If you are not able to do full push ups you can still do this exercise in the modified push up position, with knees on the floor. (fitbodyhq.com)
These figures are based on women being in the modified position but I believe men and women should be able to do full push ups. (fitbodyhq.com)
It's important to not stop, but to just continue from full push - ups to knee push - ups, and to only use your knees if you absolutely must. (tctmed.com)