FIGURE 6.5 Responses of stretch reflex model (Appendix Figure 1, page 170) to ramps of force, with 7 different combinations of Ia gain (GIa) and Ib gain (GIb), as indicated. The slope of the displacement curve marked GIa = 0 corresponds to the intrinsic stiffness of the active muscle alone. As negative feedback Ia gain increases, the stiffness also increases, but for GIa = 3, the loop goes unstable at the point indicated by the arrow on the right. The addition of positive Ib feedback to negative Ia feedback greatly increases the stiffness of the system.
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