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. 2013 Aug 5;23(15):1418–1426. doi: 10.1016/j.cub.2013.06.024

Figure 6.

Figure 6

Passive Joint Forces Are Integral to the Generation of Cyclic Limb Movements in Insects

The top panel shows the femur (gray) and tibia (black) in different phases of a movement cycle. Curved arrows show direction of tibial movement.

(Ai and Aii) Passive joint forces support flexions in the jumping legs of locust and Pseudoproscopia.

(Ai) FT joint angles during a cyclic extension and flexion movement. The resting range is indicated by white dotted lines. Movements away from the resting range are driven by either extensor tibiae (dark blue, arrow a) or flexor tibiae muscle (gray, arrow c) contractions. In contrast, return movements toward the resting range can be driven by passive joint forces (red) or by passive extensor muscle forces (light blue). The extent to which the tibia can be passively moved toward the midpoint depends on the starting position of the passive return movement and on residual forces of the flexor tibiae and extensor tibiae muscles (regions of overlap between red/gray and dark/light blue).

(Aii) Schematic of the changing forces produced by the muscles or joint at different joint angles. The schematic is not intended to characterize precisely the joint-angle or time dependencies of the forces, which are almost certainly not linear as illustrated.

(Bi and Bii) In Pseudoproscopia middle legs, the flexor muscle is stronger than the extensor, and passive joint forces contribute to both passive flexions and passive extensions (red). Colors and conventions are as in (A).

(Ci and Cii) In stick insect middle legs, the relative sizes and strengths of the extensor and flexor tibiae muscles are reversed in comparison to (A). Here, passive joint forces drive tibial extensions at flexed joint angles (red), whereas passive flexor muscle forces drive tibial flexions at extended joint angles (light gray). Colors and conventions are as in (A). In behaving animals, passive movements may be assisted by muscle contractions to enhance movement velocities, but they can occur entirely passively, without muscle contractions.