The Irish cutie shared a picture to Instagram on Friday (25 April) showing himself pedaling on an exercise bike under the watchful eye of 1D's personal trainer Mark Jarvis.
"First session back with Jarvis," he told fans. "Knee feeling ok!"
Niall later tweeted: "Long road with the knee recovery, it's not over yet! Still working hard!"
20-year-old Niall underwent a secret knee operation in January this year, only informing fans after the procedure.
At the time, he tweeted: ""Yes, I've had major knee surgery, a much larger surgery than first anticipated, but I would like to tell you it was a complete success! Considering the scale of it!"
He added: "As It was a very important time for me and I didn't want to have fans knowing my whereabouts Needed privacy obviously."
Niall's since been recovering, getting back to full health for One Direction's mammoth stadium 'Where We Are Tour', which kicks off in Bogota, Colombia tonight - although it seems a fragile knee isn't all he'll be contending with.
He politely pleaded with fans outside the group's Colombian hotel to keep the noise levels down a bit so the guys could get a bit of sleep before hitting the stage.
"I honestly think this hotel is made of paper! Haha! It feels like the people outside are in my room haha! Not a chance of sleep tonight," he tweeted.
"Guys! May I ask you to keep the noise down please ! All of our crew are tired! and we have a big day tomorrow! Don't want to sound angry! Thank you ! Much appreciated ! Big day for all tomorrow! Love you lots! Goodnight!"
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