What is the burden of proving a disability claimant is able to work?

Cir. 1996) ("If the claimant satisfies her burden of proving the requirements in the first four steps, the burden then shifts to the [Commissioner] to prove in the fifth step that the claimant is capable of working."); Bapp v. Bowen, 802 F.2d 601, 604 (2d Cir. 1986) ("Once a disability claimant proves that his severe impairment prevents him from performing his past work, the [Commissioner] then has the burden of proving that the claimant still retains a residual functional capacity to perform alternative substantial gainful work which exists in the national economy.").