Via NALC April 2017 Postal Record:

Carrying mail is tough, physical work that can take a toll on your body. Many letter carriers tolerate nagging aches and pains throughout their careers, often suffering permanent damage while attempting to “walk it off.” Fortunately, federal law provides compensation for these types of injuries, which are referred to as occupational diseases. To successfully file an occupational disease claim, you need to build a foundation for the claim by first educating your doctor about the physical nature of your work duties. Once your doctor knows what your work duties are, they need to provide a medical narrative that connects those work duties to the injury you are suffering. This is generally a three-step process.