THEORY: Laxness?

Why is [I] lax and [i] tense, [E] lax and [e] tense? I understand the
terms "tense" and "lax" are relatively poorly defined, but if a
language has spots in which vowels go that differ in length (e.g.
because of a length distinction, or open vs closed syllables), it is
common to find the "lax" one in the short position, and the "tense"
one in the long position. This is often described in terms of
undershoot: in short spots, in order to reach the tense target, the
tongue would have to move too fast (or something similar), and so it
doesn't quite reach the target; this failure may become phonologised
so that these accidents become standard allophones or even even
distinct phonemes with loss of the earlier length.

This description implicitly takes as its starting point a neutral
resting position of the tongue, something in the vicinity of [@] (or
perhaps even lower; [E] and [O] are lower than [@]). But in connected
speech, vowels don't alternate with a neutral resting tongue position,
nor (for the most part) with other vowels (which might draw them to an
average position likely around [@]). They alternate with consonants,
which mostly involve closures towards the top of the mouth, and draw
the tongue upwards (the main exceptions to this are pharyngeals, which
draw the tongue down, bilabials, which do not involve the tongue, and
possibly uvulars---I'm not quite sure). So wouldn't undershoot
encourage [i] and [e] in comparison to [I] and [E], which are further
away from where the tongue is, and where it's going to?

Am I missing something comparatively obvious here? (Maybe jaw height?
Are consonants pronounced with the jaw in a more open position, so
that for [i] you have to raise your jaw and front your tongue, whereas
for [I] you only need to mostly front your tongue, and [e] is some jaw
raising and tongue lowering & fronting but [E] is just tongue lowering
& fronting? But I think in langs with only one unstressed vowel, it
tends towards a high vowel, which seems to contradict this.)

--
Tristan.

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