Vowel backness describes how far forward or backward the principal tongue constriction lies during a vowel. Front, central and back are broad regions of this articulatory continuum. It is a separate descriptive dimension from vowel height and lip rounding.[ref]
Essentials of Linguistics (2nd edition) by Catherine Anderson and others. §3.5, Backness and the vowel quadrangle
The close vowels
[i] and [u] give a familiar front/back contrast, but they also differ in ordinary lip rounding. Comparing them alone consequently does not isolate the effect of vowel backness. Holding another articulatory dimension approximately fixed, or measuring it separately, makes a stronger experiment. A vowel chart summarizes configurations; it is not a literal scale drawing of every speaker's vocal tract. New to topics? Read the docs here!