How do you link that practice of spear and taijutsu ?
It's becoming increasingly difficult for me to answer this question. I really don't see a difference between weapon work and Taijutsu. I think, many people say "taijutsu" and mean a kind of karate (empty hand techniques). I don't really understand taijutsu this way. I understand taijutsu to mean: body skills. The body may or may not have a weapon attached to it.
Specific weapon skills- Kenjutsu, Jojutsu, Sojutsu etc teach how you can best use your body to interact with that specific weapon. While taijutsu teaches how you use your body overall. Any weapon system that is part of a larger taijutsu system, must have the same body use skills- same taijutsu, if that system has any continuity.
So, I would say the two (my taijutsu, and sojutsu) are linked in every way. The sojutsu is an extension of my taijutsu.