AleksZ Photo / Shutterstock Swiss ball; Stability ball; Physio ball; Exercise ball. It goes by many names, but the big ball that inhabits the corner of every gym is one of the most versatile pieces of equipment at your disposal. It’s not just for your core , either—the ball can be paired with weights or utilized on its own for a variety of upper- and lower-body exercises that will help you build muscle and strength. “It’s definitely an underutilized piece of equipment,” says Jennifer Romanelli , NASM-CPT, RN, a personal trainer and co-owner of Trooper Fitness in New York City. “There's a stigma around it being a tool for physical therapy , rehab, or low impact ‘easy’ workouts, but there is so much more to it.” She notes that Swiss balls are great for hammering stabilizer muscles like the adductors, as well as larger muscles like the hamstrings, quads , and glutes—all things required for lifting heavy weights. “I love combining strength and stability work,” ...