Does your horse flex through his poll? If not, the tension there can spread through the whole body. Just imagine what you feel like when your neck is stiff and sore. It is hard to look sideways or enjoy most physical activities.
Here is a test to see how soft the poll area is: Stand in front of the horse and gently push on his nose. Does he give softly? Or does he push back? Does he back away from your hand? If he gently nods his head, he is able to flex the poll.
Horses with tension around the poll may be prone to sudden reactive behavior such as rearing. The horse may also trip or stumble since the area around the first two vertebrae affects movement of the front limbs.
Horse that crib will always have tension in the poll and throat. These horses need frequent massage to keep the muscles loose enough for them to work comfortably.
Check your horses ears. Are the tips cold when the weather is not? This can be another indication of a tight poll and restricted blood flow to the area. If your horse is difficult to bridle, or suddenly pulls back when the bridle is taken off, he may need help relaxing the muscles of the poll.