They saw an increase in strength three times higher than a group given 'placebo' capsules.
Dr Maria Parr, a professor at the Institute of Pharmacy at the Free University of Berlin, said: "The advertisements for dietary supplements (containing it) suggested muscle growth, so we decided to follow this in experiments. From the data, we hypothesised performance enhancement; however, we were surprised by the extent of the effect."
She said 8lbs 13oz of spinach would have be consumed a day to get the same effect, so it is unlikely that eating the vegetable will be banned. But the findings had been sent to WADA with a recommendation that the capsules be restricted in professional sport.
"Based on the data we acquired we suggested that they consider the inclusion of ecdysterone in the list of prohibited substances," Dr Parr said. A WADA spokesman confirmed the organisation would analyse the results with a view to a potential ban. He said: "WADA maintains dialogue with scientific experts to obtain new evidence as it becomes available."
Popeye was the creation of US cartoonist Elzie Segar, who first included him in the daily comic strip Thimble Theatre in 1929.
Initially the comic dealt largely with the adventures of Olive Oyl and her boyfriend Ham Gravy. Ten years later Popeye appeared - and a romance developed with Olive. Spinach helped Popeye see off his rivals.
Popeye became so closely associated with spinach that in 1937 a statue was erected in the central square of the USA's spinach-growing capital, Crystal City in Texas.