HELL NO!!! The Niners fullback is doing straight-up Rocky-style workouts while in isolation ... squatting a whole damn tree in the snow!!!

Juszczyk's smokin' hot wife posted the footage of the 28-year-old getting in lifts out in the wilderness ... and it's pretty intense.

You can see in the videos, Juszczyk sled-pulls his wife (and others) through snow AND dirt to get some leg work done.

Then, the 6-foot-1, 240-pound NFL player gets some jump squats in ... with a dead tree on his back!!

"When the gyms are closed but Kyle still needs go workout!" Kyle's wife, Kristin, said. "😂😂"

Of course, Kyle's on a mission this offseason ... his Niners came THIIISSSS close to being Super Bowl champs last month -- and the window to try to do it again is still open.

By the way, when Kyle's not working out like a madman ... it's safe to say he ain't exactly suffering in isolation -- 'CAUSE HAVE YOU SEEN HIS WIFE?!?