The Atonement...

From Grudem's Systematic Theology

Many readers of the Gospels in the ancient world would have witnessed crucifixions and thus would have had a painfully vivid mental picture upon reading the simple words "And they crucified him" (Mark 15:24). A criminal who was crucified was essentially forced to inflict upon himself a very slow death by suffocation. When the criminal's arms were outstretched and fastened by nails to the cross, he had to support most of the weight of his body with his arms. The chest cavity would be pulled upward and outward, making it difficult to exhale in order to be able to draw fresh breath. But when the victim's longing for oxygen become unbearable, he would have to push himself up with his feet, thus giving more natural support to the weight of his body, releasing some of the weight from his arms, and enabling his chest cavity to contract more normally. By pushing himself upward in this way the criminal could fend off suffocation, but it was extremely painful because it required putting the body's weight on the nails holding the feet, and bending the elbows and pulling updward on the nails driven through the wrists. The criminal's back, which had been torn open repeatedly by a previous flogging, would scrape against the wooden cross with each breath. Thus Seneca (first century AD) spoke of a crucified man "drawing his breath of life amid long-drawn-out agony" (Epistle 101, to Lucilius, section 14).
A physician writing in the Journal of the American Medical Association in 1986 explained the pain that would have been experienced in death by crucifixion:

Adequate exhalation required lifting the body by pushing up on the feet and by flexing the elbows...However, this maneuver would place the entire weight of the body on the tarsals and would produce searing pain. Furthermore, flexion of the elbows would cause rotation of the wrists about the iron nails and cause fiery pain along the damaged median nerves...Muscle cramps and paresthesias of the outstretched and uplifted arms would add to the discomfort. As a result, each repiratory effort would become agonizing and tiring and lead eventually to asphyxia.

In some cases, crucified men would survive for several days, nearly suffocating but not quite dying. This was why the executioners would sometimes break the legs of a criminal, so that death would come quickly, as we see in John 19:31-33