If you’ve been to a boutique fitness class recently, chances are you’ve heard these three little words: “long and lean.” Because that’s the promise of so many female-targeted workout trends these days — that by focusing on tiny muscle groups and using barely-there resistance, you’ll get “trim and toned” and “long and lean” without, God forbid, bulking up.This is problematic for a few reasons: First of all, when people push the “long and lean” physique, the implied assumption is that building big, defined muscles is inherently bad or unattractive, which is just another example of our culture reinforcing a narrow standard of female beauty.What’s more maddening, though, is that this isn’t even true: Lifting heavy weights will not necessarily make you bigger; strength does not automatically equal huge-looking muscles, because that’s just not how muscles work. Enough, please.Keep reading for the truth about weight training, without the body-shaming.Simply put, it’s nearly impossible for a woman to develop huge, rippling muscles merely by adding heavier weights to her routine. One reason is hormonal: Women produce 10 to 20 times less testosterone — the primary hormone needed for muscle growth — than men, says Jacque Crockford, an exercise physiologist with the American Council on Exercise.Then there’s the matter of your muscles themselves.Muscles are made up of protein fibers and surrounding tissues. To get stronger, you need to increase the size and thickness of those fibers — a process technically called myofibrillar hypertrophy, which allows your muscles to exert more force and be able to lift, push, or pull heavier loads.There are two types of muscle fibers: Type 1 and Type 2, also known as slow-twitch and fast-twitch. “Type 2 muscles tend to be larger, whereas Type 1 fibers are smaller and less defined,” Crawford says. Men tend to have a higher concentration of Type 2 muscles than women, which may cause a slight difference in how big their muscles can grow.This means that, while you very well may notice visible changes over time — especially because building muscle makes the body more efficient at burning fat tissue, which may be covering up existing muscle — you’re not going to come out of a weight-training program looking like The Hulk (unfortunately).That said, some women will naturally have an easier time building muscle than others, depending on their own genetic makeup of Type 2 and Type 1 fibers, Crockford says. But these results won’t happen overnight, and they won’t necessarily come across as bigger — just tighter and more defined. That’s because the muscle fibers themselves, even Type 2s, don’t actually have much correlation to visible muscle size. Big biceps, pecs, and other body parts come from a process called sarcoplasmic hypertrophy — an “inflation” of the semi-fluid tissue around the muscle fibers, Crockford says. This process is triggered by muscle-repair proteins, and testosterone does help it along. But to increase its effects, bodybuilders often also take supplements, load up on protein, and do very specific workouts designed with this goal in mind.In other words, adds Jen Hoehl, an ACSM-certified personal trainer based in New York City, looks can be deceiving. “That’s why you can see girls in the gym lifting crazy weights who don’t look that strong,” she says. “And why a guy with huge arms may not necessarily be able to lift as much as you think he can.”Hoehl recommends adding three to four days a week of strength training to your routine, on top of regular aerobic exercise. If you’re not sure exactly how to start, enlist a personal trainer or join a free-weights or kettlebell class to get advice.You don’t want to go too hard before you know what you're doing and get yourself injured, but you also shouldn’t be scared to challenge yourself. In fact, you’re probably already a lot stronger than you think, Hoehl says: “I have a client who told me she only lifted eight-pound hand weights, and yesterday I had her squatting with 35 pounds. This way, she’ll continue to get stronger and support that lean muscle she’s built, rather than doing the same thing over and over and having her muscles adapt.”