A lot of people don't believe it, but it's possible to get fit while doing the simplest of routines. You could do pushups, pullups, bodyweight squats, planks, flexibility work, and a bit of cardio FOREVER, and still look and be more fit than half the guys out there. Unfortunately, there's this hard-to-dispel myth going around about how you have to switch up exercises and vary your workouts so that your body doesn't “get used to” your routine and stop getting benefits out of it. Pro tip: it's not true.
The problem is that the fitness industry has grown prodigiously recently, and with that there's just as much misdirection and confusion out there as there's ever been. Trainers are a dime a dozen, and every one of them is trying to make a name for themselves and launch a career. They're all trying to become the next Tony Horton and launch the next big exercise craze so that they can be the one that gets rich off of it. Unfortunately, this just means that there's plenty of superfluous material out there for new exercisers.
You can open up any fitness magazine and chances are you'll find two or three articles about hot new exercises that you should be trying that are going to be so much more effective than the ones that you're already using. Better, since the ones you've been using so far haven't been switched up in a while, these will allow you to try out something new and keep your body moving on to new exercises! Here's the thing: those exercises aren't necessarily any more effective than your old ones, and believing that they are only deepens your indebtedness to those people who are trying to sell new ways of exercising to you.
It's a very attractive theory, in principle. People want to believe it. If you try out a new exercise that you've never done before, you're going to find that you're much more sore the next day than if you had simply used an older exercise, even if they target the same muscles. But the problem is that people falsely equate pain with gain, and in reality that pain means something else entirely: neuromuscular adaptation.
Neuromuscular adaptation is the mental principle behind exercise. It's what happens not when your muscles get bigger or stronger, but when your mind learns to use muscles in a new way. I tell all my clients that about 75% of all strength is related to brain function and has nothing to do with muscles. The number isn't accurate, but it's in the ballpark. Two people with roughly the same height, body type, and body composition can still have vast differences in strength; this will be because one has trained longer than the other, has adapted his/her brain more to the exercise. This is why women can still make great strength gains with lifting programs despite the fact that they build muscle less easily than men. This is why guys like Bruce Lee could perform 80 lb. single bicep curls when I (at forty pounds heavier) can only do a little more than half that. It's not about how big your muscles are, it's about how well your brain recruits them.
That soreness you feel when you try out a new exercise isn't because you worked out harder; all it means is that your brain hasn't yet adapted to the exercise, and so it's using those muscles inefficiently. You can do all sorts of fancy squat variations on bosu balls, but unless you're adding any weight to the equation (and you aren't, because you're balancing on a ball!) you aren't getting any stronger. When you work out with a specific motion, your brain becomes more adept at recruiting muscles in completing that specific motion; when you start focusing on another motion, that adaptation is quickly lost. In order to really get strong then, it's best to stick with only a handful of exercises which form the core of your workout, and in which you can continually develop and get strong. Otherwise you're just twiddling your thumbs and then tossing out your gains whenever you switch on to the next exercise.
For example, there are plenty of different ways to work out the biceps, but they all boil down to curling motions. You can do curls with barbells, dumbbells, or cables, but they all end up training the same muscle. Each will train that muscle slightly differently, yes, but most people don't really know or care, and for the most part you really don't need stronger biceps for anything other than bragging. I even once saw a guy doing barbell curls with a resistance band hooked onto each end and then passing under his feet. He literally had no clue why he was doing it when I asked him. Why get more complicated when you don't have to? He could literally have added ten pounds and gotten the same thing out of the exercise, without having to go through the effort and foolishness of adding a resistance band to the equation.
Here's another reason the “you need to switch up your exercises so your body doesn't get used to them” theory is attractive: it's closely related to another, more correct theory: progressive resistance. It's true, you can't just exercise in the same way over and over and expect to make any real gains. But the solution isn't to switch up your exercises, it's to switch up your intensity. Doing more repetitions at the same weight, doing fewer repetitions at a higher weight, doing more sets, decreasing your break inbetween sets, these are all ways of varying your intensity, and more importantly, they actually work. Progressive resistance forces your brain to adapt to new loads and gain in strength, unlike exercise variation, which endlessly forces it to start over so that you never really make any gains.
I've seen plenty of articles that scientifically debunk new fads in exercise as no more effective than the old. I remember an article about those new shoes that are supposed to burn more calories by altering your gait; in the end, the study found that altering your gait forces you to walk in a new way, and thus causes you to feel more soreness, but ultimately altering your gait had no effect on calories burned nor did it improve your cardiovascular health more than those using ordinary shoes. Unfortunately, all of this information is a sort of well-kept industry secret; most people don't have access to any really scientific info the way that trainers do, and most trainers have some kind of interest in keeping their clients out of the loop so that they can look better and thus further their own careers. But the 'secrets' of neuromuscular adaptation have been known in some way or other to most fitness professionals and good athletes over the years, even if they couldn't really understand the full implications of this principle. Powerlifters, olympic lifters, and strongmen grow strong by practicing the same few, large-muscle-group exercises over and over again, building all-around strength as they go. The simpler you go, the more chances you offer your brain to adapt to your exercise, building real strength. Ultimately, given the fact that you know the right form and how not to injure yourself, it's not about how smart you exercise, it's only about how hard you're willing to push yourself. Who knew that training was ultimately about pushing yourself harder doing what you're already doing, rather than constantly picking up new stuff?
Oh wait, everyone. We've just decided to be hoodwinked for a while and let fitness professionals pull the wool over our eyes. Let's get back to that. Let's work out harder, and simpler. A strictly regimented, constant program with varying intensity is ultimately the key to success, no more and no less. Learn to stop being bad at a lot of things and instead be good at just a few.