Ronald Post, 53, was convicted of murdering a hotel clerk during an armed robbery and has been on death row for 28 years. He is scheduled for execution on January 16, a state prison official said.

Post now weighs more than 480 pounds (220 kilograms).

"Given his unique physical and medical condition, there is a substantial risk that any attempt to execute him will result in serious physical and psychological pain to him, as well as an execution involving a torturous and lingering death," says a motion filed by his attorneys.

"Ronald Post is not asking for a stay because he is obese," his attorney Joseph Wilhelm said in a statement, citing other legal issues in the case that he says are unresolved.

But Wilhelm said an anesthesiologist had determined that "Ohio's execution protocol simply will not work on Post. If it kills him at all, it could take up to 16 hours."

The lawyer has invoked Post's right under the Eighth Amendment of the US Constitution, which protects citizens against cruel and unusual punishment.

Unlike other states, Ohio only conducts executions by lethal injection, with a single dose of pentobarbital. No alternative method is offered.

In the court papers filed last week, a copy of which was obtained by AFP, lawyers say that four years ago, emergency medical personnel at a university hospital in Ohio had to try three times to insert an IV into Post's arm.

The execution gurney table also might not support Post's weight, the filing said.

Post has tried to lose weight through exercise, but joint problems have impeded his efforts.

At one facility, "he used that prison's exercise bike until it broke under his weight," the court filing says.

Post asked to be allowed to undergo gastric bypass surgery, but his request was denied. His severe depression makes dieting difficult.

Wilhelm said Post should get a stay of execution due to other mitigating factors in his case.

"Ronald Post deserves relief from his death sentence because of numerous questionable legal issues in his case. If he is executed, he deserves the quick and painless execution promised by Ohio law," the lawyer said.