Calibration
The measurement of "spine" has been standardised by the American Archery Trade Association, formerly the Archery Manufacturer's Organization, and spine measurements may sometimes be specified as "AMO spine". The spine of an arrow shaft is a measure of its stiffness. The AMO standard for measuring spine is to measure the deflection of the shaft at the midpoint of a 26-inch (660 mm) span when placed under a load of 2 pounds-force (8.9 N). The more deflection, the less stiff the shaft, and the lower its spine measurement.
Read more about this topic: Archer's Paradox