Juncker was captured on camera struggling to keep balance and stumbling surrounded by world leaders ahead of a gala at North Atlantic Treaty Organisation headquarters on Wednesday.
Pictures taken the same night show Mr Juncker being brought into the dinner in a wheelchair.
Speaking Friday, European Commission spokesman Margaritis Schinas said that Juncker "suffered from a particularly painful attack of sciatica, accompanied by cramps" that affected his ability to walk, but soon passed.
Sciatica occurs when the sciatic nerve, which runs from the hips to the feet, is irritated.
In the video, which circulated among other Polish channel Polsat, is seen as Juncker in turn support it back, then in the hands of Ukrainian President Petro Poroshenko, President of Finland, Prime Minister of the Netherlands mark Rutte.
Juncker, 63, has previously himself spoken of back and leg pain and serious injuries he suffered in a auto accident in 1989. That was unfortunately the case Wednesday night, ' Schinas added.
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'He is taking medication and feels better'.
"I don't think this is elegant and I don't think this is fair", he added.
Asked to confirm that Juncker's condition was not preventing him from carrying out the functions of his role, Schinas said, "yes, I can confirm that".
People present during the Brussels summit, which was marked by rows between U.S. President Donald Trump and European leaders over military spending, said Juncker, a former long-serving prime minister of Luxembourg, had played a full part in talks.
The German government has a "very high level of confidence" in European Commission President Jean-Claude Juncker, government spokesman Steffen Seibert said on Friday.
At the opening of a summit in the Latvian capital Riga in 2015 a light-hearted and tactile Juncker kissed leaders on the head, fiddled with their ties, saluted, and slapped them not just on the back but also on the stomach, chest and face.