Two simple models of a KT as a viscoelastic object consisting of a spring and a dashpot. (A and B) Schematic representation of the models. A spring of a stiffness κ and a dashpot of a viscous drag coefficient γ are connected in series (A) or in parallel (B). The KT (pink ellipse) is under tension F and has a length x. In A, the length is the sum of the length of the spring, x_s, and the length of the dashpot, x_d. The equations of the models are given under the schemes. (C and D) KT length as a function of time for the models in A and B, respectively, for a constant tension F that starts to act at time t = 0 and stops at t = T₁. The length of a control KT is denoted x_c. (E and F) Relaxed length as a function of the stretched length (dark green) for the models in A and B, respectively. In E, γ = 5 × 10⁵ pNs/µm, κ = 1,000 pN/µm, and x_c = 0.7 µm, whereas F and t were varied simultaneously in the intervals F = 400–1,000 pN and T1 = 7–12 min, as shown in the inset. In F, κ = 100 pN/µm, x_c = 0.4 µm, and F was varied in the interval 50–200 pN. Horizontal and oblique dashed lines correspond to complete relaxation to the control length x_c and no relaxation, respectively.