These days the term arrow spine refers to deflection of the arrow as is calcualted as above. However, the spine of the arrow is actually the weakest part of the arrow as compared to its deflextion as it is spun on its axis. Finding the arrows spine especially when using cedar arrows on stick bows is very important. Keeping the spine aligned with the cock feather is very important on stick bows. Even with my compound, I mark and align the spine of the arrow, so that they come off the bow consistant. You will find that aluminums have the most consistant spine and wooden arrow the least. Carbon would be somewhere in the middle.