The color of the spine identifies the attack angle of the spine (and blade) and is established by unique vertebrae geometry of each different color. For example, when the vertebrae fits together like puzzle pieces which defines a joint, a predetermined amount rotation is provided. In the case of Blue spines, the joint between adjacent vertebra is allowed to rotate four degrees (4°) in a first direction, and twelve degrees (12°) in a second direction. If the spine (having five joints) is assembled with the teardrops facing up, then the blade will flex upward twenty degrees during the 'toe up' return stroke, and flex sixty degrees during the 'toe down' power stroke (four degrees times five joints equals twenty degrees 'toe up', and twelve degrees times five joints equals sixty degrees 'toe down'). Blue spines are considered best for popular use. Green spines have vertebra which causes the spine to be straight (0°) in the 'toe up' direction for enhanced frog kicks when pushing away with a flat blade, yet during the 'toe down' direction the twelve degree articulation between successive Green vertebrae will allow the blade to flex sixty degrees just like the Blue spines. Yellow spines limit the blade flex to be twenty degrees in the 'toe up' direction, and seventy five degrees in the 'toe down direction (15° x 5 = 75°). Yellow spines are for the easiest kicking experience, however a user will have to kick slightly faster for a given speed versus Blue or Green spines.Spines may be mix and matched for refined customization.