There was a need for some precision in the activity too, Ruby needed her hands to guide the foam, giving the pushes or pulls, throwing her power behind some of the pieces and just launching others. She didn't work a lot with her telekinesis, it wasn't her most developed power, but it was possibly her only offensive power too.
She opted to keep it steady at first; direct medium sized pieces towards Theo's torso, keeping the speed of items steady, fair enough that he could gauge where they were going to hit and block or attack as needed. Picking up the pace a little into that, pulling already discarded foam shapes back into the mix, coming from behind rather than in front, Ruby had to narrow her focus a little before getting into the swing of things again.
Personally, she preferred this to the usual fare of running or sit ups, she hated the gym aspect of working out because she'd always used nature as her own personal gym; hiking, trails, rock climbing, those were her past times. This bullshit about running on a treadmill or doing crunches on a mat just didn't suit her. But an interactive method? Yeah, she could get behind that. Even if she had to avoid on of the foam shapes that Theo deflected.