As one who has perfected the art of avoiding exercise * at all times* this thread has left me quite exhausted.
I’m going for a power nap now/
Phew!!
Gasp!!
How about this thought?
As you move through a pushup, if you allow your back to arch even slightly, you take a lot of he weight off your arms; Your back may do some of the work after your arms straighten and then your back pulls up into line.
Jusat a thought that this might be contributing to the sense of less load.
Whoa, guys! Settle down. Physicist checking in. (How often does that make any sense to say?)
Alex_Dubinsky’s got it right. If the scales show 190 lbs at the palms, then the 250 lb center-of-mass must be three-quarters of the way up from the toes. Indeed, the lifter is only moving the 250 lbs a reduced 3/4-distance, but this is equivalent (work-wise) to moving 190 lbs the full distance. The mechanical advantage only counts once. The OP, realizing this, measured the force at his palms, saving us the trouble of getting levers involved at all.
The answer lies in biomechanics. Offering what I can to that part of the discussion: With push-ups, your shoulders are seeing 190 lbs minus the weight of your arms, your upper arms are seeing 190 lbs minus the weight of your forearms … which could all make it a bit easier, especially if you’re a big guy.
But, that’s not going to explain most of it. I don’t do bench press very often, but when I do, I find the big difference is in how low the bar gets relative to how close my chest (in a push-up) gets to the floor. That is, in my usual push-up form, my chest won’t get any closer than 4 inches or so to the floor. With a bench press, the bar can come all the way down, and those first few inches of press are by far the toughest. Could you do 8-9 reps of 190 lbs (maybe 170 lbs) if you didn’t bring the bar all the way down each rep? (My guess is no, since stability, etc., will still be playing a big role.)
Actually, since every part of your arm except the hand is being raised during a push-up, you’re still lifting a portion of the weight of your arms. It does get a bit easier because you rotate them to extend rather than simply pushing.
Easier explanation: You are very good at balancing yourself with minimal muscle twitch, because you’ve been doing it for a very long time.
You aren’t very good at balancing a 45-pound bar with 145 pounds of weights on it because it isn’t instinctive. Thus, while you may exert the same effort to extend your arm, you’re extending a great deal more lateral effort to keep your arms from being pushed, say, down and back over your head.
Yup, it’s kind of like why big odd-shaped things seem heavier and jar caps with poor grips seem like they’re stuck harder. Your mind’s perception of difficulty seems some sort of average of the effort between all the big muscles and all the little ones, and you often do not perceive which muscles are really the weakest link. (And also because, especially in the case of odd-shaped things and jar caps, your big muscles have to do extra work to make up for the little ones.)
Pasta’s point that you go lower is important too. So is the other thing mentioned: that the OP’s pushup position is different from his bench press. Boyo Jim also made a good note, that often people arch their backs when they get tired, and this alters significantly the upper-body muscles being worked (arching the back also reduces the distance the CoM travels and hence work). In general, pushups always have a bit of a downward motion to them, and benchpresses are either forward or up.
Last thing: pushups give you a large choice of where to put your hands and into which direction you push. You may not even notice it, but slight optimization in position and direction really let you minimize your exhertion (you may not even be pressing forward, but instead making use of the friction at your feet to redirect a downward or upward component). Benchpresses afford far less optimization.
you may not even be pressing forward
should be:
pressing parallel to the normal of the floor. (ie perpendicular)