To get the ball rolling, something along the lines of... "Describe how a good athlete would setup their body to resist a push of 5 lbs, 25lbs 100 lbs, 200 lbs... and so on."
So our problem,
We are standing there with our hand out to our sides, and a force is coming in, we want to resist this force.
If you were 'bad' at working with your body. I wouldn't call this athletic, or external or anything other then bad body use. You wouldn't change the structure of your body, you would simply let the force come in and use all the muscular tension you could muster to fight the force.
If you were 'athletic' facing the same problem. You would align your body to the force, activate only the muscle groups needed, and use your skeletal muscular body to the best possible results.
So that is how I would basically outline the difference between 'bad' body use and 'good' (athletic) body use in this situation.