There are actually two different types of spine — static spine and dynamic spine.
Static spine is measured by the amount of flex in the arrow when an 880-gram (1.94-lb.) weight is suspended from the center of the arrow. The arrow must be 29 inches in length and supported by two points, which are 28 inches apart. The number of inches the arrow deflects or bends due to the weight is the spine size, or spine measurement, of the arrow.
Dynamic spine describes how an arrow reacts from the stored energy of a bow as it is shot. There are several factors that determines how an arrow will react when released, including whether it is released with fingers or a mechanical release; the amount of energy applied by the bow; the bow's cam system (single, round wheel, hard or soft cam); the overall weight, static spine, and length of the arrow; point weight; nock weight and fletching weight. Even nock set material (traditional brass nock or serving nock, and the string loop), along with string and serving material, can influence dynamic spine. Because of the nearly unlimited variables in determining dynamic spine, arrows are usually measured using static spine.
Spine is very important when it comes to tuning, shooting and grouping your arrows. If you do not have the correct arrow spine for your bow setup, you are going to get erratic arrow flight and poor groups. Having the proper arrow spine is key for optimizing the grouping of your arrows and for the best possible accuracy. All major arrow makers offer a spine selection chart that will help you make the correct choice. When looking at the shaft selection chart, one of the things you might find is that you are on the cusp of choosing between different spine sizes. That’s true for me. I have a 28-inch draw length and shoot a 28½-inch shaft out of a compound bow with a draw weight of 70 pounds, usually choosing a 100-grain broadhead. This puts me on the cusp of choosing between 400- and 340-sized shafts. In most cases, I find that choosing the shaft with a bit stiffer spine — in this case, the 340 — makes it much easier for me to properly tune my bow and achieve the ultimate in accuracy. The stiffer arrow weighs a bit more and I end up giving up a few feet per second in raw arrow speed, but as a good friend once told me, “A slow hit is better than a fast miss any day!”