Dumbbells and kettlebells offer benefits for your workout, but not necessarily the same benefits. They work your muscles differently, and kettlebell exercises add more of an aerobic quality to your workout than dumbbells. You don't have to replace one type of weight for the other; adding both to your workout regimen can give you an overall, balanced plan.
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Weighing Them Down
A difference between kettlebells and dumbbells that you can spot immediately is what they look like. Kettlebells look like small cannonballs with handles attached, and dumbbells have a center bar with weights on each side. Dumbbells let you change your weights quickly; with free-weight-style dumbbells, simply slide another weight plate on each end or remove one, for example. With kettlebells, you must grab a heavier bell each time you're ready to change weights. This can limit your workout routine if the gym doesn't have the kettlebell you need, if someone's using it or if you don't want the expense of buying every available weight for home use. Some dumbbells have fixed weights on the ends, but these are usually for lighter weights, not serious lifting.
Balanced or Off-Center
Dumbbells provide a balanced piece of workout equipment, equally weighted on each side. This offers stability in your workout, giving you more one-handed options and more control of the movements than kettlebells. The bells are weighted off-center -- the handle is light on one side, while the ball is heavy on the other. As you move through exercises, the kettlebell tends to want to keep going in the original direction, pulled through centrifugal force on the ball section. It takes more muscles group to redirect a kettlebell than it does a dumbbell.
The Way You Move
Because it's an unbalanced weight, a kettlebell lends itself well to aerobic-style exercises while still offering significant resistance to build stronger muscles. For example, the kettlebell swing -- where you squat with the bell between your legs and stand up, using the force from your hips to help propel the kettlebell in front of your chest -- works muscles in your shoulders, abdominals, hips and thighs, adding an aerobic element as you squat and stand repeatedly. You can perform the move with one or two hands on the bell. A typical dumbbell exercise is a biceps curl, where you hold a dumbbell down by your side and bend your elbow to lift it toward your shoulder -- a movement in a straight line, using a targeted group of muscles only. When you try a biceps curl with a kettlebell, you need extra muscles to stabilize the movement; the ball section that starts out hanging below your hand continues to hang down as you lift your hand, eventually ending up resting on your forearm. This changes the location of the weight and requires more wrist and forearm power to control.
Some More Differences and Goals
Letting your goals dictate which types of weights you choose helps you reach those goals. Dumbbells typically allow you to use heavier weights with a more controlled movement, leading to bulkier muscles. You still build strength with kettlebells, but the fluid movements and inability to focus on one single muscle group means you're more likely to create definition and tone than bulk. If you're trying to lose weight instead of create bulky muscles, kettlebells offer a variety of exercises to combine cardiovascular exercise with resistance training, in a way dumbbells can't offer.