Re: Resistance training overview: the four basic levels
Glad a few people are getting something out of this post. As for Andrew's types of resistance I'm thinking that someone who knows how to resist internally would not really want to resist only with muscle. It would feel somewhat unnatural and perhaps even unsafe in a martial context since so much of the internal stuff is something that you embody naturally rather than turn on and off. It's also not very useful since you want to train with internal resistance as much as possible anyway.
I'd say leave the muscle-only resistance to beginners (and others) who don't have any internal skill. I like to encourage new people to try to use anything they have learned elsewhere and see what I can do against it, and so I will usually let people resist me with whatever they can initially. That's typically going to be muscle only until they start learning the other stuff. One of the goals of this is for them to realize that this doesn't work, and another nice benefit is that it allows me to practice against natural, untrained (by me, at least) resistance. Once they learn enough of the other kind of resistance to make that practice not as worthwhile, then I will usually have them start to focus on relaxing more, since that is the important and more difficult part.
You can always add the muscle you have to internal resistance to make things harder, but since aikido is not about using muscle power it doesn't make sense to me to train in this specifically. My philosophy is that you should train mainly in the stuff you want to be able to do, both as uke and nage, and this is also one reason why I don't do striking techniques much, even in full resistance freestyle. However, since not everyone is going to train the way you do, it's useful to occasionally work against things that are outside of you own training goals but within the realm of what you might face from someone really trying to get you, such as combined internal and external resistance and striking techniques.