The chakras fill the entire area they're assigned to; a person working with energy may feel sensations primarily in the front or in the back, but it all depends on their focus. Some of the most effective energy work involves making sure energy is felt not just in front or in back, but through that whole section of the physical body where the chakra is located. If you were to strongly activate your chakras, you would find that it fills that whole area of your body -- from front to back.
Kundalini is said to be a kind of dormant energy located in the tail bone, and normally moves up the spine when it is activated. This doesn't necessarily mean that all energy is supposed to move exclusively through the spine, only that kundalini usually does this.