Former Super Bowl champion Omar Bolden, an ISSA-certified personal trainer, creator of X2 Workouts, and owner of Optymyze Training, explained that because of the number of exercises you can do with a single kettlebell, they're a perfect addition to any home gym. "They're great for building muscle, increasing endurance, and strengthening the core," Bolden told POPSUGAR. He likes the JAXJOX KettlebellConnect ($229) because its smart technology allows you to adjust the weight from 12 to 42 lbs.