As a gym veteran, I frequently receive two types of questions from beginners. The question that does not relate to what protein powder they should choose focuses on a very different matter: How much weight should they be lifting on a given exercise?
It's a perfectly good question, and there's no simple answer. In fact, a number of variables are in play here, so let's go through them one at a time. Doing so will allow you to customize your training program to ensure you're using the right weight every time.
You can pick up a 20-pound bar, curl it 75 times, and, after a while, you'll become fatigued and your arms will get pumped. You'll certainly be sweating a lot. Conversely, you can pick up an 85-pound bar, curl it 8 times, then have to drop it because you can't do any more reps. In both cases, you trained "hard." But is one approach better than another?
It may surprise you to learn that the answer changes depending on your goal. If you're looking to get as strong as possible, you'll be using a heavier weight than someone who is trying to get as big as possible. And to improve muscular endurance, you'll use an even lighter weight.
  • Strength training means choosing weights that allow you to train in a rep range of 1-6.
  • Building muscle mean choosing weights that allow you to train in a rep range of 8-12.
  • Focusing on muscular endurance means choosing weights that allow you to train for at least 15 reps.
Let's take a closer look at the three training protocols.
1

Training For Strength

The biggest, strongest men and women—powerlifters, Olympic lifters, Strongmen—have one thing on their minds: getting stronger. To lift heavy objects in competition means they have to likewise lift heavy objects in practice. That means, basically, lifting really, really heavy.
Focusing on strength requires doing multijoint movements like bench presses, squats, and deadlifts. Here, more than a single set of joints are working at once, such as the shoulder and elbow joints working together on a bench press. This multijoint action recruits more total muscle mass, thus allowing you to lift heavier weights.
The actual muscle fibers being recruited during very heavy sets are called fast-twitch muscle fibers; they're the ones that are more prone to growing bigger and stronger in response to resistance training as well. However, they run out of steam fairly quickly, which is why you can't lift a very heavy weight very many times.
Rest periods between sets for main lifts are fairly long (3-5 minutes) so that incomplete recovery doesn't inhibit succeeding sets. Of course, lifting heavy means warming up well beforehand, so a number of progressively heavier warm-up sets precede the maximal weights. Strength trainers also avoid taking sets to muscle failure, a technique used primarily by bodybuilders.
For a great introductory piece on strength training, read "Your First Strength Phase: A Beginner's Guide to Getting Strong," by Todd Bumgardner.
 
Focusing on strength requires doing multijoint movements like bench presses, squats, and deadlifts.
2

Training For Muscle Size

While those who train for maximizing strength do in fact get big, their methods may not be the most effective for maximally increasing muscle size (hypertrophy). Bodybuilders and gym rats who aim to increase the size of their muscles take a slightly different approach to determining how much weight they use. Here, choosing a weight in which they can do 8-12 reps has been shown to maximize muscle gains.
But there are a couple of caveats with that statement, so let's address those first.
  • You must use good form. You've probably seen YouTube videos of guys bouncing the bar off their chest when benching because the weight is so heavy that they need to generate a little extra momentum to get it going. That doesn't count as good form. Each exercise has its own "good form checklist." Generally speaking, you have to control the weight, and only designated joints are supposed to be working. If it takes hip or knee action to curl a barbell, you're using joints that should not be involved. There's a name for it—cheating—and it violates the good form mantra.
  • Perform a "true" set of 8-12 reps. Of course, you can just put a light weight on the bar and stop at 12 reps, but that's not a true set. A true set means that you're very near muscle failure—the point at which you can't do another rep on your own with good form. If you can do a 13th rep, the weight you used was too light. Similarly, if you can do only 4-5 reps, the weight is too heavy for maximum muscle-building. The sweet spot lies in choosing a weight in which you can just do 8-12 reps on your own.
Bodybuilders also train the fast-twitch muscle fibers, usually starting with multijoint movements sorted by body part. Here the recipe calls for higher volume (3-4 working sets of multiple exercises at different angles) and shorter rest periods (60 seconds for smaller muscle groups and up to 90 seconds for larger ones).
For more detail on how to maximize your size gains, check out the 8-Week Fast Track to Intermediate workout.
 
The sweet spot lies in choosing a weight in which you can just do 8-12 reps on your own.
3

Training For Muscle Endurance

Not everyone trains to get as big or as strong as possible. You can also train at a lower level of intensity—that is, the weight you use relative to what you can maximally lift for one rep. This builds up the mechanisms within the muscle that make it more aerobically efficient, without increasing the size of the muscle. Hence, the muscle can do lots of reps for long periods of time without fatiguing. The classic marathon runner's musculature is designed to take him or her the distance.
Focusing on muscle endurance means choosing very light weights that can be done for 15-20 reps or more. The weight stimulus just isn't strong enough to maximize size or strength. That's because the muscles are engaging the slow-twitch rather than the fast-twitch fibers. These fibers are designed to be used for longer activities and don't typically grow significantly in size compared to the fast-twitch variety.
 
BCAA Powder to Build Muscle and Recover from Intense Training.