A Chill Down His Spine
Even if I just run my fingers down the horse's back, he's very "goosey."

When put under saddle, the horse starts with a few bucks before warming up.

From a veterinary standpoint, what is a cold back and what can be done to make the horse more comfortable?

A cold back or girth sensitivity can certainly be challenging. Typically these horses are not experiencing true back pain but rather have very sensitive skin. Many react as you describe: sensitive to touch, along with being very temperamental to girth tightening and/or bucking when first mounted.

With this said, I do believe it's prudent to have a full exam performed on the horse to make sure there aren't other issues that need to be addressed. However, true back pain usually manifests itself as poor performance while riding. Many cold back/girthy horses are fine after the initial saddling episode.

There are many things that I look for when examining a horse for back pain. First palpate (feel) the back looking for pain responses while also visually inspecting for any abnormal contours. Normal response to deep palpation of the muscles along either side of the vertebral column (back bone) and the vertebrae themselves is flexion of the back downward. Excessive downward flexion or none at all with irritation displayed by the horse, is suggestive of pain. Also, when the croup is palpated the back flexes upward. When pain is present in the spinal column the ability to flex is usually diminished, therefore horses with true back pain have difficulty bending in circles under saddle. There horses also have trouble collecting (rounding their backs and bringing their hind legs under themselves).

I also closely examine for lameness and evaluate performance while the horse jogs in-hand traveling straight and in circles, on both hard and soft surfaces. In addition, I like to examine a horse's performance while saddled, with and without a rider. Often, some portion of a horse's gait will be affected by back pain, such as the walk or one of the transitions (walk/trot, trot/canter). I also pay close attention to fit and positioning of the saddle - occasionally I find that the saddle is placed too far forward and is restricting movement of the horse's shoulders.

Unfortunately, there doesn't seem to be much we can do for the cold-backed horse, but we can help them by paying attention to how we tack up. Horses are usually more willing partners if their girths are tightened very slowly. Some horses react very violently when their girths are tightened too quickly. For horses, the should be slowly tightened while they are walking. Some horses respond well to lunging with saddle on to warm up their backs before mounting.
This article published here with permission Horse Illustrated July 2003 issue.