There is a lot to question here; for instance how can we really know that his legs were not broken. My take is that we can only, at best, state that there is no obvious evidence that the legs were broken.
Have a look.
Abrasions on the shoulders of the man of the Shroud, particularly on the dorsal image of the right shoulder, indicate that he carried a heavy object, such as the transverse beam of a cross. This must have occurred after he was scourged because the scourge wounds are underneath the shoulder abrasions. But if the crossbeam had been in direct contact with the scourged shoulders, the lacerations would have widened, but on the Shroud, they have kept their shape. This is consistent with the man on the Shroud carrying his cross under which was a garment protecting his scourge-wounded shoulders, as we saw that the gospels of Matthew and Mark recorded.
On Carrying the Crossbeam:
The man on the Shroud has cuts to both knees, especially to his left knee, indicating an unprotected fall onto a hard surface. A Roman crucifixion victim was made to carry the horizontal crossbeam tied to his outstretched arms and placed across the back of his neck. Which meant that when he fell, which would have been often in his scourged-weakened condition under the heavy weight of the crossbeam, on the uneven, climbing path to the crucifixion site, he could not protect his face from the impact of the fall. This explains why the man on the Shroud’s nose is swollen, displaced and had been bleeding. It also explains why the nasal area of the Shroud contain a high concentration of ground particles and dust.
On Being Nailed to the Cross:
The man of the Shroud was nailed to a cross. He has a bloodstain on the back of his left hand, which overlays his right hand, showing that his hands were pierced by nails through his wrists, not through his palms. This is anatomically accurate as French surgeon Dr. Pierre Barbet (1884–1961) demonstrated, that nails through the palms would tear through by the weight of a man’s body on a cross. The man’s left foot appears to have been forced over his right foot and both fixed to the cross by a single nail driven through the insteps.
On Having Died on the Cross:
The man on the Shroud is dead. He has a swollen abdomen which indicates he died of asphyxiation, the way crucifixion victims died. Also, the body of the man on the Shroud is in a state of rigor mortis, in which the muscles stiffen, keeping the body in the position it was immediately prior to death. Signs of rigor mortis on the Shroud man include: his head is bent forward, the chest and abdomen are "frozen", and his whole body is rigid and stiff, occupying some of the positions it did on the cross, especially his left leg. Further evidence that the man on the Shroud was dead is the post-mortem blood flows, especially from the chest wound. If the man’s heart had been beating the blood would have spurted out onto the cloth, instead of oozing out as it did.
On His Legs Not Broken:
The legs of the man on the Shroud are not broken.This is despite the crurifragium, the breaking of a crucifixion victim’s leg-bones with a heavy mallet, to hasten his death, because he could no longer use his legs to raise himself up to breathe , being the norm in Roman crucifixions. As we saw above, Jehohanan’s legs had been broken and the Gospel of John records that the Roman soldiers broke the legs of the two robbers crucified with Jesus, to bring about their immediate death(Jn 19:31-32).
On the Lance Wound:
The man on the Shroud was speared in his right side. Clearly visible on the Shroud is a lance stab wound in the man’s right side together with an effusion of blood and clear fluid. The wound is on the left-hand side of the Shroud image but because of mirror reversal it was in the right side of the man of the Shroud. The wound and its bloodstain is immediately adjacent to one of the triangular-shaped burn marks from the fire of 1532 (see "part 12"), yet miraculously was not covered by it.
The image is taken from Stephen’s blog. The caption reads: "G. Ricci, `Crucifixion,’ sculpture in wood according to research carried out on the Holy Shroud".