Fig. 5. The effect of insertion angle on the proportion of contractile force in the direction of bite force (effective force). Insertion angles closer to 90° exert 100% of their force normal to the jaw (in the same direction as bite force) and therefore have an effective force of 1.0. The twisted and untwisted conditions are modeled using the same insertion points and therefore have the same in-lever moment arms; however, the twisting of the tendon results in shallower insertion angles and therefore a lower resting effective force. As gape increases from 0 to 40°, insertion angle increases for the anterior face and decreases for the posterior face of both conditions. Because the anterior face of the twisted condition has a resting insertion angle of less than 90°, its effective force is highest at a gape of approximately 10°. Although the posterior face of the twisted system is always less efficient than its untwisted counterpart, its anterior face remains more efficient than the untwisted anterior face for larger gapes, resulting in an eventual equality of average effective force for both conditions. Ant., anterior; Post., posterior; Avg., average.