i got no idea Malda. If I had to guess id say it has an effect on them. Its a constant motion, motion requires energy, exerting energy tires anything out........
Some head nod is a given in gaited horses. Its necessary. But after 10 miles on a trail an exagerated head nod will grate on me. I would personally walk away from a horse that has an exaggerated head nod. To me it proves nothing but a tiresome ride for myself.