Four decades after the iconic Jane Fonda exercise videos sparked a fitness craze, we’ve fielded a series of flip-flopping messages about the value of strength training for women. Although Fonda told women it was okay to have muscles, her videos — rife with svelte figures in bodysuits — didn’t contain much strength training, “especially for women in their 40s through 60s,” says Dr. Elizabeth Matzkin, chief of Women’s Sports Medicine at Harvard-affiliated Brigham and Women’s Hospital. Now, women