When rigging in Houdini, do we always need to create muscles? Based from the a tutorial in sidefx, the skin geometry was attached to the muscles. The muscles are attached to the bones.

Is this always the case, or can we attache the skin geometry directly to the bones?

No you don't have to use muscles.

There are two options: Capture (Skin) by proximity/capture regions or capture (skin) by muscles.

The Capture Geometry tool gives you the three options: Proximity, Capture and Metaballs where Metaballs means muscles.

please note that you CANNOT combine ‘capture to bones’ and ‘capture to muscles’

you can hang muscles on bones to move them around but when you capture the skin it can only be one or the other.