U.S. patent #4565370 was granted on January 21, 1986 to Tony Christianson for a “foot holding device for hanging upside down.”
Mr. Christianson’s patent makes pointed reference to earlier, roughly similar inventions—especially to one by R.M. Martin, who was granted U.S. patent number 3,380,447 on April 30, 1968 for an “ankle device for supporting an individual in an inverted position” in a manner “which is comfortable and safe.” Mr. Christianson lists several aspects of the Martin invention that, to him, seem sub-optimal. He goes on to say, perhaps with a hint of pride, that [AIR 16:1]:
Another problem associated with Martin’s device is the possibility that an extended or pointed foot can slip out of the device and thereby drop the user…. The present invention solves all the problems presented by [the Martin device].
(Thanks to Perri T. Unger for bringing the Christianson patent to our attention.)