Shown below are some illustrations of another vestibular reflex stimulated by extension of the neck. In the pure form of this reflex, called the Symmetrical Tonic Neck Reflex (STNR), when the neck is extended forelimbs (or arms) extend while hindlimbs (or hips & knees) flex.
In human development, the STNR enables the baby, at around 6 months old, to get itself up off its tummy for the first time, and point its own spine upward. In evolutionary terms, after our ancestors slithered out of the primeval swamp and practiced crawling on the belly for a few millions years, it must have been the STNR, or some precursor thereof, that enabled us, for the first time, to get up on to all fours.
In an increasing number of sittings as I have got older, my sitting-zen does not seem to get this far up the developmental/evolutionary food chain, in which case I try to find a bit of ease in my pre-STNR, sub-simean slumped condition -- as opposed to the alternative of trying to show a more human form by stiffening my neck into extension.