How to improve the rig in blender to prevent weird mesh deformations?

I generated a character in makehuman and imported it blender. I have neither used Mhx rig or rigify as they will decrease performance due to lots of bones. I just used simple HumanIK rig. But, the rig doesn’t contain any IK bones, so I created them myself. But, now I am having a problem that the deformations of mesh are not that natural. On rotating upper shoulder character rotates in weird way and on rotating hip the character hip mesh gets deformed. It is hard to explain the exact things happening. Just the problem I said. So, is there Any blender user who has good experience with rigging? I just need few suggestions to improve my character rig :confused: ? You can get the .blend file from here:

You can see the problem in this video:

You can’t take 1 bone in the spine, rotate it 180 degrees, and expect it to look “natural.” The bones of rigify or mhx rig build mechanisms that smooth the curvature of the spine.

If you want to build it yourself, Nathan Vegdahl’s, Humane Rigging series is one of the best places to start.