You'll develop unbreakable bones
OK, that may be an oversell. There's no such thing as an unbreakable bone. But since strong bones are (somewhat!) important for supporting your bodyweight and helping you move from one location to another, you want to do whatever you can to make them as strong as possible.
Strength training is an excellent way to help maintain the bone you have, while even helping improve bone density. This is especially important for women, who lose an average of 53% of their peak bone mass by age 80 (guys aren't out of the woods, either, losing about 18% by the same age), which often contributes to life-altering falls and fractures. Many studies, including multi-study review articles, point to resistance training as an ideal method for staving off these losses.
Just remember, the beneficial effects of resistance training on bone mineral density are site-specific. In other words, push-ups and bench presses may help you improve the bone density of your humerus and shoulder girdle, but they won't do a damn thing for your hips, spine, or femurs. Likewise, squats and lunges can help solidify the bones of your lower body, but won't do anything for your upper body. Common sense, but people believe the darndest things.
You'll be resistant to injuries
In addition to helping you build stronger bones, hitting the weights can help reduce your chances of injury by a pretty significant amount. In fact, a review of 25 studies found that "strength training reduced sports injuries to less than ⅓ and overuse injuries could be almost halved."
So next time you sign up for your office softball team, make sure you carve out some time to hit the gym -- it may save you the pain of a pulled hammy.
You'll feel way more confident
Who's the baddest boss in town? You are, of course, especially if you just spent time pumping iron. The beauty of strength training is that you see results quickly -- and not just in the mirror, but in competence and ability as well. Shoot, if last week you could barely do five push-ups, and this week you blast through six or seven, you just received immediate, positive feedback that you're killing it. There's no denying that that feels good.
What's really interesting, though, is that you may not even have to technically get stronger to enjoy a boost in self-esteem. According to a 2015 study, overweight and obese adolescents who participated in a four-week resistance-training program improved their self-esteem more than their cardio-only peers, but their improvement in self-esteem was related as much to the feeling of getting stronger as it was to actual improvements in strength.
So yeah, if you want to feel good about yourself, spend some time in the weight room.
You'll stay lean
Yeah, yeah, this is a benefit related to physical appearance, but it's not specifically related to muscle size or strength… except peripherally. You see, muscle mass is thermogenic -- it creates heat, which means it burns calories. When you put on muscle mass, you're basically donning a calorie-burning heat blanket that runs 24 hours a day.
Granted, it won't cause your metabolism to skyrocket, but when it comes to maintaining weight and preventing weight gain, a few extra pounds of muscle mass certainly don't hurt. Muscle tissue is roughly three times more metabolic than fat tissue, so if a pound of fat burns about two calories per day, a pound of muscle burns six. It seems like a meager difference, but those are "free" calories you burn just by being a little more muscle-y.
The real benefit, though, comes from your strength-training workouts themselves. After strength training, particularly heavy strength training, your resting metabolic rate, or RMR (how many calories your body burns per day at rest), is increased for at least 24 hours by around 4% to 9%, depending on your gender (this breaks down to about 4% for females and as much as 9% for males).
If your RMR is typically 1,500 calories, that equates to a 60- to 130-calorie increase in metabolic rate, not counting any additional burn generated from your workout. In other words, strength training kicks your metabolism into high gear, helping you prevent weight gain while leaning out.
You'll become more flexible
Contrary to popular belief, lifting weights won't automatically destroy your flexibility and turn you into a musclebound freak who has to turn his entire body to look to the right. As long as you use proper form, moving your joints through a full range of motion while handling a heavy load can actually help enhance your flexibility.
In fact, a 2011 study found that strength training was every bit as effective as static stretching at improving flexibility, with the added benefit of also enhancing strength. Two birds, one stone? That's a pretty good deal.