Spring arcs made of high quality “Space Age” plastic material. Main structural components of the innovative patented Impact Protection System (IPS). Compresses, together with the T-Springs, to provide the spring effect that reduces the impact stress by up to 80%. Comes is different thicknesses adapted to the weight of the users; the Shell 8 fits the children shoe, the Shell 6 L for light adult users of the KJXR3 (sizes S & M), the Shell 6 H for heavy adult users of the KJXR3 (sizes L & XL) and the Shell 7 for very heavy or extreme users on the Pro7 (L & XL). Includes a double way safety system SRS in conjunction with the T-Springs with a locking position for extreme uses. Offer a long product life span.