They're equally perfect for cramped hotel rooms, sunny days in the park, and snowy days when you can't leave your house. They're cheaper than any gym membership, and allow zero excuses for missing a workout. Bodyweight exercises have a lot going for them. But how effective are they? Can you really build muscle from a bodyweight regimen?

First let's talk about what counts as a bodyweight exercise. I'm a welcoming person; I'll include any kind of calisthenics, plyometrics and the like that are meant to be done outside of a gym. Some fans of bodyweight exercises will also add weights, like doing pushups with a sandbag backpack, but I'm mostly thinking of the unweighted stuff today.

When you're used to thinking of more weights making a more challenging exercise, it's easy to look down on unweighted moves as frivolous, easy, maybe even boring. Aside from calisthenics' image problem (kids and old people can do it! And they spend half their time stretching!), the charges against bodyweight exercise boil down to two things. One, they're just not hard enough. And two, you have a limited choice of motions--there isn't a bodyweight equivalent of every move in the gym, and many important ones, the snatch and deadlift as obvious examples, are completely missing.

It's not difficult to come up with tough exercises for your arms: pull-ups in their many variations (including one-handed) provide plenty of challenge for anyone. You'll rarely find anyone complaining that handstand pushups are too easy. But the story is different for legs: Air squats are easy. Pistol squats are an improvement, but hardly equivalent to a gym setup where you can pile on the weights. And nothing you can do empty-handed is going to mimic a 250-pound deadlift.

If an exercise relies on your body weight for resistance—say a pushup—then you can't increase the weight while keeping the move the same. (In fact, if you lose weight, your resistance will decrease.) You can keep the exercise challenging by doing more reps, but at that point, are you just improving endurance without getting stronger in real terms? Can you actually build muscle this way?

Too many reps?

We've all heard it, and probably not many of us have questioned it: the idea that fewer reps at a high weight is going to be more effective for building strength than endless reps at low weight. What you may not have heard: this idea is based more on tradition than scientific evidence. Which makes a certain 2012 study out of McMaster University all the more surprising.

The authors write that there is "no empirical evidence" for favoring heavy weights (and thus low reps). What especially concerned them is the lack of an explanation for how the muscle could possibly tell the difference between a heavy and a light set of an exercise, if both are performed to failure. Afterward, every muscle fiber has been recruited, used, and perhaps damaged; so wouldn't the signal for repair and growth be the same?

So they set up a little experiment. They recruited 18 healthy young men, "recreationally active" but without weightlifting experience. They tested each leg separately, effectively giving them 36 subjects to work with, and assigned each to one of three workouts (so that, yes, the subjects had a different exercise regimen for their left and right leg.) The experimental workout, done on a knee extension machine, called for one of:

• Three sets to failure at 80% of 1RM
• Three sets to failure at 30% of 1RM
• One set to failure at 30% of 1RM

The researchers also fed each subject the same post-workout protein supplement.

As expected, the workout with only one set at 30% didn't build as much strength or muscle as the same weight at three sets. That confirms a hypothesis that volume (how much work you do, total) is more important than what happens in any one set. But the interesting part was that, for those doing three sets to failure, subjects in the 80% and 30% groups fared the same on measures of muscle size, protein synthesis, and strength after 10 weeks.

There was once a theory that only heavy weights can recruit Type II (fast twitch) fibers, but the researchers point out this is based on an old study that only looked at single reps. When you perform an exercise to failure, on the other hand, more and more muscle fibers are being recruited, and in the end, all of the fibers have had their turn to work.

This doesn't change things for light workouts where low weights are used but you don't go to failure; in those cases the Type II fibers are still hanging around filing their nails. Based on this, we can predict that a bodyweight workout in which you do exercises to failure and collapse into a heap of jelly will still build plenty of muscle. One that feels like a breeze, probably not – but you can always add plyometric exercises into that easy routine, which do effectively recruit Type II fibers.

A review of the evidence written in 2013 (which included the above study) concluded that low and high reps do seem to be equivalent when you go to failure, but that the question just hasn't been studied enough in trained athletes to really be sure if it's true all of the time.

Don't have time for a hundred pushups?

That said, high reps can take a long time, which may not be the best way to budget your training. So what are your other options?

You can't say body weight exercises are all too easy. Bodyweight enthusiasts work toward showstoppers like the human flag, one-armed pull-ups, pistol squats, handstand push-ups, and so on. But here we veer into territory that is, in some cases, a bit far afield from weightlifting; these moves require lots of balance and skill-specific practice. That's great if you want to be a gymnast (or just, you know, amaze your friends) but whether they fit into your overall plan depends on your goals and whether you want to pursue a whole new set of skills.

Still with me? The biggest problem, then, is that exercises are limited by position. Many of the harder exercises are just that—different exercises, not harder versions of the same exercise.

Take the pushup for example. On the surface, it's similar to a bench press. But how much weight are you really lifting? Google that question and you'll find people who are guessing anywhere from 30 to 80 percent, not realizing that there is a simple way of answering this question: do a pushup with your hands on a bathroom scale.

The interesting thing about that experiment is not so much the number you see, but the fact that it changes throughout the pushup. That's right, not only does the angle between your body and the direction of force change through the motion, but the amount of weight you're lifting changes too. When I tried this, the scale registered 71% of my body weight at the top of the movement, and 76% at the bottom. This varies considerably based on your hand placement, body shape, and who knows what else.

Wanting more of a challenge—who doesn't?--I propped my toes up on a chair and checked the numbers again: 73% at the top, 78% at the bottom. Definitely more weight, and definitely a harder exercise, but it's also not the same movement: more similar to an incline press than a flat bench. A true handstand pushup would have you taking on 100% of your weight, but would be challenging in very different ways than a shoulder press of the same amount.

The bottom line

Can body weight exercises build muscle? It's very likely the answer is yes. Even if you're not moving as much weight as you would in the gym, if you take a muscle to failure you are spurring it to grow, possibly (though not guaranteed) in a way that's equal to what a heavy barbell would do.

Does it mimic a gym workout? No way. There are lots of ways to modify exercises, get creative, and add challenge—sandbags or partners to add weight, for example—but you'll have to be comfortable with the idea that you're not getting exactly the same workout.

Is it worthwhile? That depends on your goals. Maybe you like the feeling of flow you get from doing dozens of reps of something. Maybe you have a lifelong dream of doing an iron cross or human flag. Heck, maybe you just like that it's different than what you do in the gym. It's hard to hate on something you can do in the park on a sunny day.