Fig. 1.
Model of the femur–tibia (FT) joint of the metathoracic leg. (a) Extensor apodeme attachment point on the tibia. (b) FT hinge joint and connection of semi-lunar process (SLP) spring. (c) Flexor apodeme attachment point on the tibia. (d) A more distal point on the tibia. (e) The SLP spring is attached between the femur and the tibia. (f) The SLP mass moves along the slider joint (g) oriented between the points b and g that is at an inclination of 36.9 deg. (h) Heitler's lump. The flexor muscle wraps over this lump to alter its orientation with respect to the tibia as the leg is moved. (i) The tendon lock is modeled as a spring located between points c and i (magenta line). It is only enabled when the tibia is fully flexed and flexor muscle has a tension greater than 0.15 N (Bennet-Clark, 1975; Heitler, 1974). The distance between a,b is 0.76 mm, b,c is 1.64 mm. The angle a,b,c is 144 deg. and b,c,d is 143 deg. (Heitler, 1974). The muscle model is shown for the flexor and extensor muscles. This consists of a spring (Kpe), in parallel with a tension generator (T), and a dashpot (B), in series with another spring (Kse). (1) The muscle is activated by firing of a motor neuron (MN). (2) This depolarizes the muscle membrane. (3) Changes in the membrane voltage (Vm) are converted to a tension value using a sigmoidal function. (4) The tension value is scaled based on the muscle length. (5) The scaled tension is applied to the muscle by the force generator to produce a contraction. Tibia extensor muscle/apodeme is shown as a red line, and the tibia flexor muscle/apodeme is shown in green.