What is the burden of proving that physical impairment prevents a claimant from returning to his previous occupation?

At step five of the sequential analysis, once a claimant has proven that his physical impairment prevents a return to his previous occupation, the burden shifts to the Secretary to show that the claimant can engage in other types of substantial gainful work that exist in the national economy. Gallant v. Heckler, 753 F.2d 1450, 1452 (9th Cir.1984). The Secretary must consider the claimant's residual functional capacity and vocational factors such as age, education, and past work experience. 20 C.F.R. Secs. 404.1520(f), 416.920(f) (1989).