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http://www.k-state.edu/media/newsreleases/2017-05/spaceflight5917.html
Astronauts aboard the International Space Station have decreased physical fitness because of a decrease in the way oxygen moves through the body, according to a Kansas State University kinesiology study.
Carl Ade, assistant professor of exercise physiology, and collaborators partnered with the Johnson Space Center to find that astronauts' exercise capacity decreases between 30 and 50 percent in long-duration spaceflight because the heart and small blood vessels are not as effective at transporting oxygen to the working muscle.
"It is a dramatic decrease," Ade said. "When your cardiovascular function decreases, your aerobic exercise capacity goes down. You can't perform physically challenging activities anymore. While earlier studies suggest that this happens because of changes in heart function, our data suggests that there are some things happening at the level of the heart, but also at the level of the microcirculation within capillaries."
Space: Not Even Once.
Decreases in maximal oxygen uptake following long-duration spaceflight: Role of convective and diffusive O2 transport mechanisms (open, DOI: 10.1152/japplphysiol.00280.2016) (DX)
(Score: 2) by bob_super on Friday May 12 2017, @06:12PM
Demand and offer: moving in a tiny box in space requires little effort, and gravity isn't fighting blood flow, so the body optimizes resource allocation.
People who spend many months in a hospital bed can regen their muscles and endurance afterwards, if they didn't get past a permanently damaging point (and are young enough), so I'm going to guess the answer to your question is a cautious yes.
Whether/how quickly you get back to the original level is the interesting part for space exploration.