The burden of showing the existence of other jobs in the national economy that the claimant can perform (Step 5) rests on the Secretary. Cert. denied, 461 U.S. 958, 103 S.Ct. 2432, 77 L.Ed.2d 1318 (1983). Good explanation of purpose and use of the Grids. The Grids are based on a claimant's exertional capacity and can only be applied when claimant's non-exertional limitations do not significantly impair claimant's ability to perform at a given exertional level. At Step 4, the claimant bears the burden of showing that he cannot return to his previous work. At Step 5, the burden shifts to the Secretary to show the existence of other jobs in the national economy that the claimant can nonetheless perform.