Biceps on fire

I dropped the weighted dips and pull ups this month in favor of muscle ups on the rings. Happily, I'm still able to bang them out, though I'm still not quite up to a solid set of 10 muscle ups yet. I realize now that my abs and core aren't quite strong enough for me to make it through 10. I was strong enough to force myself through a 7th rep using mostly my arms. I paid for it later though. My biceps have been sore ever since that muscle up workout a couple of days ago.

And today I went on with my iron cross practice as usual (what could be more fun on a Friday night?). My biceps are burning even more now. Why exactly do I do this to myself?