ASICS GEL-Nimbus 17 - Boys' Grade School The Nimbus is back and better than ever, offering more technology features for a smoother, more efficient ride. Clutch Collar System offers an endoskelatal collar that compliments the exoskelatal heel counter for a uniquely secure fit. Biomorphic Fit upper delivers stretch material strategically placed on the upper to reduce buckling and the potential for irritation. Propulsion Trusstic System mimics connective tissue in the foot by creating tension as foot enters propulsion stage. Vertical flex groove decouples the tooling along the line of progression for enhanced gait efficiency. American Pediatric Medical Association (APMA) Seal of Acceptance.