Concentric
This type is typical of most exercise. The external force on the muscle is less than the force the muscle is generating - a shortening contraction. The effect is not visible during the classic biceps curl, which is in fact auxotonic because the resistance (the weight being lifted) does not remain the same through the exercise. Tension is highest at a parallel to the floor level, and eases off above and below this point. Therefore tension changes as well as muscle length.
Read more about this topic: Isotonic (exercise Physiology)