Doing a push-up with your body straight and knees off the floor is harder than with them on the ground as it requires less trunk stability to maintain position. The question is: "What is the benefit of push-ups on your knees other than training the beginner or extending the exercise once the athletes strength fails to hold posture?"
In May of this year a study reported in the Journal of Applied Biomechanics measured the surface muscle activity of 8 upper body and core muscles during a push-up. There findings were ....."there were no significant differences between push-ups on the toes versus knees with respect to the percent contribution for the primary muscle groups."
As a coach this changes your thinking about the value of training push-ups from a kneeling versus extended position. Training a novice athlete with planks and bent leg push-ups will quickly hasten the desired development for chest and trunk. Teach the athlete how adjusting their legs still adequately targets the primary chest muscles and is an important consideration when Getting Strong regardless of ones overall strength. This knowledge brings a new focus and enthusiasm to the modified position.
Push-up to Get the chest Strong
The Pendulum Vertical Chest Press