Speaking during a Sway in the Morning interview on SiriusXM, she admitted: "I blew my back out. That was in the Billy Brown scene. He threw me up against a wall. All I can say is I was totally committed to the scene."
Though Viola - who plays ruthless lawyer Annalise Keating in the TV series - admits she had to ask producers to "slow down" on the sex scenes for the forseeable future, she is expecting writer Shonda Rhimes to bring them back quite soon.
She laughed: "I just told them to slow it down for a minute.
"I'm going to get back into it. I have to. It's Shondaland!
"They're gonna have me doing all kinds of stuff with all kinds of people."
Elsewhere in the interview, Viola confirmed she won't be attending the Oscars this weekend.
However, though the Suicide Squad star is disappointed by the lack of ethnic actors and filmmakers in the nominations list for the 88th Academy Awards, she is not snubbing the ceremony in Los Angeles to make a point like Will Smith and his wife Jada Pinkett Smith, but it simply clashes with her travel plans.