There's no getting around it: after the age of 40, your body starts losing around 0.5 percent of its muscle mass each year. Maintaining—and even building—strength after age 40 is possible, but requires adjustments to remain injury-free. Now is the time to lose the "no pain, no gain" mantra and start taking care of your body instead of punishing it. Pushing through pain can do more harm than good. The key to strength training once you hit the big four-oh is to work with your body, rather than against it.