Why are the crucified persons' legs broken?

Q. Why did the soldiers break the legs of the two men crucified with Jesus?

A. According to those who have studied the grim mechanics of crucifixion, in order to breathe adequately, the crucified victim periodically had to push himself up using his legs. Breaking the legs prevented him from doing that, thus hastening his death. He would suffocate.

The people asked Pilate to break the legs of Jesus and the two thieves crucified with Him so that they would die quickly, allowing their bodies to be taken down from the crosses before the Sabbath (which began at sunset). Otherwise, apparently, the spectacle would have been viewed as a desecration of the holy day. At Pilate’s command, then, the soldiers broke the legs of the thieves, but not of Jesus, because they discovered that He was already dead. Just to make sure, however, one soldier (according to tradition, named Longinus) thrust his lance into Jesus’ side (see John 19:33-34).

As it turns out, that scenario fulfilled Messianic prophecies (as John pointed out): “Not a bone of it will be broken” and “They will look upon Him whom they have pierced” (John 19:36-37; see also Exodus 12:46; Numbers 9:12; Psalm 34:20; Zechariah 12:10).