The Crucifixion of St. Peter

The cult of the martyrs is in that sense, it's an expression of the identification that Christians in the middle ages are feeling with the sufferings of Christ himself. So around the year 1000 you start to get portrayals of Christ in his full suffering on the cross. And over the centuries that follow throughout the course of the high mid-lages, the emphasis on Christ's sufferings become more and more intense. But because Christ has provided this model of suffering and triumph, they were suffering, therefore those who follow him can share in it.