Yes, flex is a Scrabble word!
...and is worth 14 points in Scrabble, and 15 points in Words with Friends
flex
Verb
1. Contract
2. Exhibit the strength of
3. Form a curve
4. Bend a joint
5. Cause (a plastic object) to assume a crooked or angular form
Noun
1. The act of flexing
What Scrabble words can I make with the letters in flex?
3 Letter Words
2 Letter Words