As part of your duties, you may be required to lift heavy objects. Do you have any physical restrictions?

"I have a back injury that sometimes gives me trouble, but with a brace, I can complete the physical tasks, as described."

If there are physical requirements for the job, they were listed in the job posting to which you applied. If there are physical requirements, the interviewer is assuming you reviewed the job posting before applying for the position, and as such, there will not be any limitations.
If you answer no and physical requirements are mentioned in the job posting, the interviewer may assume you are unprepared for the interview.

"I reviewed your job posting before the interview and am aware of the physical requirements. I have no physical restrictions and can lift heavy objects when necessary."