Passive tension can develop in the muscle's complex connective tissue.
All the curves share the same characteristic shape, because the variables graphed on the x and y axes are just scaled versions of length and force. Stress, a measure of force per area, replaces force on the y-axis. Strain, an expression of the percentage of elongation beyond resting length, replaces absolute length on the x-axis. Joint angle replaces length as the x-axis variable. To use a flexor muscle group as an example, flexors are short when the joint is flexed and elongated when the joint is extended.