I’ve been hitting the gym three times a week to build a little definition - nothing TOO high impact, just want to gain strength and look a li’l better for swimsuit season.
For no particular reason, I’ve been doing three sets of 15 for each weight exercise. A coworker says that’s too much, and I should be doing 3x12 instead. I say bollocks - what’s three reps’ difference? If anything, 3x15 means I should be gaining strength that much faster, right?
It’s been a while since i was a gym regular, and probably even longer since i did any real reading on the subject, but when i worked out the common wisdom was to do heavy weight/low reps for building muscle and strength, and lighter weight/high reps for definition and tone.
So, if you main priority of building muscle and making yourself stronger, you might take your friends advice. When i was trying to build muscle, i would usually max out at about 8-10 reps. But if you already have enough muscle, and you just want to tone it up for the beach, then 15 reps (or even more) of a lighter weight should be fine.
It sounds, however, like you’ve only just started going to the gym. If that’s true, and you’re not especially muscular, and you want to look muscular, you might be better of starting with some muscle-building exercises. Also, if it’s looking good on the beach that you want, you need to consider your body fat. While building up muscle might make you look better, those muscles won’t show so well if they’re covered by a bunch of fat. Working out with weights can increase your metabolism and help reduce fat, but if you really want to burn some fat, you should do some cardio as well.
Take this advice for what it’s worth. I don’t claim to be an expert, but this is what worked for me back when i was younger and fitter. Also, be aware that, when you ask a question like this, you’ll often gets dozens of answers, many of them apparently contradictory. This is one of those areas where many people have strong opinions, and many believe that their way is the only way. Do what works best for you.One piece of advice is (or should be) universal, however: maintain good form in your lifting, even if it means you have to lift lighter weights.
Generally lower reps with more weight are preferred for muscle building. The reasoning being that if you can lift something 15x, your not using enough weight.
The idea is to fatigue the muscle to the point where it has to rebuild itself over the next 48 hours.
However, if huffing and puffing and pushing yourself to the breaking point for each set of reps you perform isn’t your style, higher reps are fine.
Thanks for the advice so far! Actually body fat isn’t a concern at all, I’m skinny like a chicken leg. The good news is that this means even a LITTLE muscle definition looks a lot better than it otherwise would.
Sounds like I should go for 8-10 reps then! I’ll try that this week.
First, direct to your question, if you can do 15, but can’t do 16, then the weight is probably OK. But if you can do 20 but stop at 15, you’re not doing much to build muscle. You build muscle not by simply moving weights around but by exhausting the muscle. If you are doing three sets of 15, I’m guessing you need to increase the weights.
There is some conventional wisdom, but as in many sports, every individual is different and there are a bazillion opinions.
One fact that must play into weight training, but I’m not sure how, is that there are two types of muscle fibers, slow twitch and fast twitch. Fast twitch is for power, and slow twitch is for endurance. Different people have different distributions of these two types of fibers, and training can also alter the balance.
Arthur Jones, the guy who invented Nautilus machines, espoused the idea that you should do just one set, and set the weight to where you could do at least 8 but not more than 12 reps in good form (going to “momentary muscle failure”). I later suspected that this approach was pitched by health clubs that figured using their capacity to get 300 members to do one set would make more money that 100 members doing three sets. 
I had a friend who was a successful amateur competitive bodybuilder. She told me to do three sets of successively increasing weight and decreasing reps. The first set was considered a warm-up set of around 12-15 reps, then the third set should be 8-12. I suddenly got past my plateau and increased my weights. I concluded that the “one set” theory is OK but very limited.
Last year I decided to add an extra set. Got past another plateau.
Recently I decided to do three sets but work each set to momentary failure instead of just the last one. Started making progress immediately.
Some people do the sets opposite, figuring they exhaust their muscles on the first heavy set, and will do another set of a lighter weight to squeeze out more reps. I have found, counterintuitively, that even if my muscle feels exhausted from lifting a lighter weight 12-15 times, I can turn around and pump out 8 reps of a much heavier weight. I would guess that it’s because different fibers come into play depending on the load on the muscle. But IANAPhysiologist.
Last, although increased muscle size will increase definition somewhat, definition has more to do with lowering body fat than increasing muscle.