What ot do if at work I am expected to move fixturing that is over 100 pounds but I am a 59 year old female with arthritis?

An employer needs to make "reasonable accomodations" for employees with disabilities (e.g. arthritis), but not due to age or sex/gender; therefore, if there is something the employer could do, which is not too expensive or disruptive to its operations, to allow you to do your job--such as by providing a dolly, or pulley, or other assistive equipment; or having you  move lighter products while others move heavier ones (and doing so does not disrupt operations)--they probably need to do this under the law.

On the other hand, if there is no reasonable accomodation to your arthritis which would let you do your job--anything they could do would be too costly or disruptive--or even with a reasonable accomodation, you still could not do the job (such as because you are too physically small, or your age does not permit), then the employer could terminate you. The law does not require an employer to pay or retain employees who can't do the job for which they are hired; it's simply the case not every employee may physically be able to do every job (for example, firefighters can be legally required to be able to carry an adult male down a ladder; people who cannot do that do not have to ve hired as firefighters). It may be that you have a job which is not possible for  you.