As the name implies, "Eternal Physique" refers to the flesh and blood of the entire body that has been completely transformed by the variable pressure muscle.
And its effects are also extremely simple—before the depletion of Origin Quality, Ian will not feel tired, and he will hardly get sick or poisoned, unless the virus is some kind of essence construct or caused by high-level alchemy.
Moreover, the Eternal Physique can perfectly utilize most 'strengthening potions.' The body fluid of the Flame Giant Locust is one of the main materials for stamina potions, and its muscular structure has inherited the corresponding abilities, with the same dosage of potion, the duration on the bearer of Eternal Physique will be extended by about 50% to 100%.
Of course, this is not without cost—the Eternal Physique has a peculiar resistance to most ongoing healing potions.