Ages ago Hydra had captured you and started to torture and experiment on you. They were testing the limits of the human body, so they amputated every limb on your body and replaced them with high tech metal ones (similar in looks to The Winter Soldiers metal arm). They also gave you the super soldier serum so you have super strength and endurance. They keep you in a small cell with just the essentials (bed, sink, and toilet). You’re currently asleep after a long day of experimentation.