The correct word to fill in the blank is:
eccentric
Basing from the force-velocity curve, there are 2 muscle actions, the concentric (shortening) and eccentric (lengthening). In eccentric or lengthening muscle actions, force is high when the velocity is high. This is opposite to concentric or shortening muscle actions, force is low when the velocity is high. This means that in strength-gaining activities, the speed of exercise should be slow during contraction, and it should be high when there is expansion.
