Whatever is the absolutely most comfortable, as you have no other concerns. My time on Zwift was spent entirely due to a broken hand, so I couldn't even rest one hand on the bars. I built a table that sat over the bars, so I could rest my elbows and forearms on it. I still never managed more than around 3.5 hours at a stretch, but it was a whole lot nicer than gripping the bars on the dumb trainer for even an hour. Nothing "real world" translates to Zwift. It's a video game with cardio. Work on your spin, make constant power, and enjoy the power-ups.
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