Warning: spoilers ahead for Captain America: Sentinel of Liberty #1!
Following the apparent death of Steve Rogers, Bucky Barnes (aka the Winter Soldier) became Marvel's new Captain America, managing to replace the Star-Spangled Avenger despite lacking the Super-Soldier Serum that gives Steve his enhanced abilities. Now, Captain America: Sentinel of Liberty #1 explicitly confirms how the Winter Soldier is able to keep up with genuine superhumans.
Bucky Barnes has one of the more tragic origins and backstories compared to other superheroes in the Marvel Universe. He was famously once Captain America's sidekick and a member of the Invaders in World War II. But after an accident, Bucky was lost at sea just like Captain America. Except Bucky wasn't found by the Americans like Cap. He was found by the Russians and turned into their Winter Soldier, an assassin who was fitted with a cybernetic arm and periodically woken from cryogenic sleep to go out on missions only he could accomplish.
Related: Captain America: Bucky's Nickname Is Secretly Based On A Real Hero
In Captain America: Sentinel of Liberty #1 by Jackson Lanzing, Collin Kelly, and Carmen Carnero, Bucky and Cap seek to stop a threat to a parade. While searching for the threat, Bucky points out that while Captain America was on ice, his life was defined by decades of going toe to toe with superpowered beings and even gods. The Winter Soldier was frozen between missions, meaning that his life was a near-unending gauntlet of impossible experiences more extreme even than Steve Rogers' long and accomplished career.
This explains how the Winter Soldier is as strong and fit as he is. While he may not have the Super-Soldier Serum in his veins, he spent the prime of his life
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