The verb grind is usually inflected ground in the past tense and as a past participle. Grinded might be considered incorrect in some contexts, but it has grown more common over the past several decades. It’s especially common in American sports commentary and writing, where grind means to overcome adversity by playing hard. Based on historical Google News searches, this sense of grinded has been around at least since the 1960s.
And though spell check disapproves of grinded, it is actually not a new word. It has been used historically in various senses (the OED lists a number of examples from the 19th century and earlier), and a Google Ngram suggests the word was more common in the 19th century than it is now. Still, use of grinded seems to have bottomed out in the middle of the 20th century, and it might sound wrong to many English speakers, especially outside the U.S.
Grinded in sports
Lester grinded his way through six shutout innings to capture the duel of former All-Stars. [Boston Herald]
The Denver Nuggets last night at Pepsi Center grinded out a 104-90 victory over a streaking Sacramento Kings club that had won four straight games. [Denver Daily News (article now offline)]
He ground the cereal into a powder and served it later to Al Roker on the “Today” show. [Arizona Republic]
Kapoor suspects the whole chili (with seeds and stems) was ground together instead of just the flesh. [Toronto Star]
Since last November’s election, the economy has ground to a halt. [Independent]