HEAVY: Ronald Post claims his over 200kg weight means his execution would be too slow to be humane.

A condemned Ohio inmate who weighs at least 218kg wants his upcoming execution delayed, saying his weight could lead to a "torturous and lingering death".

Ronald Post, who shot and killed a hotel clerk in northern Ohio almost 30 years ago, said his weight, vein access, scar tissue and other medical problems raise the likelihood his executioners would encounter severe problems.

He's also so big that the execution gurney might not hold him, lawyers for Post said in federal court papers filed Friday.

"Indeed, 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," the filing said.

Post, 53, is scheduled to die January 16, for the 1983 shooting death of Helen Vantz in Elyria.

A spokeswoman for the prisons department had no comment on the pending litigation.

Inmates' weight has come up previously in death penalty cases in Ohio and elsewhere.

In 2008, federal courts rejected arguments by condemned double-killer Richard Cooey that he was too obese to die by injection.

Cooey's attorneys had argued that prison food and limited opportunities to exercise contributed to a weight problem that would make it difficult for the execution team to find a viable vein for lethal injection.

Cooey, who was 1.7m and weighed 121kg, was executed October 14, 2008.

In 2007, it took Ohio executioners about two hours to insert IVs into the veins of condemned inmate Christopher Newton, who weighed about 120kg. A prison spokeswoman at the time said his size was an issue.

In 1994 in Washington state, a federal judge upheld the conviction of Mitchell Rupe, but agreed with Rupe's contention that at more than 181kg, he was too heavy to hang because of the risk of decapitation.

Rupe argued that hanging would constitute cruel and unusual punishment.

After numerous court rulings and a third trial, Rupe was eventually sentenced to life in prison, where he died in 2006.

Ohio executes inmates with a single dose of pentobarbital, usually injected through the arms.

Medical personnel have had a hard time inserting IVs into Post's arms, according to the court filing.

Four years ago, an Ohio State University medical center nurse needed three attempts to insert an IV into Post's left arm, the lawyers wrote.

Post has tried losing weight, but knee and back problems have made it difficult to exercise, according to his court filing.

While at the Mansfield Correctional Institution, Post "used that prison's exercise bike until it broke under his weight", according to the filing.