I think the natural assumption is that she’s standing up, using her legs to operate the legs, and her arms out as if into sleeves … but she also complains about being cramped inside the suit. The thing is much larger than any of the hunters, and it would certainly require a whole lot of reflexes and co ordination of movement, especially for an elderly lady who has spent her entire life as a scientist.
I wonder if perhaps she’s using a control mechanism, like joysticks or screen interfaces. To me it seems more likely that someone like her would be more adept at controlling a computer than using her own limbs and body to run, jump, and shoot. Perhaps she’s sitting in there more like one would in the cockpit of a vehicle actually seated, and using levers, pedals, buttons and what have you. Makes it a bit more like a video game for her, rather than an extension of her own body. A little bit more feasible. Granted, nothing about this game is realistic, but I personally think it would be pretty neat if she were more of a pilot than a warrior