Is it type of a plagiarism?
First, it isn't plagiarism if you cite it. Plagiarism means using someone else's work as if it were your own. What you have done is paraphrase something from the paper. This is allowed as long as it is accurate. Your statement seems a bit too definite, however. The paper makes claims and provides some evidence, I suspect, but it is a bit strong to state it as you did - as if the papers claims are correct in every respect. 

Maybe: 

"According to [1], Knee flexion was ..."