Crucifixion was a method of execution used by the Romans for slaves, pirates, and enemies of the state. It was considered a most shameful and disgraceful way to die (Phil. 2:8). Condemned Roman citizens were usually exempt from crucifixion except for major crimes against the state, such as high treason. The victim was either tied or more often nailed to a large wooden cross and left to hang until dead. Iron spikes were driven between the wrist bones and ankle bones. Gravity prevented the victim from using the chest muscles to breathe. Shallow breathing was only possible using the abdominal muscles. A painful pushing up with the legs was required to exhale. The intention was to inflict rapidly increasing pain to achieve a prolonged, agonizing death. Death usually came by asphyxiation which was hastened in some cases by breaking of the legs below the knee, as was the case with the two thieves. Crucifixion is referred to in scripture as being "lifted up" (John 3:14; 8:28; 12:32), as "hanging on a tree" (Acts 5:30; 10:39; 13:29; Gal. 3:13), and as "stretching forth the hands" (John 21:18).