There are 3 types of muscle actions:
An eccentric muscle action occurs as a muscle lengthens.
This is referred to as the negative. In most circumstances this occurs during the lowering portion of the lift. Like in the above example of a biceps curl.
Concentric Muscle Action
The concentric aspect of an exercise is the “positive” or lifting portion of the lift. It’s the opposite of the eccentric.
Instead of the muscles lengthening, the target muscle is shorting to produce force.
Think of a biceps curl where you lift the weight from your thigh to your shoulder.
Isometric Muscle Action
An isometric contraction occurs when force is produced while the muscle maintains a constant length.
The joints do not move.
A good example is your spine musculature in a bent over row or deadlift. You move at the hips but the length of your spine remains constant.
Or in the case of the guy on the left, holding the dumbbell still at the midpoint of a biceps curl.