When modeling appendages that will need to deform in animation, remember that at least two rings that circle the entire diameter are needed to maintain volume.
The first example in the image below has three rings in the front that merge into two in the back. The top loop is 100% weighted to the top bone and the bottom 100% to the bottom bone but the muddle half loop is a 50/50 split between the two. This is the best solution with the lowest poly usage.
If you have to go lower than that, two full rings are necessary like in the middle example. The top loop is weighted to the top bone and the bottom loop on the bottom bone. This ensures both sides of the joint maintain their volume.
The last example is what to avoid. It has two rings that merge into one in the back. The merged area is given 50/50 weight and you can see that it collapses the volume of the joint. Always maintain two rings if you can.