A still from the video of Swedish rider Patrick Kittel riding the stallion Watermill Scandic, taken at Odense in October.
The video shows a horse being schooled before the World Cup round in Odense on October 18 by Swedish rider Patrick Kittel. The animal, Watermill Scandic, is shown in a hyperflexed position with its tongue flopping outside its mouth. The horse's tongue appears blue.
FEI secretary-general and chief executive Alexander McLin has written to member nations to inform them of the organisation's investigation.
McLin acknowleged the matter "has attracted great attention on social media".
"A video featuring an international rider using Rollkür or hyperflexion - a questionable and potentially abusive training method - in the warm-up arena at the FEI World Cup Dressage in Odense, Denmark, two weeks ago was posted on YouTube," McLin wrote.
"So far, it has almost 100,000 views and has generated strong debate on social media and especially Facebook on the issue of animal welfare. The FEI has also received numerous emails."
McLin outlined the FEI's position: "We are taking the issues raised in the video and in the comments made by members of the public on social media and by email very seriously and have opened a full investigation. The conclusions of this investigation will be made public in due course.
"FEI's main concern has always been and will always be the welfare of the horse."
The stallion Watermill Scandic, who has won two Kur classes this season. © http://www.hbcstal.nl
"Such procedures always take time as it is important that they are done in a thorough and fair way. The FEI is aware of the high-level of interest in the issues raised in the video and the subsequent online debate and would like to thank everyone who has contacted us for their feedback. We understand the concern and we will make the findings of the investigation public as soon as we are in a position to do so.
"The FEI is also giving the issue of Rollkür special attention. This topic has been put on the agenda of the FEI Bureau meeting, which is taking place on November 15 in Copenhagen. Important developments will be announced as soon as possible."
McLin said his letter was intended to provide information to member countries on the FEI's position.
"Members of the public may contact your national federation to express their disagreement or ask for your support and action against this method. We hope the above information will be useful for your communication and are at your entire disposal if you have any specific queries or require further information."
The British Horse Society wrote to the FEI shortly after the video received wide attention, demanding an investigation into the use of hyperflexion in dressage training.