I drew the curves first in the study below. Somehow they reminded me of part of a leg bone near the joint. It was only natural to insert the ‘ball’ part of the ‘ball and socket’ joint, to complete the idea.
The design came out fine, only it wasn’t exactly the picture I had in my head. I really wanted it to be shaded like a steel ball bearing, to contrast with the organic shapes in front of it.
This is why it is so important to practice, practice, practice.