Through many years at the gym, I’ve often seen women using the same 5-pound dumbbells for a variety of exercises: chess presses, shoulder presses, rows, bicep curls, lateral raises, front raises and more. Since weights should be thought of as tools, this is tantamount to using a screwdriver for every task, when sometimes you need a hammer or a wrench.
For example, the weight you use for a chest press should be heavier than the weight you use for a shoulder press, which should he heavier than the weight you use for a lateral raise.
As you work out consistently, you should grow stronger over time, necessitating the use of heavier weights for each exercise. This is especially true in the early stages of a workout program, when strength gains are most pronounced.
It’s not just women either. I’ve often seen men using the same 25-pound dumbbells for an exercise that they’ve been performing for years. They make no progress because their program is not built on progression.
Your body is a demand-based system. That is, the more you demand of it, the more it will produce. If you don’t steadily, but incrementally, increase the demand, your workouts and your results will stagnate.
Aside from the fact that you shouldn’t be lifting the same exact weight for a variety of exercises, you shouldn’t be lifting the same exact weight for the same exercise, week after week, month after month.
In short, weight training is meant to be progressive. You shouldn’t be lifting the same amount of weight today that you did one year ago. You should have gotten stronger; you should have progressed.
However, sometimes your body needs a break from heavy lifting. Take it easy once a week and work on muscular endurance instead of just strength. Give your muscles a break, while still remaining active in the gym.
How do you know if you are (or are not) using a high enough weight, aka, working out at the proper intensity?
First, let’s define intensity, which is based on the maximum number of repetitions that can be performed with a given weight. The one-repetition maximum (1RM) is an ideal baseline:
• High Intensity = 80–100% of your 1-rep max (1RM); typically 1-5 reps
• Medium Intensity = 60–80% of your 1-rep max (1RM); 6-12 reps
• Low Intensity = 10–40% of your 1-rep max (1RM); 15 or more reps
The National Strength and Conditioning Association (NSCA) provides the following guidelines:
• Hypertrophy (muscle growth) = 6-12 RM
• Strength = 6, or fewer, RM
• Power = 1-2 RM
In order for your muscles to undergo an adaptation from training (i.e., to grow stronger or bigger), they must be stressed by working against a weight/load/stimulus that is greater than what they are accustomed.
Don’t stop an exercise simply because you have reached 10 or 12 reps. Perform each exercise to the point of momentary muscular failure, whether that’s 12, 10, eight or six reps.
Increasing weight means a reduction of reps. If, for example, you can chest press 40-pound dumbbells 10 times to failure, that means you shouldn’t be able to press 50-pound dumbbells 10 times (if you can, you should be working out with the 50s, not the 40s). It’s likely that you can only press the 50s six times or eight times, or whatever it might be.
If the current weight you are lifting isn’t challenging, you need to increase that weight. Strength training is meant to be challenging, because the whole point is to “overload” your muscles so they become stronger.
Simply put, if you can do more than 12 repetitions, you should increase the weight. Everyone could stand to be stronger. 
If you determine that it’s time to increase your resistance for a particular exercise, do this by adding no more than 10 percent at a time. For example, if you’re currently lifting 50 pounds, you’d increase that by five pounds to 55 pounds. This incremental increase should keep you safe and prevent injury. It’s important to maintain good form with a new, higher weight.
Remember, if you don’t make your muscles work harder than they’re accustomed, they won’t get stronger (which is the whole point). If you train progressively, your muscles will grow stronger in order to meet the increasing demands you are placing on them.
Lastly, recognize that some days you will feel like the Incredible Hulk, while on other days you will feel like a mere mortal. It’s okay; it’s all part of the process. Don’t beat yourself up over a bad training day. We all have them from time to time.