We've all known a tender-footed horse. Work over hard, rocky or rough surfaces makes the horse obviously short-strided, while grassy or otherwise soft ground conditions give him a noticeable improvement. Tender feet in horses have many causes, some of which are preventable.
Some horses with hoof pain have diagnosed conditions such as navicular disease or ringbone. Soft surfaces are more comfortable for the horse because there's less concussion. Other horses have normal X-rays but poor quality hoof walls, flat soles, a clubfoot or trimming issues that cause discomfort. Evaluation by a competent veterinarian and farrier can set those on the right track. You may even need to get a second opinion from another farrier or veterinarian to rule out these causes.
However, some tender-footed horses that are properly trimmed and have good quality hooves with negative X-rays may still have ouchy feet. If the horse is also overweight, prone to developing a prominent fatty crest or has fat pockets above the eyes or bulges of fat in odd places such as the base of the tail or around the withers, consider the possibility that a metabolic problem may be causing the foot pain.
Insulin resistance is a condition similar to early stages of type II diabetes in people. This isn't the type of problem that requires insulin injections. The problem is that the cells don't respond normally to the insulin the body produces.
Insulin resistance can happen in horses, too. In older horses, it's a common complication of Cushing's disease, a tumor in the pituitary gland. In younger horses, it can be found in individuals that are "good doers" or "air ferns," holding their weight or even getting overweight on feeding programs that don't have the same effect in other horses.
In a nutshell, their metabolism is geared to putting on weight and saving their calories, rather than efficiently burning them. When these horses are put on a strict diet, they have a great deal of trouble losing weight because their body interprets the drastic calorie cut as an "emergency" situation and responds by getting even more insulin resistant.
The extra weight these horses carry certainly puts considerable strain on their feet (and joints), but that's not the whole story. In the worst-case scenario, insulin resistance puts the horse at high risk for laminitis, an inflammation of and damage to the laminar connections in the hoof that help hold the coffin bone in place. If this is severe enough, the horse will founder. Even if full-blown laminitis/founder doesn't develop, the insulin resistance can result in low-grade, painful inflammation and interference with blood flow to the feet.
If you suspect this might be going on with your horse, ask your vet to run an insulin and glucose blood test. Don't feed any grain the day of the blood test. Hay is OK. The glucose usually won't be elevated, but if insulin is, your horse is likely insulin-resistant. A diet of moderate calories (no added fat or large amounts of high-fat ingredients), correct mineral supplements and no grain can help your horse get to a normal weight and may alleviate the foot tenderness.