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.