Hello Zisi Wolff,
The simple short answer is: weakness in the pecs and synergists. Start with wall push ups standing close to the wall, progress away from the wall. Progress to push ups on a table, then a bench, and slowly work your way to floor push ups on the knees.
You could try doing the push ups first before tiring that area. Your client may like to push with rubber resistance.
Natalie aka NAPS 2 B Fit.
There are many variables here with regard to why your client cannot do a pushup. It is difficult for us to determine without evaluating your client. You are the best one to observe and determine the reason–then you can work on modifications.
The pushup works chest, shoulders, triceps, core, and even legs. Then, you take bodyweight into consideration, and that adds another variable. Pushup position is another factor. Here is a basic progression: Wall pushups, on knees, on toes an incline, on toes on the floor, and then on toes on a decline.
The good news is if it’s an exercise that will incorporated into a routine, then you can watch your client progress through the levels.
could it not just be a matter of amount of resistance? In a push up, the person has a lot of weight to lift, usually much more than you would hand her in a dumbbell or barbell. Then there is the matter of shoulder girdle stability. Most chest exercises are done supine where the bench/floor provide at least some external stabilization.
Nancy made some good suggestions for alternatives to move in the direction of a push up.
As a matter of interest: why is it important to you that she does push ups?
Have you tried any gentler modifications to the push-up, i.e. a push-up at the wall?
Also, you mention that your client can do lots of shoulder and arm exercises, but you haven’t said anything about her strength and ability to fire the muscles in her chest. Have you tested her chest strength in other movements, such as a chest fly?
What I’ve done with beginners who couldn’t do push-ups on the floor, is started with resist-a-ball “presses,” where they hold a resist-a-ball straight out from their body, hands a little lower than shoulders, and then press in towards the center. That gives them an idea of whether their pectorals are firing and if they can “feel” their chest muscles. Then, I put them on a wall or a high bench or a bar on the weight rack and have them do a push-up on that incline, trying to get the same “feel” in their chest muscles as they did while doing the resist-a-ball press.