Rubber "bell" boots, sometimes called overreach boots, protect a front foot or leg in the case of "clipping" from a rear foot.

It is not abnormal for a horse to have some asymmetry in one leg due to one leg being weaker than another.  Dr. Adrian Harrison from Denmark indicated in "The Horse" magazine that placing something loose around the fetlock of the weak leg causes the horse to feel something on the leg and will engage that leg, strengthening it more. He said whatever is applied to the leg does not have to be heavy but can be as simple as a bell boot. By strapping the boot loosely on the weaker leg during normal training, riders can help their horses build muscle in that leg. This technique is helpful for a leg that may have been injured or has an opposite dominant leg. Dr. Harrison indicates this really helps horses that have previously injured a leg and need to get it back into shape. 

The concept is proprioception, which is an individual’s awareness of their body parts. The bell boot allows the horse to remember that specific leg and use it more. The Danish team was able to use biotechnology to confirm that proprioception is working to increase muscle activity.  In the study, eight amateur dressage horses were sound but had slightly weak left hind limbs as measured when circling to the left. Bell boots were used for 60 minutes every three days in the left hind leg for six weeks during normal workouts. After six weeks, the legs were tested, and the right and left legs were found to have the same amount of strength. Most horses have one leg they prefer to use more than the others, and although this may not cause a lameness, it could affect performance, and many times experienced riders can feel the difference. If there is an imbalance, it could lead to more serious problems if not checked.