Both 俯卧撑 (fǔwòchēng) and 伏地挺身 (fúdìtǐngshēn) apparently mean "pushup" according to MDBG. Google Images doesn't give me a clue as to how these terms are different. Are they exactly the same? Are they regional variations?
俯卧撑 is more commonly used in Mainland Chinese Mandarin, 伏地挺身 in Taiwanese Mandarin.
From Baidu Baike 俯卧撑
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.