The Sinew Coils, also called The Steel Within, is one of the twenty-two gene-seed implant organs of the Adeptus Astartes, and one of the three unique only to the Primaris Space Marines.
A Primaris Space Marine's sinews are reinforced with durametallic coil-cables that can contract with incredible force, magnifying his strength as well as giving his body another layer of defence. A Primaris Space Marine can crush a man's skull in his hand, break Flak Armour to flinders, or even bite through a metal cable should the need arise.
See Also
Sources
- Codex: Adeptus Astartes - Space Marines (8th Edition), "The Making of a Space Marine", pp. 10-11