The brachiocephalicus is the prime muscle for forward movement. This muscle starts at the top of the head and travels down the neck to the top of the front leg. When the horse is ridden correctly from behind, this very long muscle can do its job freely. However, if the rider has a death grip on the horses’ face, lengthenings, circles, and canter departs will all be hampered. The jaw and neck will be stiff. The horse becomes exhausted more quickly as it tries to move its legs without the help of the rest of the body. You can see the horse flicking his toes instead of lengthening his frame. I also see this in school horses who have inexperienced riders using the reins to balance.
Even when the rider is soft, the neck can still become stiff, especially after a lot of collected work. Sports massage can provide relief, allowing energy to flow through the entire body.