Simple question, not so simple answer.
The human body ‘moves’ certain ways. Like a suit of armour there are certain ‘weaknesses’ in how things are put together. There are key ‘spots’ that have to be flexible and that move in certain ways. If you armor these spots you exchange mobility for protection. If you were to totally protect them, you couldn’t move (look at late period jousting armour where the guy had to be winched up onto his horse.)
Strength and physical conditioning CAN act as a kind of armor — especially when it works with structure. On the other hand, it’s less effective if you are moved along the lines the joints are supposed to move. (That’s the complicated part of your question.)
If you lock your arm out and I push into your fist, strength and structure will let you resist.
If you bend your arm and I push it’s going to become a contest of muscle. Me pushing to bend your arm further, you pushing back.
If however, locked or bent, I do a circular action that causes your arm to operate in the ball and socket way your shoulder is designed. you’re going to have a hell of a time resisting. And even if you do, all it’s going to take is for me to reverse direction (e.g., go from clockwise to counter clockwise.)
This is the difference between trying to force the body vs. moving it along the lines of how it moves. Put a push pin in that, we’ll come back to it.
A big element of this is knowing how the body works. This so if you’re pushing you’re not pushing against his structure and his ability to resist. Try pushing straight into a horse stance from the side. Ain’t going to happen unless you’ve go 100 pounds on the guy. (But… change the angle just a bit…)
You go in the right way easy peasy. Go in the wrong way and it’s hard work, no matter how good of shape the guy is in.
Another complicating issue is how familiar the guy you’re doing it to is with being moved this way. I know some big strong men (who rely on strength) that moving them is easier than a 20 dollar whore — because they don’t know what you’re doing so they don’t tighten up when you do it to them. I know other people who do recognize it for what it is — and these fuckers are like trying to put an armbar on an octopus.
Here’s where being in shape comes in. While it’s ALWAYS easier to moves someone along the way their body moves how hard or easy that is can be strongly influenced by their physical conditioning and reaction time.
So someone who is in good shape and can resist in time is going to be a bitch to move this way, but it’s still easier than trying to move him another way.
The problem with that however, is –odds are– if he’s that good you need to abandon your plans to do that move. Because he’s slowed you down enough that something real nasty is coming in from another vector.
Then comes the fine balancing act of recognizing when the guy is just blindly trying to resist vs. the SOB knows what he’s doing and is countering you. Because one is keep going and the other is GET THE HELL OUT OF THERE!!!