Adam Clarke on John 21:18

Thou shalt stretch forth thy hands - Wetstein observes that it was a custom at Rome to put the necks of those who were to be crucified into a yoke, and to stretch out their hands and fasten them to the end of it; and having thus led them through the city they were carried out to be crucified. See his note on this place. Thus then Peter was girded, chained, and carried whither he would not - not that he was unwilling to die for Christ; but he was a man - he did not love death; but he loved his life less than he loved his God.