Fig. 5. Movements of the insertions of the flexor and extensor tibiae muscles relative to the pivot of the femoro-tibial joint as the tibia moves about the femur. (A) Drawing of the femoro-tibial joint of a left hind leg viewed anteriorly. The distributed insertion of the tendon of the extensor tibiae muscle is shown. The outline of the femur and tibia are colour-coded in orange and red, respectively, and the pivot of the joint is represented by a black dot. The tendon of the extensor tibiae muscle is blue and that of the flexor is green. (B) Six diagrams made from scale drawings of the femoro-tibial joint angles from 163° (extended) to 21.5° (flexed). The outline of the joint is drawn only for the most extended position of the tibia, but the lump over which the flexor tibiae tendon slides is drawn in each. At the most flexed angle, which was never observed during natural movements of the joint, the action of the extensor tendon is over centre. (C) Graphs of the extensor and flexor lever arms (expressed as fractions of tibial length) at different joint angles measured from the forces that have to be applied to the tendons to effect joint movements. The lever arms of the flexor and extensor are equal (lever ratio=1) at a joint angle of 55°.