Choctaw, Arrow spine is the amount of flex or bend in an arrow at the center point of the shaft. This is usually calculated using a 28" arrow. The arrow is supported on each end and then pressure or a weight is applied to the center. The amount the arrow sags or bends in the center is considered the spine. Using the same amount of weight or pressure on the center point the more the arrow sags the weaker the arrow spine and the less flex the stiffer the spine. This gives you a basic idea on the flex of the arrow with all the components added. You can reduce the spine of an arrow by changing components. A heavier tip or broadhead will weaken the spine. Installation of extended nocks like the Beiter will stiffen the spine. This is why an arrow selection chart will ask many questions as type of cam, draw weight and length of draw. Each of these items have an impact on the static weight and excelleration of the arrow when released. The harder the cams, length or tip weight will determine how much flex there will be at the center of the arrow upon release.