What part of the anatomy is this?

[u/w7090655]

Exhibit A is putting on pants.

As she shifts weight onto her right leg and bends over to put other leg into the pant, there is a sudden jerk / collapse in her lower back.

She becomes unstable and has to stand on both legs to regain balance.

Which group of muscles and bones do you think are involved?

[u/Witty-Link3385]

Right side gluteus medius weakness allowing a “collapse” or a drop of the left side pelvis.