Do I need to exercise before an arm lift? If not, will it affect my result?

The better shape you are in before surgery, the better the result your surgeon can give you. This applies to all plastic surgery, not just brachioplasty. There are limits to how much can be safely liposuctioned at the same time as skin excision so the thinner your skin flaps are going into surgery the more toned you will look coming out. 

I believe that the healthier you are before surgery, the better you'll do.  In the case of arm lifts, I feel that this is particularly true.  General weight loss will maximize the amount of extra skin that can be removed, good cardiovascular health will help prevent against anesthesia and postoperative complications and good muscular tone can optimize the aesthetic outcome.  I congratulate you for considering the benefits of exercise in preparation for your arm lift - consider it a valuable investment in your operation.  I wish you the best!

Exercising before and arm lift should not have any real effects on the results. The amount of muscle that could be gained or lost is relatively insignificant compared to the way your skin will stretch under the new tension.

Before your arm lift for arm reduction surgery, you should be as healthy as possible. If you're not currently exercising I would suggest some form of cardiac exercise. This will get you "healthier" for the upcoming surgery.  In regards to arm exercise, I do not think arm exercises are particularly necessary. An experienced, board-certified plastic surgeon should be able to sculpt your arms to achieve optimal results.  Please seek a plastic surgeon with significant experience in arm lift or brachioplasty surgery. Sincerely, Dr.Katzen.

I see no issue with you exercising to your hearts content. It's always better to be in shape for your procedure.