What is the difference between 俯卧撑 and 伏地挺身?
俯卧撑 is more commonly used in Mainland Chinese Mandarin, 伏地挺身 in Taiwanese Mandarin.
From Baidu Baike 俯卧撑

俯卧撑（英文：push-up；Press-up。中国大陆称作俯卧撑，中国台湾地区称作伏地挺身，港澳地区称作掌上压）

Also, 俯卧撑 gives more results, probably this the preferred expression.

If you search for both expressions on TW domains (with site:tw [expression]) on Google, 伏地挺身 gives roughly 4 times more results than 俯卧撑. On PRC domains (site:cn), the situation is opposite, 俯卧撑 is roughly 10 times more common than 伏地挺身. Of course many Chinese and Taiwanese sites use other domains than .cn and .tw, this is just to illustrate the regional difference on a limited set of websites.