changes.

Military press requires equipment, and if you use a machine you have the same guided-motion/loss-of-benefit as bench-presses vs. pushups. The motion seems a bit the same, though. Anyway, I was mostly joking. Handstand pushups are hard.

13

From physcis forums

14

Yes, I meant to say that that a lever changes the way the work gets done, not the amount. ie. the ratio of force and distance for the given amount of work as I said later in the post. Sorry for the poor choice of words… buddy. :wink:

15

that’s great, but the OP was about a direct measurement of the weight on the poster’s arms. No need to calculate that force a priori from body mass. It has been observed.

I salute the poster for that excellent move.

We have to stop talking about this levers nonsense. IT DOESN’T APPLY.

16

I’m not a physicist or an engineer, but I am a weightlifter.

If you’re comparing your pushups to a free weight bench press, you should take into consideration the fact that you’re exerting quite a bit of effort to keep the bar steady, in addition to that required to push it away from your body. Another example of this is that most people can bench press substantially more on a Universal machine (where, as in a pushup, the weight is laterally stable) than with free weights. In my case, I can bench probably 15% more on a Universal.

17

It seems like doing a bench press would also involve a little extra effort over a pushup because in addition to lifting the weight on the bar you also lift the weight of your arms. Probably not a huge contribution for most people, but there nonetheless.

18

That’s really funny because the guy goes on and on about estimating the center of mass of the body to figure out how much weight is being lifted, when he could have just gotten into a push-up position on a scale like the OP.

19

Because all that does is tell you how much of the weight of your lever isn’t resting on the fulcrum. In this case, the lever is also the load. Remember, the OP wanted to know why he could do pushups but not bench press what he thought was the equivalent weight. To say that the answer is that only 190lbs of his 250lbs weight is on his arms ignores his original question. He’s figured that much out, and now wants to know why that 190lbs it’s easier to lift. The answer is that he isn’t moving that weight through the same distance. His arms are moving some 28 or so inches to move his center of mass some distance less. If you think 190lbs of this 250lb man is actually situated in the vicinity of his shoulders, you might want to draw a picture of what such a man might look like.

I don’t really care if captain foreskin thinks that levers aren’t the explanation.

20

Why the hell are you talking about the distance the center of mass moves, when the weight is at the arms? You wanna talk about how far the center of mass moves, talk about how much the center of mass weighs. It moves less than the arms precisely as much as it weighs more than at the arms. 190dist-at-shoulders=250dist-at-CoM

Listen, private lever. Go try passing high school physics.