The P90X program requires very few pieces of equipment, which is one of the reasons it's such a popular at-home workout. A pull-up bar and some type of resistance-training equipment is all you really need. While resistance bands are an inexpensive and versatile option, you may prefer dumbbells. Learning how to size your dumbbells correctly will get you the results you want without injury.
Types of Dumbbells
There isn't a lot of variety when it comes to dumbbells. Traditional dumbbells come in all different weights from 1 pound to over 100 pounds. A full set would cost you a pretty penny and take up a lot of real estate in your house. That's why you need to determine what size weights you need ahead of time, and only buy those weights. As your strength increases, you can add on.
Another option is to go with adjustable dumbbells. These cost less than a full set of weights and take up a lot less space in your home. They'll give you a bit more room to grow as you get stronger.
Different Weights for Different Goals
Before you start to figure out what weight you need to lift for each exercise, you should know what you're trying to achieve. If your goal is to build mass and strength, you're going to want some heavier weights. If you just want to get toned and increase your muscular endurance, you don't need to go so heavy.
You don't want to go too light or too heavy. While choosing a weight that's too light won't necessarily mean you won't get results, you won't get the results promised by the P90X challenge if the weights aren't challenging enough. Going too heavy can lead to compromised form, poor technique and injury.
The P90X fit test lists prerequisites for how many reps of an exercise women and men should be able to do with a specific weight, if weights are required. For biceps curls, men should be able to curl 20 pounds for 10 to 15 reps to failure. Failure means the last rep you can do with proper form. Women should be able to lift 8-pound dumbbells for 10 to 15 reps to failure.
Biceps curls are a good baseline for figuring out what weights you'll need for other upper-body exercises. Typically, you'll need to go a little lighter for exercises like front and side raises, where you're extending your arms out in front of you or to the sides. You'll likely need to go a little heavier for overhead presses, and heavier still for back exercises.
Weights for lower body exercises will need to be even heavier. Most of the lower body exercises in P90X don't require more than your bodyweight, however you are given the option to use weights to increase the challenge.
Do Your Research
Don't wait until you've already pressed "play" on your first daily workout to decide what weights you want to use. If you didn't buy a full set, you'll need to plan well ahead of your first workout anyway. If you belong to a gym, you can go there to experiment with some different weights. You can also go to a sporting goods store to do this.
Start with 8- or 20-pound biceps curls. If you can do 10 to 15 curls without a problem, go up in weight. Try the same thing with overhead presses. Start with 10 to 25 pounds for a woman or man, respectively, and increase from there. Do the same with rows. Find the weight that is very challenging to lift with good form past the ninth to 14th rep.
You're going to get stronger -- guaranteed -- if you're doing P90X correctly. This means eventually you're going to have to go up in weight. In time, you may be doing biceps curls with the weights you bought for shoulder presses, and you may need to get some heavier weights for back exercises. You can always size up when the time comes and keep your old weights on hand, give them to a friend or donate them.