Oh, I think there's been a mistake made. Performance gains can be made through "interVAL" training, not "interNAL" training. Interval training is when you intersperse high and low intensity activities throughout your training session. For instance, at one station you might jump rope, throw a medicine ball at another, hit a heavy bag at another, and so on.
You should be doing that regardless of whether you do aikido, martial arts, internal strength, etc. that's just basic fitness.