I use side reins all the time on my horses. Moderation and gradual conditioning are the keys. They're really useful for building muscle and teaching balance. My TB's trot has gotten much more controlled and rhythmic since I started using them. I don't think they restrict movement at all, in fact, they've had the opposite effect on my horses. I was always taught that side reins are only a lunging tool, never for riding. Although I have seen people ride with them.