These pieces of equipment can have very detrimental effects if they are adjusted tightly, used strongly, or if used for long periods of time. Horses may become hard-mouthed and heavy, and they will begin to travel on the forehand if the rider can not keep sufficient impulsion. Additionally, many horses that are continuously or incorrectly ridden in draw or running reins may never learn to engage the hind quarters and lift their withers for self-carriage, and this habit may permanently damage their training. In extreme cases, horses may develop neck and back pain from being forced to hold an artificial position for long periods of time. The incorrect use of draw reins will produce horses who are overflexed on short necks with correspondingly flat backs and disengaged hind legs.