This is an excerpt from Morning Strength Workouts by Annette Lang.
Free weights (dumbbells and barbells) add resistance to exercises, thus stimulating your muscles to gain in strength, size, or endurance, depending on how you tailor the exercises to your individual goals. Gravity is still pulling straight down, so sometimes when using free weights you have to manipulate your body's position relative to gravity in order to get a good exercise.
Let's look at a chest-press exercise, which is essentially the same pushing exercise as a push-up, working your pecs, anterior deltoids, and triceps. If you stand with a dumbbell in each hand and hold your arms up and out in order to perform the exercise, you will not be targeting the muscles you want as effectively as if you lie down on your back (supine) and push the dumbbells up away from gravity. Since gravity is pulling straight down, the muscles that are lined up against that force will be most challenged. In the standing example, your shoulder muscles, or deltoids, would do the most work just in order to hold the weights up.
Free weights are great tools to incorporate into a workout, because you can use them anywhere. Sometimes, though, a limiting factor comes into play: one's ability to hold the weights. When doing lunges, for example, it is difficult for some people to hold a dumbbell that's heavy enough to meet the needs of the exercise—for example, to reach fatigue in 8 to 10 repetitions. If you have trouble holding on to heavy weights, you might need to do more sets with a lighter weight, do another exercise first to fatigue the targeted body part, or do the exercise slower to get more time under tension.
So, which are better: body-weight exercises against gravity or free-weight exercises? The best answer is always “it depends.” Neither is better; they are just different. Let's look at the push-up and chest press with dumbbells again. For many people, the chest press is harder. For starters, you need to be able to hold a dumbbell in each hand and negotiate the movement above your body. This is challenging until you learn the specific pattern you need to do the exercise. If you are limited in your wrist, elbow, or total-arm strength, then the weight you pick needs to be light enough for these areas to hold and might not be enough to overload your chest muscles. A push-up, on the other hand, is easier in terms of your wrist, elbow, and total-arm strength because you are pushing against the floor, which gives you more stability. The challenge with a push-up is the core strength in addition to the upper-body strength that you need to push your body away from gravity. You can make this easier by doing the push-up on your knees instead of toes, essentially making your body shorter.
If your goal is to increase pure strength with a weight you can lift, as well as to improve proprioception and your body's ability to work together as a whole, free weights are the way to go. Conversely, if your goal is to increase your relative strength, then picking a body-weight exercise instead of a free-weight exercise is a good choice. If you want to vary your program, then alternating the exercises is a great idea.
This is an excerpt from Morning Strength Workouts.