a, A generic Kelvin–Voigt system exposed to a constant force F epid , and its predicted elongation change for F seam = 0.85 and four different values of α DV based on the equation \({F}_{epid}={F}_{seam}\;{\alpha }_{DV}\). b, A similar system exposed to two forces, F epid and an oscillating force F muscles , and predicted elongation change using F epid = 0.85 and F muscles with an amplitude equal to 1 and the behaviour depicted in the blue-boxed inset. For simplicity, we will refer to the amplitude of F muscles as F muscles . As the pulsatile force induces both compression and stretching (see Fig. 1c), its net input on elongation is transient and the system oscillates around the maximal value reached without F muscles . In all other panels (except in a), F muscles was set as a periodic function with positive and negative steps of duration 6 s modulated by a cosine function, alternating with periods of null value of duration 15 s (b, inset). c, A Kelvin–Voigt system with mechanical plasticity introduced according to equations (1), (4), (6) and (7) in the Supplementary Information, and predicted elongation change using F epid = 0.85, F c = 0, F muscles = 3 and four distinct values of the plasticity factor β, or using F epid = 0.85, F c = 0, β = 0.10 and four distinct values of F muscles . d, A Kelvin–Voigt system in which the plasticity is defective (β = 0), and in which there is actin tearing according to equation (7) in the Supplementary Information, inducing a progressive reduction of F epid , and predicted elongation change with an initial value of F epid = 0.85, the tearing factor γ = 0.15 and F muscles = 3; the inset outlined in blue shows the behaviour of α DV (t) over time. In a–d, the elastic constant of the spring is k = 1, the initial resting length has the value λ(t = 0) = 1, and the viscosity is η = 10. e, Result of the fit for the following genotypes: WT, unc-112(−) alone spc-1(−) alone, spc-1(−) pak-1(−) double, unc-112(−); spc-1(−) pak(−) according to equations (1), (4), (9) to (11) in the Supplementary Information. The values of the parameters are specified in paragraphs 1.5 and 1.6 in the Supplementary Information. The shallow decrease in length for the curve of unc-112(−); spc-1(−) pak-1(−) after 150 min is due to a deformation of the embryos under the effect of unc-112 knockdown but not to retraction, which is why the fit has been evaluated on the first 150 min of the curve.