Answer
20. Isometric contraction.
Work Step by Step
An isometric contraction is one in which there is only an increase in muscle tension, but no associated increase in length of muscle fibers, or change in angle of associated joint. An examples is holding a side plank yoga pose for several minutes. Another example is straining to lift a barbell that one is unable to budge.
In an isotonic contraction --by contrast with an isometric one--muscle tension, and the length of muscle fibers change. Isotonic contractions occur when one does pull-ups, hoists a book bag, or raises a dumbell of mangeable weight.