You can also do it in the gym, if they have one of the "machines" shown at the very right in this picture: https://www.fit-star.de/Resources/Public/Content/Specials/He... - not sure how they're called or how common they are, but you put your lower arms on the armrests, grab the handles, rest your back against the blue ball and just let your legs hang. Then you can do some ab exercises.