I imagine that the Coronavirus lockdowns closing gyms has something to do with this, but there’s a growing movement towards bodyweight strength training (also known traditionally as calisthenics).
I ran into this recent Medium Elemental piece, which as recent others have done says that you don’t need weights to get in shape. It basically recommends you stick to basic exercises like push ups and pull ups.
And yes, in principle, you can get swole on as little as the Fundamental Few: Push ups, pull ups, squats, lunges, core exercises e.g. planks, sit ups, crunches, Russian twists, etc.
All of these exercises are safe, healthy and useful for most to do, except for push ups and pull ups. Most people do not have the needed muscular strength to minimally complete push ups or pull ups.
Consider that for push-ups you are basically bench pressing most of your entire bodyweight. Could you rack that much weight on an Olympic bar and bench press that right now?
The problem with the knees-down modified push-up is that it often bastardizes your head-to-hips-to-lower-body alignment for the push up. Many who do this often need to angle their thighs and butt upward into an anterior pelvic tilt to complete the exercise… which is not how your hip flexors and core need to be aligned to properly execute a full push-up. This alignment sends much of your body weight down your thighs to your knees, effectively cancelling most of the strength training in the exercise.
With pull-ups, along with needing a stable elevated bar to do a pull up, there’s not much you can do bodyweight-wise to modify the pullup and make it easier. You either pull your entire bodyweight up until you get your head above the bar, or you don’t.
To mirror the push-up metaphor, imagine I set a Lat Pulldown machine to about 80% of your bodyweight. Could you pull that weight down safely with proper form right now?
If you can’t, you don’t have the strength to safely execute a pullup. In fact, many people who force-train themselves to somehow complete them (looking at you, Crossfitters) often set themselves up for long-term injury problems because the force of pulling a weight your muscles aren’t yet equipped to form-properly pull torques, shears and jams all of your relevant joints.
It’s possible to somehow pull your bodyweight up to a pullup bar before your muscles can safely execute a bodyweight-equal lat pulldown… but not safely with proper form, and not without creating undue pressure in your joints (which by the way are not designed to bear weight the way muscles are; remember that your joints are not muscles).
I would even attest that this is the source of the vast majority of injuries in strength training, Crossfit and various athletics. People force their bodies to do exercises before their bodies are ready and/or have the needed strength to do those exercises.
Presuming you don’t have an easy and effective way to do a home bodyweight alternate to these exercises… e.g. modified inverted rows, elevated or incline push ups, or other possibly-bizarre modifications that suit your specific needs… and you do want to eventually perform push-ups and pullups but currently they’re very hard to do, then it’d do you more good in the short run to train the relevant muscles for those exercises on strength training machines if you possibly can. You could even train to eventually execute a pullup or push-up the way you would spend 12-20 weeks training for a marathon.
If you can’t do a pull-up, or can barely do a pull-up, you should do Lat Pull-Downs and Seated Weighted Rows (with proper form, of course!) at the gym as much as you can, at as high a weight as you can reasonably pull for 3-5 sets of 6-12 reps, until you can eventually pull-down at least 80% of your bodyweight. I realize this could take a while… but so would rehabbing from a blown out shoulder (and the latter is a lot more painful and expensive).
If you can’t do a full-form head-to-toe push-up… I’m not opposed to doing modified varieties like wall or incline or knee-down push-ups for practice. But you should perform Incline Bench Presses (a form-safer version than the regular flat back bench-press), pressing shoulder exercises like Shoulder Presses and Front/Lateral Raises, some sort of triceps exercises like Triceps Push-Downs, and possibly another chest pressing exercise such as Chest Flys, at the gym as much as you can, at as high a weight as you can reasonably pull for 3-5 sets of 6-12 reps… until you can bench press about 70% of your weight.
Because a proper push-up requires a strong, aligned core, I would also recommend doing regular ab work… in particular static planks, since your body will in fact have to basically do a plank during every push up. (The regular elbows-down variety is perfectly fine. You can modify to straight arm planks and even perform reps where you alternate between both, if you safely can and want to. This will help.)
If you can’t get to a gym, I would definitely incorporate any of these exercises you can in bodyweight form or using objects around the house. Mark Lauren wrote a great book on how to do this called You Are Your Own Gym. Even following his written routines would be a softer landing into serious bodyweight training than just trying to do cold-turkey push ups and pull ups before you’re ready.
But, main point is… bodyweight exercises can be the easiest form of strength training, if you already have the necessary upper body strength to safely do a bare minimum of all the exercises. And many don’t, plus are risking injury even trying to do them anyway. These people would be better off developing the upper body strength to eventually be able to do them all safely, using machines at the gym or other modified exercises.