This is an excerpt from Women's Home Workout Bible by Brad Schoenfeld.
Dumbbells are the heart of any home gym. In addition to the benefits previously mentioned, they confer several other advantages that can't be derived from other exercise modalities, including the following qualities:
Improved muscular balance. After nearly two decades as a fitness professional, one thing that stands out to me is the number of women who have strength deficits between muscle groups. For some women, it's a function of right or left dominance; for others, it's an adaptation to previous injury; and for others still, it's an imbalance that comes from using one side more than the other in the course of everyday activities (a woman often will be stronger on the side where she holds her purse). These imbalances can't be corrected with machine training (or barbells, to a lesser extent), because your stronger side forces up the weight at the expense of the weaker counterpart. In fact, imbalances can actually be exacerbated by the exclusive use of machines. Dumbbells, on the other hand, require that all of your muscles work equally. Over time, this brings about uniform muscle development, making both sides of your body equivalent in terms of strength and proportions. If the weaker side doesn't do its fair share, you won't be able to complete the lift.
Increased range of motion. With machines and barbells, your range of motion is limited by a fixed hand position on the bar—you can only move the weight as far as your grip will allow. Not so with dumbbells. Because your hands function independently of one another, you can lift through a greater range of motion. For example, during a chest press with dumbbells, you can press the weights both up and inward. This extra range recruits more muscle fibers, leading to greater muscular development and better flexibility.
Reduced joint strain. Dumbbells are better suited to move in line with the natural action of your body than exercises performed on a bar. The reason: You aren't locked into a preset pattern of movement. If necessary, your joints can adopt a more circular pattern of motion, which places less strain on soft-tissue structures and diminishes the chance of injury.
Dumbbells can be classified into two basic categories: fixed and adjustable. You're undoubtedly familiar with the fixed variety (sometimes called stand-alone dumbbells). You can find them in practically every commercial gym in America, lined up in racks against walls or mirrors. The simple design includes a small bar with weighted plates attached at each end; no mess, no fuss.
The primary benefit to fixed dumbbells is that you don't have to spend time adjusting weights. Simply pick them up and you're ready to go. The downside is that they can clutter up your workout area. A complete set of dumbbells up to 25 pounds (11 kg) includes 16 separate weights. If space is an issue, dumbbells can be cumbersome.
Fixed dumbbells (see figure 4.1a) are usually made of cast iron or solid steel and sometimes covered in neoprene, chrome, vinyl, or rubber. I'd advise you to stay clear of plain cast-iron dumbbells. The paint tends to chip off with frequent use, and they are prone to rust. Also, avoid the old, bulky style of dumbbells that are filled with sand. You'll wind up with a big mess if the outer covering breaks. Trust me; it's not much fun trying to vacuum grains of sand out of your carpet or off your hardwood floor!
Adjustable dumbbells are similar to miniature barbells in that you add plates to the ends of a steel bar. The weights are then secured with clips or collars to prevent slipping. Although this style of dumbbell is generally less expensive than the fixed variety, it is inefficient. You'll waste valuable training time while adding and subtracting weights. Housing the plates becomes unruly when you progress to heavier poundages. I don't recommend them.
However, a special type of adjustable weight called a selectorized dumbbell (see figure 4.1b) warrants consideration. Although the exact shape and structure varies from one model to the next, the basic design consists of a handle attached to a series of nested weight plates. You choose the weight by either turning a knob or inserting a pin into the nested plates. This engages the desired amount of weights to the handle.
The primary advantage of selectorized dumbbells is their space efficiency. A pair of selectorized 'bells can take the place of a dozen or more pairs of the fixed variety. Just stick them in a closet or tuck them away in a corner, and they're out of harm's way—a big plus if you have a small workout area. The trade-off is that changing from one set of weights to another can take 30 seconds or more. At first, this might not seem like a big deal, but the additional time can be detrimental to circuit training or time-sensitive techniques such as drop sets, supersets, and giant sets.
This is an excerpt from Women's Home Workout Bible.