Hs questions (when to do them, and how to know which muscles are failing first)

I have two handstand questions:

- Should I do them before the main strength training of the day, or after?

- At first I have a good form, with good straight line hands to toes. After a couple of sets I'm beginong to arch my back and can't keep the straight line. What is failing here, the back, abs, shoulders?

Show your handstand first or we do not know what we are talking about.

Just checking are you doing the mobility condition in good form? 

A video would be better to diagnose your issues.

Typically I'd say to do your handstand work before your strength work. Handstand is skill work, so you should be fresh mentally and physically.

If you have a clean handstand that gets arched after a while, typically it would be the shoulders and traps failing first, as arching usually occurs so that you can solicit the bigger chest muscles. But it can also be the abs failing to maintain hollow. A video would be best here.