You have likely heard the saying that rotating and replacing your car's tires every so often helps it to drive smoother and wear down less quickly. The same is true of your workouts; the more you mix them up and introduce new tools, the more your body will respond to stay fit. Muscles, like the rest of you, get bored from time to time and need a little freshening up to keep the rhythm going. When you hit a wall in your fitness plan, try introducing a kettlebell to mix up the routine.
Kettlebells consist of a ball weight hanging from a stationary handle. Thus, the weight's center of gravity hangs out away from the body. Some kettlebells do come with swivel handles for use by experienced lifters. The Swivel handles can be tricky, especially when doing presses or momentum-based workouts. Many people find that a kettlebell is actually harder to lift than a regular dumbbell because the displaced weight changes the momentum of the lift. This can help people who really want to work on form and stabilizer muscles since there is a new variable in the workout.