Ah you see, that's ACTUALLY the unarmed combat animation - if you put in a X-Com soldier you can see how he knees the human in the face
(j/k)
That's pretty helpful to visualize what may be "off" indeed!
That's the "unarmed" movement animation - when armed it will have only one set of arms holding the (two-handed) weapon which will only move up and down with the torso, creating a stiffer appearance.
There's a couple of things at work that creates some of those flaws:
I believe I may have accidently copy-pasted the legs/bodies of some of the sprites in that line out-of-synch compared to my drafts (that I keep in another section of my work sheet). There's actually 1 pixel height difference too much between the first 4 frames and the last 4 frames. I never intended that to be an animation test so I rushed it, just to see so that each set of arms lined up with the bodies and didn't block the legs - and to compare them to each other.
For the first draft I also worked on the assumption that the torso's where at a steady height, but from studying the Muton sprites they actually move the torso like so: 0, 0, +1, +2 (or -1, -1, 0, +1) - that goes for the arms and legs as well so I believe the game is juggling a little on purpose to create a greater sense of movement. Or the game itself may be using some inbuilt Y-coordinates to further change how the sprite moves once in game.
I also now noticed that while the torso and arms are doing the Y-dance, the "non-moving" leg ISN'T which creates that "jumpiness" because I copied it wrongly.
Let's see if I can correct it.
Edit:
New draft
Corrected the 1 pixel left:right height difference.
Modified the rightmost arm a little to show more of it, making the "jump" between it and the frame before it smaller.
Had to elongate the two rightmost legs after correcting for the one and two pixel "drop".
Corrected some minor shading errors on some of the legs.
"Repaired" the fifth leg, making it look thicker.