Jack doing a backflip
A backflip was a gymnastics move.
In 2374, Jack demonstrated his ability to do a backflip to Julian Bashir, asking him if he could do the same. The doctor then answered him, saying, "I doubt it." (DS9: "Statistical Probabilities")
In 2377, Harry Kim complained that his stomach was full of alien parasites doing backflips. (VOY: "Workforce")