If you want something out of box for free you can use Blender and download rigged models here: https://sketchfab.com/3d-models/rigged-characters-482b7772d3.... Or find poses and animations from https://www.mixamo.com.

Or you can use Houdini's free version, it comes with a fully-rigged mannequin and a realistic proportioned human. You can't export them in free version but if you're a 2D artist you can just screenshot and use it as a reference.

Houdini has everything from full-body muscle simulation to writing a OpenCL image processing kernel there. All free if screenshot-as-export is acceptable in your use cases. If you need actual rendering it's $25/mo billed annually.

This website's pro plus subscription is $15/mo.

(Not saying there is no room for another tool. Just stating the alternatives.)