what is the type of contraction in which the muscle fibers produce increased tension but do not shorten?

Answer 1

Isometric contraction

Answer 2

isometric. 'iso' means same; the "meter" is a unit of length, so in isometric contraction, the muscle theoretically stays the same length, but tension is developed. You can do isometric exercises by working against an immovable object (e.g. trying to pick up a desk that is bolted to the floor, would be exercise for your biceps).