In todays world of sue or be sued  an employer  can only ask certain questions related to health issues like the ability to lift heavy items. However, a good human resource person worth their salt can created a job description that is easy for a potential employee to understand and follow-up with questions that are within the "federal guidelines.

Obviously an employer can ask an applicant about; previous job experience which will give the employer a heads-up on physical capabilities. The employer can require an applicant to prove the ability to do manual labor like lifting and other physical requirements and even has the right to pay for a physical examination if the prospective employee agrees. However there are much simpler ways  to ask questions about physical abilities.

An employer has the right to discuss duties and responsibilities  of the job asking questions relevant to the job like lifting heavy . Now an employer can not ask this question;  Do you have any disabilities? Instead he can ask;  Are you able to perform the specific duties of this position?

Employers can also have available to potential new employees a hand-out about: "What is the correct lifting techniques". So the potential employee can see and realize the job requires some lifting.