The car idling next to me at the traffic light yesterday morning blared “summertime…and the livin’ is easy” loudly out its windows. Three young women sat in the car, swaying and singing aloud. I couldn’t help but smile, even in my caffeine-depleted state.Summertime. Sunshine and swimming. Bathing suit season. I think after many, many years of dreading that phrase, I might have just found the Holy Grail of how to rock a bikini—and in the process, lower my risk of diabetes and osteoporosis and probably a whole host of chronic diseases that I really don’t want to think about.It’s as simple as this: strength training.I have long known about cardio training, and the need to get at least 30 minutes of exercise a day. The 2008 Physical Activity Guidelines for Americans recommends 150 minutes of moderate-intensity aerobic exercise weekly. What I had not absorbed, when I learned of these guidelines, was that they also recommended muscle-strength exercises at least twice a week. Strength training with weight-bearing exercises are crucial for bones, among other things, and therefore is really an important component of staying healthly, especially for women.Oh, I knew that to prevent osteoporosis, weight-bearing exercise was the way to go, and that it also supports a healthy metabolism by burning fat and building muscle. But I also knew that some of my favorite cardio exercises such as walking and biking fell into that category, so I conveniently forgot about strength training. And then last year, I hurt my knee. Suddenly, out went all those long walks and spinning classes.Unable to do the higher impact cardio workouts that also satisfied my weight-bearing requirements, I decided earlier this year to try a core barre class, which uses elements of pilates and ballet to strengthen and tone muscles.For the first month, I spent more time on the floor in a heap than actually participating. These classes are hard. Seriously hard. Using just our own body, and sometimes small hand weights, you work your core, your arms, your legs and your butt. Muscles I didn’t know I had ached the day after class. But I keep going back. It’s amazing.After just one month I felt taller, I felt stronger, and my clothes fit better. Now, several months in, I’ve changed my body composition, I’m sure of it. For the first time, this summer I’m not so worried about that “bathing suit season.” And I’m not so worried about osteoporosis, either.MORE: The Real Reason You Don’t Have Your Bikini Body Has Nothing to Do With Exercise