... So what about lower-body exercises such as squats? This is where it becomes virtually impossible to find antagonist pairings. Unless you relegate your lower body workouts to the leg extension and leg curl, there really aren't any good lower body pairings – at least on the surface.
I get around this issue by pairing upper and lower body exercises. If squats are performed, I'll pair this with an upper body exercise. Importantly, I try to avoid pairing a neurally demanding lower body exercise with a high fatigue upper body exercise. For example, full squats could be paired with hammer curls, but not push presses. And deadlifts could be paired with skull crushers, not pull-ups. ...