Thorax is the rib-cage. I am looking at the angle of the ribcage/spine, like if you balanced a stick along her spine in her ribcage.

Yeah, that first MTB'er is seriously rocking! Ouch! Ouch! Ouch!

I noticed the second one looked like she had locked elbows, now I gotta go check out her rocking, too.

Obsessions take so much work!