If my friend fell on my treadmill at my apartment but did not appear to be hurt, can she sue me?

She can sue you only if she can prove liability as well as damages. 

Liability will arise if there was a dangerous condition caused by you or treadmill (your responsibility to maintain).

Damages: she will have to prove injury. It will have to be proved by something more than her word alone- for example, pictures, visits to doctor, record of treatment, etc. If there is no evidence, there is no case/lawsuiit.

Even if she were injured, that does not necessarily--or even likely--make you liable. Liability depends on fault--on violating some duty of care, either intentionally or through negligence (unreasonable carelessness). Without fault, there is no liability--an apartment lessor or owner is not an insurer for her guests, and only may be liable to the extent she is at fault.

Even if you invited her to use your treadmill, unless you caused the fall through careless or deliberately--e.g. you pushed or tripped her; you invited her to use the treadmill even though you knew it was defective--you would not be liable to her. Not every time a person is injured is someone else obligated to pay.

Moreover, even if you were somehow at fault, you'd only be liable to the extent she can prove she suffered injuries caused by your fault--she can't simply make them up, so to speak, but must have evidence, such as medical evidence, of harm, and could only recover an amount commensurate with the actual harm.