If there's one thing that I've frequently complained about weightlifters is that many seem stuck in the same rut when it comes to making an exercise more difficult: add more weight. That's the reason why so many can't seem to comprehend how Bodyweight could strengthen the body as well as weights could. They don't see how you could possibly get stronger if the weight (your body) stays the same. The truth is that there are a myriad of ways to increase the difficulty of strength training that don't involve more weight, or iron.
Bodyweight trainees have a similar rut. Many only know one way of making an exercise more difficult: add more reps. Sure, this works for a while but eventually, the max strength benefits are maxed out and the exercise becomes good for endurance. Unfortunately, they don't seem to see or understand that. So, they continue on training with high reps, believing that they're going to get bigger and max-strength stronger by cranking out rep after rep. It kind of makes BW the laughing stock of the strength training world, not taken seriously by the serious strength trainers.
Like I said, high reps are good for endurance but it's value for building muscular bulk and max strength is negligible. I know because I've tried it before. I got into very good shape but eventually I stopped getting the benefits that I sought. So, I had to look elsewhere. So, I started playing with ways to make the exercises that I did more difficult. I looked for new ones. I tweaked the new ones that I found. I made some basic, even crude, pieces of equipment. Other times I'd buy stuff that looked like it would have some merit. Travel always has a way forcing me to be creative with my training, as well as demanding basic but brutal workouts that don't take too long.
Based on my work, I've found that exercises performed at no more than 25-30 reps per set are good for getting max-stronger and increasing muscle size. I also found out that muscle confusion is incredibly important. I daresay it's more important with BW than it is with weight training. Gaining strength in different directions, working from different angles, makes for some astonishing strength gains.
There are other ways to make a BW exercise harder. Most of them fall under one, basic idea: make the exercise more unstable. It's a pretty simple to understand why this works: your muscles now have to work to keep the body in place while moving. Tension becomes your ally. You can do this in a number of ways:
1. Placement of your limbs. Generally speaking, moving limbs together makes a BW exercise harder. Move your hands closer together to make push-ups harder. Move your feet apart to make them easier. Move one hand downward, on a towel, to make a pull-up harder.
2. Stuff! TRX. Push-up T's. Rings. Ab Wheels. These things make a lot of exercises much more unsteady, translating to more contraction. The nice thing about these tools is that most don't take up too much space, making them ideal for travel. The downside is that some are expensive. The upside is that they are pretty easy to improvise if you're handy. In any case, an investment in a bit of gear pays strength dividends.
3. Single Limb Work. Do I need to tell you that dropping a limb out of any exericse will make the exercise A LOT HARDER? Hopefully, I don't. If you're going to bitch that BW doesn't have good max-strength building exercises, then do 6 one-arm push-ups with your feet on chairs, 3 for each arm. Then, get back to me.
There's something else that merits mentioning since we're talking about the fallacy of getting strong using high rep calisthenics and that's the notion that high reps cause injury. I can't give a conclusive answer on that one but I can speak from my own experiences. I've never injured myself by doing high rep (30+ rep) calisthenics. I do, however, see where some people could, or probably will, injure themselves by training in such a manner. Many people sacrifice control of the movement in order to make it easier to do more reps. That's stupid, and it merits my normal adomonition: Speed - Control = INJURY
If you really commit yourself to BW training as a comprehensive method of strength training, then you'll realize that the possibilities of getting more powerful are hard to fathom and don't begin and end with simple number of repetitions. Those of us who do know that with the right modifications, there isn't a lot that you can't do with BW that you can't do with weights. As far as the average (and above-average) strength trainer is concerned, Bodyweight is weight lifting's equal for getting strong. You just have to get creative and get past the high reps as the only method of progression.