Are daily pushups, situps and squats adequate exercise for someone who has no desire to get 'fit' but wants to stay healthy?
Body weight exercises can be very effective & helpful when trying to build functional strength, improve flexibility, decrease body fat, & improve overall physical health. However, I say this only if it's done correctly. Body weight routines are still considered resistance training (weights) & any time you challenge your body with resistance you MUST also give your body what it needs to recover. You need to coupled your regime with a healthy diet & you must give your body adequate time to recover before you attempt to repeat a muscle group again. Resistance training can be broken down into 4 basic movements: push, pull, squat, & lunge. Every form of resistance training can be reduced down to these 4 basic movements whether we're talking machines, free weights, body weight exercises, etc. you're either pulling resistance toward your center mass, pushing it away, squaring or lunging. The only thing that separates these different forms of resistance are 4 basic progressions & how they're applied to the movement. The 4 progressions are: tempo, stability, range of motion, & load. For example, take a bench press for example & how it compares to push ups. Both are a push movement, both are focused on the same groups of muscles (chest & tricep), the only difference are the progressions & how they are applied. The bench press can be more stable & you can add much more weight (load) than you could with a push up, where as the push up can be less stable & could more easily increase your range of motion than you could typically with a bench press. So now that you understand how resistance works now it's important to understand how important recovery is to your progress. I'd suggest breaking your body weight regime into a 3 day routine. "Push day" where guy focus on all push movements, then the next time you exercise (whether that be the next day or the day after) on day #2 center your workout around pull movement, followed by day #3 where you focus on squats & lunges. This way you aren't over working one individual muscle group & your allowing your body to recover before you work out that group again. Then as you continue with your routine, begin researching new & creative ways to challenge yourself using the 4 progressions we talked about earlier. Increase your tempo, start implementing stability with the introduction of balls, bands, etc. You'll soon find that there's a whole lot you can do with very minimal equipment involved. I hope this helps & you stick with it. Have fun!