The physical element is certainly important, but I think the mental component is more important. Getting used to violence is not an easy thing for most people, and why should it be? It's awful. Luckily, (or maybe, unluckily) some occupations expose you to it on a regular basis.
What we always tried to do in the dojo, was push people physically throughout their training career. Then add components of mental pressures through surprise scenarios. I adhere to the belief that your training should be more physically and mentally challenging than anything you experience other than an actual self defense situation in real life.
If you never have to push yourself in the dojo, or deal with any fear or mental pressure, being in a dangerous and frightening scenario outside may not go the way you hope.
I mean, you don't push people all at once from the beginning, that wouldn't be of any help. And if you match them up for any sort of sparring, obviously you pair them accordingly to help them grow. Once they've grown, you throw them to the wolves. Everybody should spar with everybody, especially sparring people bigger, stronger, faster, smarter and more experienced than you.
Martial Arts is a physically demanding undertaking. I think it should be.
good which one?
