Eye strain question: effective focal distance of VR?

When you focus on something close to your face, your "eye muscles" (Ciliary muscle) contract. When you focus on something in the distance, they relax.

With VR headsets, what is the effective focal distance? Sure you get the perception of depth from stereoscopy, parallax etc, but are your eye muscles still contracted to focus at a very short fixed distance the entire time? Does that lead to eye strain when playing for long periods?