The push up is one of the greatest exercises you can do. It is a full-body movement that can be done anywhere using your body weight. That said, the push up can be a difficult movement to master for some people, often requiring modifications at first which many people start on their knees. If you do push ups with your knees on the floor, you will get better at one thing and one thing only – doing push ups with your knees on the floor. Sorry to be the one to break it to you but by doing this modification you are not ever going to get better at doing full push ups. You are not building the proper muscles necessary for growth.
In the proper push up position, the feet are together, glutes are squeezed, and the back is in a neutral position. In other words, there should be no sag in the back and everything should be tight.Then, the entire body should move in straight line, with no sag and with your elbows at approximately a 45-degree angle. At the bottom of the movement, your forearms should be vertical to the ground. The core and glutes are stabilizing the lumbar spine, the back is engaged, and the chest is set up in an optimal position for work. If you think about it, it is really just a moving plank.
So by doing them on your knees let’s look at what is going on!In this position, you are taking your legs out of the equation entirely. It becomes more difficult to engage your glutes. Right now just try to squeeze your glutes in this position – it isn’t as easy. You also don’t need to stabilize your lumbar spine as much, and you couldn’t anyway as you are effectively turning off the stabilizers of the hip which play a huge role in lumbar spine stabilization.Even more, when you then start to lower to the bottom of the push up, the on-the-knees position causes most people to lead the movement with their elbows. This means they are rarely able to end the position with the forearms vertical to the floor. This puts unnecessary stress on the elbow, wrist, and shoulder, which could potentially lead to injuries, not to mention it isn’t the optimal position for strengthening the chest muscles.
So what if you really can’t do a full push up ? If you can’t do a full push up, don’t do more push ups from your knees expecting to get better.Instead, modify the full push up position so your body learns to properly stabilize and work together. How do you do that? The incline push up. Use a bench or barbell on a rack and elevate your hands to the point where you can perform the full movement while maintaining a neutral back and strong core. As you improve, you can lower the position of the bar until you eventually are back to the floor.
Remember, your core is your pillar. It is where all of your strength and power comes from. If you don’t have a strong pillar, you can’t expect to efficiently and effectively move your limbs. This is why performing the push up in any way other than a plank position is not going to help you improve your push ups.
reference
https://breakingmuscle.com/bodyweight/why-knee-push-ups-wont-make-you-any-stronger