First of all, it should be said that such behavior among scaly creatures with limbs is a way of communication. 
Yes, you and I are used to communicating with sounds, but not all lizards have the ability to reproduce them.
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So many of them use rhythmic movements to transmit information to their fellows and representatives of other species.
My muscles will speak for me.
At the same time, these "push-ups" can mean completely different concepts. 1) This can happen when a lizard feels threatened or needs to assert dominance in its territory. In this case, the signal is intended for fellow lizards. 2) Sometimes lizards do push-ups to show potential partners their best features, such as beautiful color patterns. This is also a signal for representatives of their own species, but of the opposite sex. 3) And sometimes this is a warning to predators. Animals perform push-up exercises when they are in a threatening pose facing the enemy. This shows the predator that the lizard is aware of the presence of an enemy and is ready to fight back. 4) There is also a version that such movements help regulate body temperature. As cold-blooded creatures, lizards need to conserve heat, and such movements help achieve their goal.
Oh, yes, how nice it is in the bathhouse after a workout!
Of course, push-ups are just one manifestation of "body language". There are still a lot of different gestures and body movements that have a semantic load, but people are still far from fully understanding them.
Sports for everyone.
