You've just completed 5 sets of 15 reps and you're sweaty, but you could definitely go another round. If this is you, you probably need to be lifting heavier weights. Or maybe, you did 3 sets of 10 reps and are wondering if you can/should go up in weight. The easiest way to determine how much weight you should be lifting is to figure out two things. First, what is your one rep max. Second, what are your fitness goals? Do you want to be able to lift really heavy, or do you want to tone your muscles and trim up. If you want to lift as much as possible, you should be lifting at about 85% of your one rep max. When lifting at 85% of your one rep max, your last rep in a set should feel like you might break form at any moment because you're muscle is just so burnt out. If you're looking to simply have toned and defined muscles to look good in a bikini, then you should be lifting at about 70% of your one rep max. This should allow you to do a few more reps than your 85% of max and still have your muscles completely burnt out by the last rep. Lower reps build strength while higher reps pump muscles up and give them volume, but too many reps is simply cardio. To figure out your one rep maximum, use the table below to find the number of reps to failure that you can do at whatever weight you wish (just make sure it's heavy enough to reach muscle failure at no more than 12 reps since the table stops there).
Once you have this, divide the weight you used by %1RM. So, for example, if I used a 15 pound weight and was able to lift it for 9 reps, then I would divide 10 by 0.76 to find out that my one rep max is 20 pounds. So, if I wanted to be able to lift heavier weights, I would increase my weight to 17 pounds. However, if I wanted to tone my muscles, then I might decrease my weight to 14 pounds. Now that you know your 1 rep maximum... should you be lifting heavier?
Reps %1RM 1 100 2 95 3 90 4 88 5 86 6 83 7 80 8 78 9 76 10 75 11 72 12 70