Not exactly the same. The key is that the Immortal's aging process will halt, regardless of their current period of development -- e.g., Kenny is trapped forever as a child. There's reason enough to believe that, aside from the aging process, their anatomy combats anything potentially harmful (disease, poison, toxins, etc.); however due to the fact that they can suffer a mortal death suggests that their body is not necessarily frozen in time, as one would expect from a vampire, for instance. Aside from violent death, Immortals can die of thirst, starvation or suffocation -- they'll revive afterward, of course, but it's still possible. This is why they still eat and drink and breathe. Accordingly, it's entirely possible that their bodies can gain/lose weight and muscle definition. Because their survival hinges on their combat skill, it's necessary for them to train and keep themselves in top shape, though I expect Duncan would do this regardless. While they have accelerated healing capabilities, they can't regenerate lost appendages; it's doubtful that they could re-grow an internal organ, though they seem to repair the same way as a surface wound would (with all of the gunshots Duncan's taken over the years, many bullets must have punctured his heart, lungs, kidneys, etc). I wonder what happens to the bullets?
A more curious question is Immortals and their mental health. Their regenerative abilities apparently do not extend to repairing nerve imbalances or any abnormal mental activity. Many of the evil Immortals are depicted as insane, though perhaps they were insane prior to their first "death".
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