Praise then darkness, and creation unfinished

I have a sore muscle in my left upper arm. I'm fairly sure it's the triceps: when I went to do the triceps exercise on Tuesday, at first I didn't get the left side of the machine to move, and then (according to the trainer) I put more energy on the left than the right.

I'm trying to rest it, which is difficult because so much of which arm gets used for things like holding poles on moving trains is long-ingrained habit; I skipped the gym today, and will probably do almost entirely lower-body stuff on Saturday, and maybe a bit of right-arm only for biceps and such.