I am looking for a word, an adjective, to substitute for 'back and forth' to describe the exerted forward reach then recoil action of a horse's head in a gallop.
'Reciprocate' and 'oscillate' are both problematic - one is precise, machine-like controlled, the other determined organically by gravity, or robotically like a metronome. 'Annuent' is an antiquated word for 'nod', as in a human head nod of assent (based on annuent muscles in head/neck) and fits nicely ... but is unconventional... e.g. "The annuent up-down motion of the horses' heads towards the finish post, ended when the camera froze the inside horse a nose in front".
Any thoughts on a better one?