How come Saiya-Jins were able to access MSSJ post-Cell?

[Legendary Saiya-Jin]

If they were always in base in Post-Cell, how would they be able to access MSSJ without effort let alone SSJ2 and 3? Don't you think they'd have to re-train themselves to hold SSJ again before they could re-utilise MSSJ/ SSJ2 and beyond powers?

[Kamiccolo9]

Maybe they trained in SSJ? We see Vegeta training in his SSJ state in the Buu Arc, and I'm sure Goku did some training as a SSJ between the Cell and Buu Arcs. Gohan didn't really train, but I doubt that he never went SSJ at all during the gap.

[Bussani]

I think they only had to stay Super Saiyan constantly for long enough to get used to it--or master it, if you prefer. They're not going to lose that skill just because they don't stay Super Saiyan constantly after that. Maybe the skill would degrade if they never used Super Saiyan again for the rest of their lives, but we know that isn't the case.

[Kaboom]

Yeah, I don't think Full-Power Super Saiyan is something you can really lose. The whole point was getting the form to where it comes naturally for you. I suppose maybe YEARS of doing a whole lot of nothing could start to negatively affect it (and if you want to get really hypothetical, might help explain Gohan's difficulty in using SSj2 during the Boo arc). But it's like any other learned skill. You'd no sooner lose the newfound ease of Super Saiyan by not maintaining it 24/7 than you would lose the ability to ride a bicycle by not riding it every day.

[Saiga]

Given that Trunks wasn't always in Super Saiyan at the Cell Games, yet shows SSFP features when he returns to his timeline, I think that once you've mastered the form you don't need to keep maintaining it. Maybe Goku and Gohan kept maintaining it after their RoSaT training because they spent less time in there than Vegeta and Trunks did.

[miguelnuva1]

I think Goku and Gohan just staying in the form in the Cell games because they needed to get their bodies use to SSJ outside the ROSAT and just because there was no need to go back to normal.

Once a Saiyan becomes a MSSJ they can't go back to SSJ.

[Kaboom]

I figure Goku and Gohan kept themselves in Super Saiyan even after leaving the room just to keep themselves as "fresh" and ready for the Cell Games as possible. Likely as part of the light amount of training they did outside the room during those 10 days.