Remember this one? In the pic, Harry takes the elliptical for a spin while flashing a goofy wide-opened mouth expression that just screams "Heyyyyyyy" to us.
One Direction personal trainer Mark Jarvis is a saint for retweeting the pic and we'd like to see more of the guys in action (and shirtless), thank you very much.
Cybex International fitness equipment shared the photo, writing: "#TBT to @harrystyles doing work on an Arc Trainer! #OneDirection #1D #CybexStrong"
As for the 1D training sessions, sounds like Harry is in top form, with Mark tweeting: "Cracking day. @craigoduffer first fight camp session went perfectly & @Harry_Styles is just crazy fit. My supersoldier army is growing! :)"
Earlier this week, he tweeted: "Tough gym session today. Getting tour fit for the team! @Harry_Styles @NiallOfficial @zaynmalik @Louis_Tomlinson @Real_Liam_Payne @NRGFuel"