When measuring skeletal muscle tension that develops during isometric contractions, it is observed that:

The total tension that is developed on stimulating a muscle isometrically is the sum of the passive tension of the unstimulated muscle and the active tension exeed by stimulation. The passive tension increases monotonically with the length of the fiber. Active tension, however, increases up to the resting length of the fiber and then declines as the length is increased fuher. The total tension therefore also shows first an increase and then a decrease as a fiber is lengthened.