When I play multiplayer I cant hold or relax all I can do is contract and extend

Go into your game settings and change mouse buttons to 1.

Ooooooooh Never Knew that.....

Or you could just hit x...

That's for whole body though. It'll mess up his move since it'll take forever to change joints back to the way they were before the last move.

Other option is to roll the mouse ball thing. The scroll wheel.