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.
Now, let’s compare this to a push up with your knees on the floor. In this position, you are taking your legs out of the equation entirely. It becomes more difficult to engage your glutes (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.
These reasons for not doing knee push ups may not seem like a big deal, but they are. For many people the reason they struggle with push ups isn’t necessarily their upper body strength. Of course, it’s part of the struggle, but what is often a bigger limitation is that people lack the core strength and/or the proper movement patterning to execute this movement correctly. This is why performing this movement on your knees will in no way help you improve your full push up and will potentially cause you more harm than good in the long run.
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.