Could it be that you are nervous and or tense sometimes when you knit.
Judge: Madame, either you will have to cease knitting or I will have to cease judging. Which shall it be?
Miss Marple: It helps me to concentrate, milord.
Judge: It does not help me, madame.
Miss Marple: Oh. Very well. (puts away knitting)
Judge: Thank you, Madame.
The lyf so short, the craft so long to lerne.
~ Geoffrey Chaucer - The Parliament of Fowls Line 1.