The problem comes when the switch is pulled. Your going along fine then suddenly you loose a gear, or two. Management likes to talk like lawyers (because that's how there job is setup and they are trained to do) and so don't many UPS drivers. Management will say "where's the decline over time...none, so, your just slowing down on purpose." The sudden drop in production is what everyone will be jumping on. So, what do ya do if your feelin it and your falling behind?
1, Adjust your methods to avoid stressful and physical exertions. Like clearing a walk path in your cargo area. Unload anything that gets in your way. Don't try and pickup and roll over the top heavy or irregular size items. If you can't push or drag an item off the truck then move whatever you have too so you can. Use your hand truck more. Put boxes on the cab floor before exiting and them pull them close to you.
2, Adjust your load. Have your loader leave out all oversize, overweight's, irregulars off the vehicle, load last, and do those stops first to get them out of your work area. Clear your path and work space in your cargo area.
3, Adjust how you handle boxes. If your carrying a box over your chest area then stop. If you have to pull it down and let drop to the floor of your cargo area a heavy or large box from the top shelf then do it. If you can safely slide it down into your power zone then, ok, don't let it drop, but, if it in anyway threaten your body, LET IT GO.
4, Take your necessary restroom breaks. Holding it in is harmful to your body and washing you hands and face regularly is helpful and a healthy practice. When men get past certain a certain age they need to urinate more often and feel the need to go immediately regardless of the volume. So go.
5, REPORT all injuries NO MATTER how small. Get copies and keep records. You don't have to loose time on Workmen's Comp. Losing a day or 3 to recover without becoming a paid claimant helps the honest narrative. The very fact that your reporting strains, pains, or injuries that linger more over time is a red flag to corporate and to your attorney should it progress and become debilitating over the short or long term.
Management will always push back that's there job. If you stay consistent in your explanations, actions, and motivations an argument for age and physical conditions becomes self-evident.
Work smart not hard.