Proper Hoof Biomechanics - hoof function during movement:
A healthy hoof is meant to expand upon impact, with the leg fully extended and the heels landing first. If the hoof does not function this way because of mechanics, ie. long or under run heels or heel pain associated with Navicular Syndrome then we lose proper mechanics. When we lose proper Biomechanics, the hoofs circulatory system is incapacitated and cannot function the way that it was made to. As the hoof loses circulation it starts to contract and atrophy until the heels are so close together that proper function is mechanically impossible. This is the state that we find many of our horses in today.
The Traditional Pasture Trim: A pasture trim is a "normal" barefoot trim performed by a Farrier which involves carving out the concavity or healthy "bowl" shape of the sole. The bowl shape is optimal for correct biomechanics and shock absorption, but if you try to carve it instead of letting the horse grow it, much damage can be done. The reason that concavity is not present in many cases is because the inner structures of the hoof have moved down in the hoof capsule, the coronet band is pushed up, and there is less sole than there needs to be in these situations. Carving this shape into the sole causes soreness at a minimum and sometimes complete lameness until the horse can grow in that missing sole.
Barefoot Versus Shod - Impacts on the Hoof:
Shock Absorption, Concussion and Reverberation-
Barefoot: The barefoot hoof absorbs 70% - 80% of the impact of the horses weight as it descends. Video of Barefoot Hoof in Action
Shod:
- In a shod hoof the
shock absorption that normally happens in a barefoot hoof is diminished
by 70%- 80%. The
concussions forces that a shod horse receives on pavement at a walk, is
three times that of a barefoot horse on
the same surface at a trot.
- Studies done by the Swiss Cavalry in Zurich found that a horse shoe on pavement sustains vibrations up to 800 hz which is enough to damage living tissue. Video of Shod Hoof in Action
Circulation-
Barefoot: As
the hoof expands upon impacting the ground the blood is allowed to rush
into the hoof and when
it is pick back up into it's passive phase the hoof constricts and
blood is squeezed back up
the leg. 
Shod: The horse shoe is nailed on in the passive, contracted stage of movement when it is off the ground causing constriction of the blood supply. The circulation becomes stagnated or in a state of numbness similar to having a rubber band around your wrist. So when the shoes are finally pulled off it is quite normal for the horse to be uncomfortable for awhile while the hooves "come alive" again with circulation, also the normal sole callusing is absent and has to be rebuilt- so to speak. To the right you can see a Thermographic image of a horse with only one shoe on, allowing us to see the lack of circulation in that hoof.
Protection- A healthy hoof wall and callused sole form a light weight "shell" that is nearly impervious to sticks and stones - no pun intended :)
Traction- The bare hoof is amazing in it's traction abilities and far surpasses the shod hoof on many types of terrain including wet cement, mud, ice and rocky ground.