Breaking the legs of victims on the cross was a common practice by the Romans to speed up death. Soldiers would use the steel shaft of a short Roman spear to shatter the person’s lower leg bones. This would prevent the individual from pushing up with his legs so he could breathe, and death by asphyxiation would result in a matter of minutes.
When the soldiers approached Jesus, they determined that he was already dead, and one used his spear to pierce Jesus’ side to confirm it (see John 19:34). This fulfilled another Old Testament prophecy about the Messiah: his bones would remain unbroken (see Psalm 34:19–20).
Adapted from interview with Dr. Alexander Metherell