Fig. 4.
Static error with spindle feedback only. Graph shows expected minimal static error in elbow joint position with the use of feedback control based on muscle spindles only. Data are based on contractile element (CE) length feedback of the biceps muscle only. Reference positions were 45° shoulder position and 90° elbow position. When not activated, at these positions the biarticular biceps muscle is at optimum length and has an estimated moment arm at the elbow of 0.023 m. The muscle parameter values were identical to those in Kistemaker et al. (2006): maximal isometric force (Fmax) = 414 N, optimum length (lCE) = 0.137 m, slack length (lSE) = 0.204 m. Force of the series elastic element (SE) was modeled as a quadratric spring with 4% (bottom line), 5% (middle line), or 6% (top line) SE stretch (ΔlSE) at Fmax. FB, feedback.