When we think of muscles flexing we perhaps think of bodybuilders flexing their biceps during a bicep curl. Although this certainly is one of the types of muscle contraction it is not the only type. There are in fact three types of muscle contraction concentric, eccentric and static.
Muscle tension whilst shortening or lengthening:
Muscle tension with no change in muscle length i.e pausing at the bottom of a barbell back squat